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Ask the Experts Editor's note: The "Ask the Experts" column has migrated to the "Ask the Weather Guys" column on the blog. For now, the column will be online only, but may reappear in the USA TODAY newspaper weather page in the future, depending on space considerations.
Send a weather question to the Weather Guys Answers to frequently asked questions Questions and answers archive Q: Was the lowest barometric pressure in U.S. history recorded during a hurricane? Yes. The lowest barometric pressure ever measured on the U.S. mainland was 26.35 inches, set on Sept. 2, 1935, in Long Key, Fla., during the Category 5 “Labor Day” hurricane that killed more than 400 people. This is the third-lowest barometric pressure ever recorded during an Atlantic hurricane, behind only Wilma in 2005 and Gilbert in 1988 (both readings were recorded offshore). The world’s low-pressure record of 25.69 inches was set over the Pacific Ocean, during Typhoon Tip in Oct. 1979. For comparison, standard sea-level barometric pressure is 29.92 inches, while the highest pressure reading of all-time was 32.01 inches, set in the Soviet Union in Dec. 1968. This USA TODAY resource page has more about understanding air pressure. (Answered by Doyle Rice, USA TODAY's weather editor, April 28, 2008) Q: Are ocean tides always at the same time intervals? A: No. Neither high tides nor low tides occur at precise 12 hour intervals. This would only be possible if the Earth were completely smooth and covered with a uniform depth of water, with the only tidal influence being a stationary moon. Taking the revolution of the moon into account, high tides are separated by 12 hours, 25 minutes, with a tidal day defined as 24 hours, 50 minutes. Other variables such as the relative positions of Earth, sun and moon, as well as geographic variables such as the presence of land masses and the topography of the ocean floor, can further influence the arrival times of the tides. Read an FAQ about oceans, tides and waves on this USA TODAY resource page. There is also an excellent writeup entitled "Our Restless Tides" on this NOAA webpage. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 27, 2008) Q: Why does the weather usually become cool and clear after a thunderstorm? A: Thunderstorms often occur along or slightly ahead of a cold front. Since the front is a boundary where cold, dry air is impinging upon warm, moist air, you’d feel the tangible change in the air mass due to the frontal passage. In addition to a change of temperature on a warm, humid day, the rain from the thunderstorm can clear the air by flushing out pollutants and particles that can lead to haze. Learn more about storms and fronts on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 23, 2008) Q: When is the latest it’s snowed in some northern U.S. cities? Measurable snow (which is defined as 0.1 inch or greater) has been recorded as late as April 15 in New York City, April 27 in Philadelphia, May 3 in Detroit, May 10 in Boston, May 11 in Chicago, and June 2 in Denver. Snow in Denver in April and May isn’t unusual –- the city typically receives more than 10 inches of snow each year in those two months. This data comes from the always-handy National Climatic Data Center’s snow climatology database. (Answered by Doyle Rice, USA TODAY's weather editor, April 21, 2008) Q: What is Doppler radar? How is it different from other radar? A: RADAR (a term coined as an acronym for Radio Detection and Ranging) operates as follows: a radio wave is transmitted, bounces off of distant objects and is detected when it returns to the transmitter location. The time it takes the wave to make the round trip tells how far away the object is. Doppler radar operates on the same basic principle, but it detects not only an object's distance, but also its motion by measuring the frequency shift between the outgoing wave and the returning wave. An object moving toward the radar would increase the returning wave's frequency while an object moving away from the radar decreases the wave's frequency. For weather purposes, this provides important information about the speed and direction of winds within thunderstorms. Learn more about Doppler radars on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 20, 2008) Q: What causes the yellows, oranges and reds in sunrises and sunsets? A: The same scattering of visible light by air molecules – which makes the sky blue during the day – also causes the yellows, oranges and reds at sunrise and sunset. Light from the horizon travels a much longer distance through the atmosphere and most of the shorter wavelengths are scattered out of one's line of vision in the process, resulting in a yellowish-orange sunrise or sunset. Red sunsets can also occur, often when small particles in the atmosphere from fires or volcanic activity contribute to the scattering. While some mistakenly attribute the brilliant colors to clouds, cloud cover only serves to reflect the light coming from the sun, it does not cause the coloration of the sunrise or sunset. Get the whole scoop on what makes the sky blue on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 16, 2008) Q: What’s the sunniest city in the USA? A: Yuma, Ariz., takes the prize as the nation’s sunniest city, by either of two ways this statistic can be measured. First, the city averages 242 clear days per year, the most of any major U.S. location. Another way of measuring “sunniness” is by the percentage of possible sunshine a city receives each year. By this measurement, Yuma again is the winner, as the city receives 90% of the possible sunshine annually. Other very sunny U.S. cities are also in the Desert Southwest, including Phoenix, Tucson, and Las Vegas. These charts from the National Climatic Data Center show how many cloudy vs. clear days there are for many U.S. cities, as well as the percentage of possible sunshine. (Answered by Doyle Rice, USA TODAY's weather editor, April 14, 2008) Q: How does a dip in the jet stream – which brings cold air south – create a low-pressure trough? A: Cold air is denser than warm air, meaning that more air molecules are clustered close to the surface of the Earth. Since air pressure is the weight of the atmosphere above a certain location, air pressure decreases more rapidly with increasing altitude in cold air than in warm air. At a given height in the atmosphere, say 30,000 feet where the jet stream is often located, the air pressure will be lower in cold air than in warm air. Learn more about how troughs affect weather on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 13, 2008) Q: How can there be a relative humidity reading when the air temperature is well below freezing? A: Water, especially in its energetic gaseous form known as water vapor, is always on the move. Even at air temperatures well below freezing above a frozen lake, there is water vapor in the air. Since relative humidity is the ratio of the amount of water vapor in the air compared to the maximum amount of water vapor possible in the air at a given temperature and pressure, there will always be a relative humidity reading. There's plenty more about the phases of water on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 9, 2008) Q: Have there ever been any planes lost – and crewmembers killed – while on hurricane hunting missions? A: Yes. Since hurricane hunter flights began in 1944, four airplanes have gone down in storms: three in Pacific typhoons and one in a Caribbean hurricane. All 36 men aboard the four airplanes were killed. The deadliest crash was in Sept. 1955, when nine crewmembers and two journalists died during a flight into Hurricane Janet over the Caribbean. The most recent was in Oct. 1974, when a crash killed six in Typhoon Bess over the South China Sea. The other two crashes were in Oct. 1952 (when 10 were killed flying into Typhon Wilma) and in Jan. 1958 (when nine died during a flight into Super Typhoon Ophelia). Airplanes no longer fly into western Pacific typhoons. There's more about the Hurricane Hunters on this USA TODAY resource page (Answered by Doyle Rice, USA TODAY's weather editor, April 7, 2008) Q: What determines how high air rises before it starts to sink? A parcel of air cools as it rises, initially at a rate of 5.5 degrees for every 1,000 feet. As water vapor in the air parcel condenses and releases heat, the cooling rate drops to 3.3 degrees per 1,000 feet. Comparing the temperature of a parcel of air rising through the atmosphere with the temperature of the surrounding air determines if that parcel will continue to rise. If the parcel is warmer, it will continue to rise. Once the parcel is colder, it will sink to its original position. Learn more about convection in the atmosphere on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 6, 2008) Q: Would planting more trees around the world offset global warming? A: Many climate scientists do not count on tree planting as a way to preclude future global warming. Trees act as a carbon sink by absorbing carbon dioxide during photosynthesis in the growing season. While it would seem that increased carbon dioxide emissions would favor additional tree growth and carbon uptake, research suggests that the world's forests are reaching a saturation point. Trees also act as a carbon source when bacteria breaks down dead trees and leaf litter. Warmer global temperatures will likely increase this rate of decomposition, releasing more carbon back into the atmosphere. Many climate change scientists see the planting of new forests as only a stopgap measure to temporarily reduce carbon emissions, not a permanent offset. These scientists prefer an approach that limits a source that we can control, that is, the burning of fossil fuels. Such an approach would require a fast leveling of emissions and steep reductions in the future to stablilize the growing carbon dioxide concentrations in the atmosphere. Learn more global warming and climate change on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, April 2, 2008) Q: Since saltwater is a better conductor than freshwater, do more lightning strikes hit the ocean? A: Lightning strikes are more likely over the world’s oceans due to their enormity, making up about 71% of the Earth's surface area, rather than their conductivity. Both freshwater and saltwater are poor conductors of electricity, though saltwater does have less electrical resistance due to the dissolved sodium and chlorine ions. The conductivity of any object has little or nothing to do with its likelihood of being struck by lightning. If two people stand on an exposed hilltop during a thunderstorm, one holding a golf club and the other a wood baseball bat, both would be equally at risk of being struck by lightning. Find out more facts about lightning on this National Severe Storms Laboratory website. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 30, 2008) Q: If a cold front is the front edge of a mass of cold air at the ground, is there a cold "back" (the back edge of the mass of the cold air)? If not, how is the back portion or end of the cold air mass defined? A: A front is simply a boundary between two different air masses. Dramatic changes in temperature, humidity, barometric pressure and wind speed and direction are all indications of a frontal passage. Once the front passes, while there will be differences within the cold air mass, these differences pale in comparison with differences between air masses. The cold air mass can be thought to move out of your area when a new, distinctly different warm or cold air mass moves in. Learn more about cold fronts on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 26, 2008) Q: Can Lake Superior create its own weather? A: Lake Superior is the largest of the Great Lakes, both in surface area and in average depth. As with any large body of water, Lake Superior moderates temperatures for coastal areas, keeping them warmer in winter and cooler in summer than they would otherwise be. In addition to this moderating influence, Lake Superior can also generate lake-effect snow, contributing to the heavy annual snowfall in northwestern portions of Michigan's Upper Peninsula. The relatively warm waters of Lake Superior in the late fall and winter can also intensify storms that move toward the Great Lakes from the west. Such a storm sank the S.S. Edmund Fitzgerald in November 1975, claiming the lives of 29 crew members. Learn more about the famed wreck of the Edmund Fitzgerald on this NOAA webpage. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 24, 2008) Q: What influences the day-to-day speed and position of the jet stream? The shape, altitude and intensity of the jet stream are determined by the temperature contrasts and pressure differences between the air masses – polar air to the north and tropical air to the south – that bound it. The jet stream's general shape is not a straight band of wind around the globe, but rather an undulating series of troughs and ridges, due to the uneven heating of land and oceans. These dips in the jet stream high above help form the storms we experience at the Earth's surface. Learn more about wind and jet streams on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 23, 2008) Q: I soaked two like-sized towels with two cups of water each, hung one outside in the wind, and one in my shower. Why is the one outside soft and dry in an hour, and the one inside more stiff and still wet in places? How does the wind help to release the water molecules? A: When towels dry, the water molecules in the towel receive enough energy from the air to break the bonds that keep them in the liquid state. Assuming that the air temperature is about the same inside and outside, this would mean that water molecules in each towel have equal amounts of energy available for evaporation. However, the number of water vapor molecules in the air around the towel can affect the rate of evaporation. There is a greater concentration of water vapor molecules around the wet towel in the bathroom compared to the towel outside, where wind whisks water vapor molecules away as quickly as they leave the liquid state. This is the same reason that a breeze helps to cool you down on a hot day. When you sweat, a thin layer of liquid water forms on your skin, and helps to draw heat away from your body. This heat is used to evaporate some of the sweat. A breeze increases the rate of evaporation and this increases the rate at which heat is removed from the body, helping you to feel cooler. Imagine that kids exiting a school bus are water vapor molecules. If kids getting off the bus keep moving and head for the school, the rest of the kids on the bus will be able to get off quicker. However, if kids that get off the bus then immediately stop and congregate around the bus stop, it will take the rest of the kids still on the bus a longer time to exit. There's plenty more about evaporation and condensation on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 19, 2008) Q: What’s the difference between the vernal and spring equinox? A: They are two terms for the same event (“ver” means “spring” in Latin.) Here in the Northern Hemisphere, it’s the exact moment when the sun is directly above the equator, which signals the beginning of astronomical spring, always around March 20. In the Southern Hemisphere, the spring equinox occurs on or near Sept. 23. This year, the spring equinox will be Thursday at 1:48 a.m. ET, for people in the Pacific Time Zone, it occurs tomorrow at 10:58 PT. And no, contrary to a very persistent myth, the spring equinox has nothing to do with balancing an egg on its end. According to meteorologist Jan Null of Golden Gate Weather Services, "there is no scientific basis for this myth, and it has been disproved numerous times....All you need to balance an egg is a raw egg, a hard, flat surface and a steady hand. And it will work any day of the year." For more about this, visit this Bad Astronomy page. .(Answered by Doyle Rice, USA TODAY's weather editor, March 17, 2008) Q: Why does the smoke from a wood fired chimney sink during low pressure and rise during high pressure? A: High pressure is typified by clear skies, dry air, calm winds and cool nights. Cool, dry air near the ground is denser than the warm air leaving the chimney. The chimney smoke will rise until moves into air of equal or lesser density. In advance of a low- pressure system, the air will typically have more water vapor. This water vapor will condense on small particles in the chimney smoke, creating small water droplets that are of equal or greater density than the surrounding air. This smoke will spread horizontally with the prevailing wind without rising much in the air. Keep in mind that atmospheric pressure is just one of the variables that play a role in the behavior of chimney smoke. Wind speed and the temperature profile of the atmosphere are also important factors. Learn more about air quality on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 16, 2008) Q: Could global warming be reduced by reflecting sunlight back into space with mirrors? A: Air molecules, clouds and the Earth's surface naturally reflect about 30% of incoming solar energy. Some scientists have proposed increasing this percentage by building giant mirrors in space or laying reflecting film in the deserts. Others have advocated floating white plastic islands in the ocean or releasing reflective sulfate particles high in the atmosphere. In most cases, these are considered emergency measures in the event that greenhouse gas emissions cannot be reduced sufficiently by effective policy-making to curtail global warming. Learn more global warming and climate change on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 12, 2008) Q: Is there any truth to the proverb 'No weather's ill if the wind be still?' A: Most of the precipitation in mid-latitude storms occurs along and near the cold and warm fronts of the storm system as warm, moist air clashes with cooler, denser air. Winds tend to increase in advance of these fronts and can signal impending inclement weather. A calm wind is more typical of high pressure, where sinking air in the atmosphere tends to minimize cloud formation and precipitation. One caveat is that summertime pulse thunderstorms can quickly form in low-wind conditions, especially on a warm, humid day. Check out this USA TODAY resource page to find out how wind measurements and barometer readings can help you forecast the weather. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 10, 2008) Q: Doesn't this cold, snowy winter mean global warming isn't happening? No. Climate change is measured over years, decades and centuries. The day-to-day and month-to-month variability of weather will still be with us, even as the globe continues to warm, which most scientists think is due to the excess carbon dioxide in the atmosphere caused by the burning of fossil fuels. Incidentally, although it was cold in some spots, the U.S. winter of 2007-08 was actually slightly above the long-term average temperature. Although final global numbers for winter won't be available until next week, some locations enjoyed their warmest winter ever, including Sweden, Finland and Latvia, according to wire reports. In December, January and February, the average temperature in Stockholm, for example, was 36 degrees — the highest mark since recordkeeping began in 1756. (Answered by Doyle Rice, USA TODAY's weather editor, March 9, 2008) Q: Can a lake or river affect the path of a tornado? A: No. Once formed, a tornado is unlikely to be deterred by a lake, river, hill or valley. Past tornadoes have crossed many major rivers east of the Rockies, including the deadliest tornado in U.S. history, the Tri-State tornado of 1925. This twister – which killed 695 people – not only roared across the Mississippi River as it crossed from Missouri into Illinois, but also the Wabash River as it traveled from Illinois into Indiana. Learn more about the Tri-State tornado on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 5, 2008) Q: What was the coldest temperature in the USA this winter? A: The USA’s coldest temperature this winter was –72 degrees, recorded on Feb. 7th and 8th in Chicken, Alaska, a tiny town near the Yukon border. Check out Chicken's website for more about this oddly named town. This reading was just 8 degrees above the all-time U.S. record low temperature of -80 degrees, set at Prospect Creek Camp, Alaska, on Jan. 23, 1971. The lowest reading so far this winter in the contiguous 48 states was –40 degrees, recorded at International Falls and Embarrass, Minn., on Feb. 11. International Falls is a well-known cold spot, and recently was awarded a trademark for the "Icebox of the Nation." This USA TODAY resource page lists the low temperature records for all 50 states. (Answered by Doyle Rice, USA TODAY's weather editor, March 3, 2008) Q: How long does it take for the average cumulus cloud to disperse? A: Cumulus clouds are the puffy, cotton ball clouds you typically see during fair weather. They result from water vapor that condenses into cloud droplets with rising columns of air called thermals. When the cloud is no longer supplied by rising water vapor, it will dissipate as the cloud droplets evaporate into the drier air around the cloud. While the lifespan is dependent on environmental conditions, most cumulus clouds last 5 to 40 minutes. Find out more about cloud development and life cycles at this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, March 2, 2008) Q: On level ground, from how far away can lightning be seen? Heard? A: Assuming a flat, unobstructed view and cloud-to-ground lightning emerging from the base of a twomile-high thunderstorm, the bolt would be visible from over 100 miles away. A taller storm could potentially be seen from a greater distance. However, trees, buildings, clouds in the foreground and terrain usually prevent such an unobstructed view, so the lightning we see is much closer. To estimate the distance you are from a lightning strike, count the seconds between when you see the lightning and hear the thunder. Take that value and divide by five. The result is the approximate distance, in miles, of the lightning. Quite often, a flash of lightning is seen, but no thunder is heard, this so-called "heat lightning" is from a storm that's too far away to be heard. Since sound waves are bent and dispersed as they propagate through the air, thunder is typically heard from storms that are closer than 10 miles, though, under the right conditions, storms can be heard as far as 20 miles away. Learn more about how lightning creates thunder on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Feb. 27, 2008) Q: What area of the USA has the lowest average temperatures in the summer? A: With average temperatures that range from the upper 40s to upper 50s, Alaska is the coolest U.S. state in July, typically the USA’s hottest month. Surprisingly, another cool spot is the Mauna Loa Observatory in Hawaii, which has an average July temperature of only 47 degrees. In the continental USA, the coolest summer weather is in western Wyoming and the Cascade Mountains of Washington, where average July temperatures are in the upper 50s. This map (PDF) from the National Climatic Data Center shows the average July temperatures across the lower 48 states. (Answered by Doyle Rice, USA TODAY's weather editor, Feb. 25, 2008) Q: Does snow cover make the air above it warmer or colder? A: Fresh snow cover makes it colder. Highly reflective to visible wavelengths of sunlight, snow reflects a lot of the energy that would be absorbed by bare ground during the day. Snow is also an effective emitter of infrared energy, increasing the amount of energy lost from the surface of the Earth. This greater energy loss during the overnight hours can make snow-covered mornings colder than mornings with no snow. This USA TODAY weather focus graphic also explains how snow can compound cold. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Feb. 24, 2008) Q: Why do most thunderstorms move from the southwest toward the northeast? A: Most thunderstorms get their motion from steering winds located between 10,000 to 20,000 feet above the surface. Many supercell thunderstorms and squall lines that form in the central USA are the result of surface winds that flow from the south and stronger steering winds that are from the southwest. Quite often, thunderstorms will develop in warm, humid air ahead of a cold front moving from west to east. While the favorable area for thunderstorm development moves from west to east as the system evolves, the individual thunderstorms will move from southwest to northeast due to the steering winds. Learn more about severe thunderstorms on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Feb. 20, 2008) Q: Was January an unusually cold month across the USA? A: Yes – the USA’s temperature in January was about 0.4 degrees below the long-term average, according to the National Climatic Data Center. Temperatures in much of the West were below average, while the Midwest, South, and Southeast were about average. However, temperatures were above normal in the Northeast, which had its 20th- warmest January on record. Globally, January was slightly warmer than normal, with a reading of nearly 0.4 degrees above average. You can read the full report about January's weather on this National Climatic Data Center's web page. (Answered by Doyle Rice, USA TODAY's weather editor, Feb. 18, 2008) Q: What is a secondary low? A: A secondary low-pressure area sometimes forms to the south and east of a weakening primary surface low. This can occur when jet stream winds are strong to the south and east of the primary low, creating a new surface low as the old surface low weakens. Another contributing factor in secondary low formation is terrain. In the winter, it’s common for a primary low west of the Appalachians to give rise to a secondary low along the East Coast. Such secondary lows can develop into strong "nor’easters" that bring heavy snow to New England. Learn more about storms and fronts on this USA TODAY resource page. You can also check out this graphic about why a storm center appears to jump. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Feb. 17, 2008) Q: How long does it take the sun to move its own diameter across the sky? A: There are two different motions that one has to include to answer this question. The first is the diurnal (daily) motion the sun makes on the sky due to the Earth's rotation. We can get a fairly accurate estimation of this motion by assuming that we are on the equator and it is the vernal or autumnal equinox (hence the sun is on the celestial equator). Then all one needs to do is to divide the sun's angular size in degrees (about 1/2 a degree) by 180 degrees (the horizon to horizon distance passing overhead) then multiply this fraction by 12 hours (1/2 a day) which is the amount of time it takes the sun to move across the sky on these two dates. That gives 0.5 deg/180 deg x 12 hours = 0.0333 hours = 2.0 minutes -- the sun moves one its angular diameters in 2 minutes of time due to the Earth's rotation. However during this time interval, the sun is also moving on the sky due to the Earth's orbital motion about the sun. The sun is moving eastward on the sky as the Earth orbits the sun (which is in the opposite [westward] direction caused by the diurnal motion). We can calculate the offsetting orbital motion with the following calculation. The sun moves around the sky with respect to the background stars (360 degrees), following the ecliptic, in one year's time (1 sidereal year = 8765.76 hrs). As such, this eastward motion of the sun works out to be 0.5 deg/360 deg x 8765.76 hr = 12.2 hours -- the sun moves one angular diameters in 12.2 hours eastward with respect to the background stars due to the Earth's orbital motion. This rate is 0.3% of the sun's diurnal motion (0.3 seconds of time) and hence can be ignored in our final answer. Check out these FAQs on the sun and the seasons on this USA TODAY resource page. (Answered by Donald Luttermoser, professor of physics and astronomy at East Tennessee State University, Feb. 13, 2008) Q: How fast does the wind have to blow to be considered a blizzard? A: Wind is only one part of the “official” definition of a blizzard. According to the National Weather Service, a blizzard is defined as a snowstorm with winds of 35 mph greater, along with snow and blowing snow that reduces visibility to less than one-quarter of a mile for at least three hours. Blowing snow is falling snow and/or snow already on the ground that’s been picked up by the wind. For more cold, hard facts about winter, check out this USA TODAY winter weather glossary page. (Answered by Doyle Rice, USA TODAY's weather editor, Feb. 11, 2008) Q: How far in advance can weather be forecast, and how accurate will that forecast be? A: It depends on what you are trying to forecast. If you want to know exactly when rain will arrive at your house, such a forecast might only be possible from a few hours to a day before the event. For forecasting the locations of storms, reasonable accuracy can be expected about three to five days out. Weather can also be forecast months in advance, but the objective is different. Rather than trying to pinpoint individual storm systems on a day-to-day basis, long-range forecasts attempt to forecast the temperature or precipitation over the span of a month or season, compared to long-term climate averages. Learn more about weather forecasting on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Feb. 10, 2008) Q: Do Alberta Clippers ever hit New England? A: From their origin in western Canada, Alberta Clippers often cross the border and zip across the northern Plains, eventually heading toward the Northeast or Mid-Atlantic. While over the northern Plains and Great Lakes, these fast-moving storms typically do not produce heavy snowfall, but can bring gusty winds and surges of colder air. If conditions are favorable, some clipper systems slow and intensify upon reaching the East Coast and can result in heavy snowstorms for New England. Learn more about Alberta Clippers on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Feb. 6, 2008) Q: When it rains, why do raindrops fall down one drop at a time, instead of the entire cloud of condensed water falling all at once? A: While processes within the cloud preclude condensation of all the water vapor into one mass, another reason that raindrops fall individually is air resistance. While cloud droplets are spherical due to the surface tension of water, raindrops, which are typically at least 100 times larger than cloud droplets, become deformed due to air resistance. Most raindrops take on the shape of a hamburger bun, while larger drops become elongated and form a loop not unlike a parachute before breaking up into smaller drops. Therefore, even if all the water in a cloud condensed into a sheet, it would break into individual droplets along the way. Learn more about rain on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Jan. 16, 2008) Q: Is the ice in the polar ice cap fresh- or saltwater, or a combination? A: Even though it comes from salty ocean water, the sea ice that forms in the Arctic and Antarctic is fresh water. And while the presence of salt lowers the freezing point of ocean water, it can and does freeze. However, the salt molecules are rejected back into the liquid as the ice forms, resulting in freshwater ice floating in a briny solution. The land ice over Greenland and Antarctica is also fresh water, and results from the accumulation of snowfall over long time periods. Learn more about the formation and chemistry of ice on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Jan. 14, 2008) Q: When was the "greenhouse effect" first recognized? A: In the 1820s, French scientist Jean-Baptiste Joseph Fourier first recognized the atmosphere’s role in maintaining a climate livable for humans on Earth. Without the atmosphere’s warming influence, the Earth’s average temperature would be near 0 degrees, far colder than the actual temperature of 59 degrees. Building on this work by Fourier and others, Swedish chemist Svante Arrhenius published his "hot-house theory" in the early 20th century, which would come to be known as the "greenhouse effect." The sun radiates energy to Earth in the form of solar radiation. Some of this energy is reflected back to space by the Earth and the atmosphere. The remaining energy is absorbed by, and thus warms, the atmosphere and the Earth. The Earth then reemits the energy back to space in the form of infrared radiation. This is where the natural greenhouse effect comes into play. Greenhouse gases, including water vapor and carbon dioxide, can absorb some of this outgoing infrared radiation. The heated greenhouse gases then radiate infrared radiation in all directions, sometimes back to Earth. Any energy trapped between the Earth and atmosphere in turn heats up the Earth. A USA TODAY online graphic also explains this process. Recent human activities have enhanced the greenhouse effect by releasing more greenhouse gases in the atmosphere, especially carbon dioxide. This enhanced greenhouse effect can be linked to global warming, which scientists have been recently studying. Check out this USA TODAY resource page for more information on climate change. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Jan. 13, 2008) Q: What are some of the cloudiest U.S. cities in January?
Probably the best way to measure this is by the percentage of the possible sunshine that a city receives each year, as measured by the National Climatic Data Center. By this measure, the cloudiest January location is Quillayute, Wash., which is cloudy 78 percent of the time, on average. Other cities that are cloudy at least 70 percent of the time in January include Seattle and Spokane, Wash.; Portland, Ore.; Grand Rapids, Mich.; Elkins, W. Va.; and Cleveland. On the other end of the spectrum, the sunniest U.S. city in January is Yuma, Ariz., which is cloudy just 16 percent of the time. Other sunny January cities are Tucson and Phoenix in Arizona, Las Vegas, Nev., and Key West, Fla. (Answered by Doyle Rice, USA TODAY's weather editor, Jan. 7, 2008) Q: What determines the rate or size of falling rain? A: A raindrop’s size as it leaves a cloud is determined by several factors, including the availability of water vapor and the intensity of updrafts within the cloud. Larger drops tend to result from the vigorous updrafts within a thunderstorm. Because larger drops usually fall faster than smaller drops, cloudbursts have the largest average drop size, fastest average fall speed and most intense rainfall rates. Mist or drizzle involve much smaller drop sizes, slower fall speeds and lower rainfall rates. Learn more about rain on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Jan. 6, 2008) Q: What's the difference between a wave's crest and its trough? A: The crest is simply a wave’s highest point, while the trough is its lowest point. The length of a wave is measured from crest to crest or from trough to trough. The wave height is the difference between the crest and the trough of the wave. The size of a wave is typically determined by the wind speed, the length of time the wind has blown, and the distance the wind has blown over water, known as fetch. Increasing any or all of these variables leads to increases in both the length and height of a wave. Learn more about how wind creates waves on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Jan. 2, 2008) Q: Where are the warmest places in the USA to vacation in January? A: The warmest location in the USA in January is Honolulu, where the average high in January is a balmy 80 degrees, and the low 65. Other locations in Hawaii have similar January temperatures. If you can’t make it to the Aloha State, try South Florida. Miami’s January average high temperature is 77, and the average low about 60; in temperate Key West, the averages are 75 and 65. Other rather warm spots in the continental USA include Phoenix (with an average January high of about 67 degrees) and Brownsville, Texas (with an average high of about 69 degrees). This map (pdf) from the National Climatic Data Center shows the average daily high temperatures for January across the USA. (Answered by Doyle Rice, USA TODAY's weather editor, Dec. 30, 2007) Q: What's the difference between sleet and hail? Aren't they both ice? A: Sleet is wintry precipitation that results when falling snowflakes are partially melted by warm air and then refrozen into small grains of ice as they fall into subfreezing air near the Earth's surface. Hail is more typical of summertime thunderstorms, as it forms in rising air currents, which carry water droplets high into a thunderstorm. There they freeze and grow as other drops collide with them. Air rising at 37 mph is needed to hold up a ¾" hailstone. Hailstones grow until they are too large for the storm's updrafts to hold them up, then they fall to the ground. Hailstones can fall at speeds well over 100 mph. Hail forms differently than sleet. Whereas hail are chunks of ice that form in the storm cloud and fall to Earth before melting, sleet starts as snow or ice in the cloud, melts during its descent, then refreezes as an ice pellet before hitting the ground. There's more about hail formation on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 26, 2007) Q: About how many thunderstorms are there each day worldwide? A: There are an estimated 40,000 to 50,000 thunderstorms each day worldwide, with about 1,800 going on at any given moment. The majority of thunderstorms occur in tropical regions over land where warm temperatures and water vapor, key ingredients for thunderstorm formation, are abundant. Tropical regions of South America, Africa and Asia can see more than 200 thunderstorm days each year. In the USA, thunderstorms are most frequent in Florida and along the eastern Gulf Coast. Learn more about thunderstorms on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec.23, 2007) Q: How does winter begin at a specific time? A: Astronomical winter in the Northern Hemisphere begins at the winter solstice, which is the exact moment when the sun is directly over the Tropic of Capricorn in the Southern Hemisphere. This year, that occurs at 1:08 a.m. ET this Saturday, Dec. 22. Although no governmental body has designated it, the start of the astronomical seasons have been described by many in the media as the seasons' "official" start. Meteorological winter, however, began on Dec. 1 and ends on the last day of February, which in 2008 is leap day, Feb. 29. Fun fact: When preparing the winter fuels outlook this year, the energy experts who prepare this information had to take into account leap day in their outlook. Surprisingly, just one extra winter day can actually make a noticable difference in the USA's annual energy usage. (Answered by Doyle Rice, USA TODAY's weather editor, Dec. 19, 2007) Q: Can there be rainfall without any wind? A: It would be unusual to have rain without wind. Not only is wind associated with the fronts that produce showers and thunderstorms, but rain can generate its own wind. Rain-cooled air descends along with raindrops and can form gust fronts as the air hits the ground and spreads out in all directions. In the case of drizzle, which are raindrops of less than .02 inch in diameter, the falling speeds of individual drops are relatively small and wouldn’t generate much wind. Learn more about rain and drizzle on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 17, 2007) Q: At what temperature will water pipes freeze and burst in an unheated crawl space under a home? What about outside water spigots? A: While there are many variables, including the amount of insulation, proximity to living space, and amount of airflow on pipes, research has found that unprotected pipes are more likely to burst when the outside temperature drops to 20 degrees or below. The best protection is to properly insulate pipes or install electric heating tapes or cables. For outside spigots, hoses and diverters should be detached and cut-off valves, if available, should be shut. Learn more about the phases of water on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 16, 2007) Q: I'm convinced that on relatively clear and calm days, when a cloud passes in front of the sun, the wind velocity often appears to pick up. Is there a reasonable explanation for this or am I simply mistaken? A: Sunlight being blocked by an isolated cumulus cloud would have little impact on winds at the surface of the Earth. However, it‘s very possible that it could result in brief cooling, similar to the cooling felt during a light breeze, as a spot on the ground goes from direct sunlight to shadow. As we all know from experience, it's warmer in the sunshine than in the shade. When standing in the sunshine, not only do we sense the temperature of the air around us, but we also absorb infrared energy directly from the sun. There's plenty more about air temperature on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 12, 2007) Q: On a typical summer day in the Northern Hemisphere, which city often has the highest temperature? A: According to Christopher Burt's book Extreme Weather, remote oases in the Sahara Desert of Mali and Algeria, along with Death Valley, Calif. -- where average July high temperatures are about 116 degrees -- usually endure some of the hottest summer days anywhere in the Northern Hemisphere. As for more populated cities, some notorious hot spots include Jacobabad, Pakistan, with an average July high of 114 degrees, and Abadan, Iran, where a typical August day has a high of 113 degrees. Compare these scorching summer high temperatures to those in Phoenix, the hottest major city in the USA, where the average high temperature in July is "only" about 107 degrees. (Answered by Doyle Rice, USA TODAY's weather editor. Dec. 10, 2007) Q: On Nov. 22, while fishing on Long Island, I found a weather balloon floating. It had a phone number on it so I called and verified it was let go on Oct. 31 in California. Can you explain this? Was it in the jet stream or something? A: Weather balloons, which can reach altitudes of more than 20 miles, are steered by winds at various levels of the atmosphere as they ascend. In order to make the more than 2,500-mile-journey from California to Long Island, N.Y., this balloon likely rode the mid-latitude jet stream. While the location, altitude and wind speeds within the jet stream vary from day to day, the mid-latitude jet stream can usually be found somewhere between 25,000 and 35,000 feet, with winds that often exceed 100 mph. Weather balloons are released twice a day, at 00:00 and 12:00 UTC, from a network of nearly 900 stations worldwide. The majority of these stations are located in the Northern Hemisphere, with 69 stations located in the lower 48 states. These balloons give forecasters a vertical snapshot of the atmosphere. Using a 6-foot-wide helium or helium-filled balloon to carry an instrument package, measurements of temperature, pressure and relative humidity are transmitted by radio waves back to Earth. According to the National Weather Service, a balloon flight can last more than two hours, with the balloon achieving altitudes up to 115,000 feet and traveling more than 125 miles from its release point. After the balloon bursts, a parachute slows the descent of the instrument package, minimizing danger to lives and property. Should you find an instrument package, also known as a radiosonde, follow the mailing instructions that will return it to the NWS for reconditioning. The NWS estimates that only about 20% of the 75,000 radiosondes released annually are recovered and returned. Learn more about weather balloons on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 9, 2007) Q: Is it true that our magnetic poles change every so often? A: Unlike the geographic North and South Poles, the magnetic poles are not directly opposite each other on the globe and can change position, intensity and even polarity. Since its location was first discovered in 1831, the position of the north magnetic pole has been moving north through Canada and will reach Siberia in a few decades. The Earth's magnetic field last flipped its polarity 780,000 years ago, though the long-time average for such reversals is about 300,000 years. There's plenty more about the Earth's magnetic poles on this NASA resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 5, 2007) Q: Does it ever thunder and lightning during a snowstorm? A: Yes, and when it does, the phenomenon is called "thundersnow." Thundersnow occurs less frequently due to the relative lack of warm and moist air that typically fuels thunderstorms during the rest of the year. That said, thundersnow can and does occur, especially during lake-effect snow events, as the relative warmth of the lake water can enhance convection, resulting in lightning and thunder. Thundersnow has also been reported in many parts of the Plains and Midwest. Since thundersnow results from vigorous convection, it can also result in localized areas of very heavy snowfall. Learn more about thundersnow research on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 3, 2007) Q: What is a microburst? How does it occur? A: A microburst refers to a blast of downbursting winds. These winds are concentrated in an area less than 2.5 square miles and can sometimes exceed 150 mph, resulting in damage akin to a tornado. Unlike tornadic winds, which tend to swirl debris in all directions, objects toppled by microburst winds tend to fall in a straight line. Due to their small size, microbursts can be difficult to detect even using Doppler radars and are almost impossible to predict. Because of their intensity, microbursts pose a danger to aircraft. Dry microbursts occur when the air beneath a raincloud or thunderstorm is very dry. Much of the precipitation associated with the storm evaporates before reaching the ground. The process of evaporation cools the air, making it denser and heavier than the surrounding air. The only evidence of the microburst might be a cloud of dust kicked up by the winds. In contrast, wet microbursts occur in more humid locations when the downbursting winds are accompanied by heavy rain. In the humid East, wet microbursts are most likely while dry microbursts occur more often in the West. Learn more about microbursts on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Dec. 2, 2007) Q: How does the weather change when there is an occluded front? A: An occluded front typically forms when a faster-moving cold front catches up to a slower-moving warm front. When the air behind the cold front is colder than the air ahead of the warm front, the occluded front will behave like a cold front, with brief, heavy rainfall and a wind shift to the west or northwest. When the air behind the cold front is not as cold as the air ahead of the warm front, lighter, but more prolonged, precipitation can be expected, similar to the overrunning precipitation produced by warm fronts. Learn more about storms and fronts on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 28, 2007) Q: What is a tornado’s average time on the ground? A: According to the Storm Prediction Center, most tornadoes last from 5 to 10 minutes, although they can exist for as few as several seconds to more than an hour. The longest-lived tornado on record is unknown, since so many of the supposedly long-lived tornadoes reported in the early 1900s and before were likely more than one tornado. One long-lived twister was the infamous Tri-State Tornado of March 1925, which may have been on the ground for nearly four hours as it tore a path of death and destruction across Missouri, Illinois and Indiana. (Answered by Doyle Rice, USA TODAY’s weather editor, Nov. 26, 2007) Q: Why is humidity often highest around sunrise? A: After a night without the input of any solar energy, it is often coolest at sunrise. Relative humidity -- a percentage that measures how close the air is to being saturated -- increases as the air temperature drops closer to the dew point temperature. Relative humidity tends to be highest during the coolest part of the day and reaches its lowest value during the heat of the day, when the difference between air temperature and dew point is greatest. Learn more about humidity on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 25, 2007) Q: How does the sun affect vision at sunrise and sunset? A: An unobscured sunrise or sunset can cause traffic delays and/or accidents if the sun falls within the field of vision of east- or westbound drivers. While the sun's rays travel a longer distance through the atmosphere and are thus not as intense on the horizon as at its zenith, the light can still be blinding to a motorist. If the roadway is oriented due east-west, the glare will be most problematic near the fall and spring equinox, when the sun rises at due east and sets at due west. Check out these FAQs on the sun and the seasons on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 19, 2007) Q: How cold does it have to be to calculate a "wind chill?" I live in Jamaica and in December and January we may get a "norther" when the temperature can drop to about 60 degrees and the wind could be blowing 40 mph. A: Just as summer's "heat index" communicates the combined effect of air temperature and humidity on human comfort, winter's "wind chill" value combines air temperature and wind speed. Wind chill is only defined for temperatures at or below 50 degrees and wind speeds above 3 mph. Wind chill values don’t include input from the sun, thus bright sunshine could increase the wind chill temperature by 10 to 18 degrees. A wind chill chart and handy wind chill calculator are included on this National Weather Service webpage. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 18, 2007) Q: When did we start keeping weather records in the USA? A: Rev. John Campanius Holm is credited as the first to keep a record of systematic weather observations, starting in 1644 in Wilmington, Del. Founding Fathers Ben Franklin, George Washington, James Madison and Thomas Jefferson were also keen weather observers and kept personal weather diaries. In 1814, a network of weather observations was established at Army posts across the country. By 1848, the Smithsonian Institution used the new telegraph technology to assemble a network of volunteer observers, providing them with standardized equipment. Learn more about weather history on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 14, 2007) Q: What is Indian summer? A: Indian summer, which has no exact definition, is generally described as a period of unseasonably warm weather in mid- or late autumn. It usually features sunny, mild days and cool nights. A killing frost and seasonably cool weather should come before this warm spell, in order for it to be called a true "Indian summer." And although there are several theories about the origin of the term, no single theory has been proven. Check out this USA TODAY resource page for more about Indian summer. (Answered by Doyle Rice, USA TODAY's weather editor, Nov. 12, 2007) Q: What are the dry and moist adiabatic lapse rates? A: An adiabatic lapse rate is a physical constant that meteorologists use to forecast the weather; a "lapse rate" describes how quickly air cools as it rises in the atmosphere, while "adiabatic" means that heat isn't being added or subtracted to the air. When no condensation occurs, the “dry” adiabatic lapse rate is 5.5 degrees for every 1,000 feet of ascent. When condensation occurs, latent heat is released and the air doesn't cool as fast. This “moist” adiabatic lapse rate is about 3.3 degrees per 1,000 feet of ascent. Learn more about the temperature profile of the atmosphere on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 11, 2007) Q: How long does a raindrop take to hit the ground? A: The lifetime of a raindrop can vary, depending on the altitude of the cloud and the size of the drops. A large raindrop with a diameter of about 0.25 inch can fall at speeds near 20 mph. Assuming that the raindrop's terminal velocity is reached shortly after leaving the cloud base, determining the lifetime is just a matter of dividing the fall distance by the terminal velocity. So a large (0.25 inch diameter) raindrop’s fall from the base of a 10,000-foot cloud would take about six minutes. There's more about rain on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 7, 2007) Q: Are there any parts of the USA where a white Christmas is guaranteed? A: Only five towns have had snow on the ground every Christmas since weather records began: Marquette and Sault Ste Marie, Mich.; Hibbing and International Falls, Minn.; and Stampede Pass, Wash. In addition, northern parts of Minnesota, Wisconsin, Michigan, New York and New England have a better than 90% chance of snow on the ground on Dec. 25, as do the highest elevations of the Rockies, Cascades, and Sierra in the West. This USA TODAY resource page has a map that shows where a white Christmas is most likely, as does this weather.com page. And here's a link to the original report (PDF) from the National Climatic Data Center. (Answered by Doyle Rice, USA TODAY's weather editor, Nov. 5, 2007) Q: In addition to the USA, which areas of the world also see frequent tornadoes? A: Canada, particularly southern sections of its prairie provinces of Alberta, Saskatchewan and Manitoba, is no stranger to tornadoes. Strong tornadoes can also occur in Bangladesh, where tornado deaths are common due to population density and poor building construction. Tornadoes also occur from Western Europe into Russia, though many of these are rather weak. Other countries that see tornadoes include South Africa, Argentina, Japan and Australia. Learn more about tornadoes on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Nov. 4, 2007) Q: What's the difference between "rain" and "showers?" A: When a forecast calls for “rain” or is worded "rain likely," rain should be expected to fall steadily over a wide area. This often occurs when an overcast sky brings a lingering rain to a region. The term "showers" indicates that the coverage of rainfall will be spotty. Since showers often come from convective clouds that are sometimes short-lived, showers can also indicate that rainfall in any one location will be brief. Learn more about rain on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 31, 2007) Q: Why do we have hurricanes? A: Hurricanes are one of the ways that the Earth's atmosphere keeps its heat budget balanced, by moving excess heat from the tropics to the middle latitudes. Hurricanes can be described as huge machines that convert the warmth of the tropical oceans and atmosphere into wind and waves. They aren’t very efficient machines, however, as only a small percentage of the disturbances that could spawn hurricanes actually do develop into storms. This page from the Atlantic Oceanographic and Meteorological Laboratory shows what's needed for tropical disturbances to develop into hurricanes. (Answered by Doyle Rice, USA TODAY's weather editor, Oct. 29, 2007) Q: Does dew or dense fog that collects in my rain gauge count as "precipitation"? A: No, if you are a volunteer who reports observations to your local National Weather Service office, dew or heavy fog should not be recorded as rainfall. On rare occasions, heavy dew might result in 0.01" showing in your rain gauge. In this case, dump it out or allow it to evaporate during the course of the day, but do not record it as observed precipitation. However, if the precipitation is the result of a light drizzle, it is counted as official rainfall and should be recorded. Learn more about measuring weather on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 28, 2007) Q: How do prevailing winds affect the movement of fronts in the USA? A: The prevailing winds here in the mid-latitudes are from the west. Therefore, most storms move west to east across the country. Attached to these eastward-moving storms are often warm and cold fronts. A cold front – where cooler, drier air impinges on warmer air – tends to move from northwest to southeast as the storm develops. The storm's warm front lifts toward the north as warm, moist air overrides cooler, denser air to the north. Learn more about winds and jet streams on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 24, 2007) Q: Which states have been hit by the most hurricanes? A: By far, the state that receives the most direct hits from hurricanes is Florida, according to the National Hurricane Center. Of the 284 hurricane hits in the USA since 1851, 114 have been on the coast of Florida. That’s 40 percent of the total number of hurricane hits. Other hurricane-prone states include Texas, with 60 hurricane hits, followed by Louisiana (52), North Carolina (46), and South Carolina (31). Of all the Category 4 or 5 hurricane hits in U.S. history, 83% have been in Florida or Texas. This list from the Atlantic Oceanographic and Meteorological Laboratory shows the hurricane hits for each state, and also details the hurricane hits by categories. (Answered by Doyle Rice, USA TODAY's weather editor, Oct. 22, 2007) Q: What's the diameter of a rainbow, and is this dimension constant? A: In order for a rainbow to be visible, droplets of rain should be in front of you while the sun should be at your back. Just as the distance that a rainbow appears from the observer can vary, the diameter of a rainbow is also not constant. If you can determine the distance between your location and the rain (perhaps by using a radar image), the height of the rainbow is about 0.9 times that distance. Since the rainbow's height is approximately the radius of the rainbow, the diameter can be found by doubling that number. Learn more about rainbows on this University Corporation for Atmospheric Research webpage. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 21, 2007) Q: Do clouds keep temperatures warmer or cooler during the overnight hours? A: Air temperature largely results from the balance between energy received by Earth from the sun and energy lost by the Earth to space. At night, although there is no energy input from the sun, the Earth continues to radiate energy. On clear nights, much of this energy radiates into space, even though some energy is absorbed and radiated back to Earth by gases in the atmosphere. Since cloud droplets are better at absorbing infrared radiation than air molecules, more of the energy radiated by the Earth is absorbed and radiated back on cloudy nights. This extra energy received by the Earth keeps the temperature warmer. Learn more about the warming influence of clouds on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 17, 2007) Q: What kind of weather changes does a cold front bring as it moves through? A: A cold front is a boundary between cool and warm air, with the colder air replacing the warmer. As the front arrives, barometric pressure falls and then rises after it passes. Winds ahead of a cold front are often from the south or southwest, while those behind the front – in the cooler air – tend to be from the north or west. In the spring, summer and fall, an arriving cold front can also trigger thunderstorms. Cold fronts are represented on weather maps by a blue line with triangles, which point in the direction the cold air is moving. One type of cold front is called an "anafront," which is described on this USA TODAY resource page. (Answered by Doyle Rice, USA TODAY’s weather editor, Oct. 15, 2007) Q: Why is it so humid in the Southeast and not in the Southwest? A: Humidity is typically highest near bodies of water, where water vapor is abundant. In the Southeast, both the Atlantic Ocean and Gulf of Mexico help contribute to higher humidity readings. In the Southwest, westerly and northwesterly prevailing winds create a drier air mass for much of the year, keeping humidity levels much lower. However, during the "monsoon season" in the summer, winds flow from the south and send humid air into the Southwest from the Gulf of Mexico, Gulf of California and eastern Pacific Ocean. Learn more about humidity on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 14, 2007) Q: Why are rapidly dropping temperatures often accompanied by a rising barometer? A: A barometer essentially measures the weight of the column of air that extends from the ground to the top of the atmosphere. A rising barometer reading coincides with rapidly dropping temperatures because cold air molecules are less energetic than warm air molecules and can pack more closely together, making cold air more dense than warm air. Learn more about air pressure on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 10, 2007) Q: What’s the difference between the old Fujita scale and the new Enhanced Fujita Scale? A: This new scale, which was instituted in February, is a more accurate method for assessing tornado damage. Since the F-scale’s inception in the early 1970s, engineering studies have shown that it significantly overestimated the wind speeds in tornadoes. For instance, an EF-4 tornado has estimated wind speeds of 166-200 mph, while an F-4 had estimated speeds of 210-261 mph. This Storm Prediction Center page has much more about the new EF-scale. (Answered by Doyle Rice, USA TODAY's weather editor, Oct. 8, 2007) Q: When lightning strikes water, are fish killed? A: A swimmer or boater does not have to be directly struck by lightning to be at risk on the water. Because water is a good conductor of electricity, lightning strikes the surface and spreads out in all directions, but does not penetrate very deep into the water. Therefore, for a fish to get zapped, it would have to be directly beneath the strike point or close to the surface, relatively close to the strike point. Find out more facts about lightning on this National Severe Storms Laboratory page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 7, 2007) Q: With an apparent increase in the number of Category 5 hurricanes, is there a case for a Category 6 rating to be introduced? Since wind speeds increase by approximately 20 mph between categories, this might be for those with wind speeds over 175 mph - a figure not far off from Felix. A: The Saffir-Simpson Hurricane Scale has 5 categories, ranging from a Category 1 with sustained winds of 74-95 mph, up to a Category 5 that begins at 156 mph. In the official hurricane database, the hurricanes with the highest wind speeds are Camille in 1969 and Allen in 1980, both with 190 mph. Given that the intervals in each category are about 20 mph, one could make the case that a 6th category is needed for those with winds from 180 mph and higher. However, such extreme intensities are exceedingly rare. (Neither Dean nor Felix would have reached that threshold.) Even with the projected changes of about a relatively small 5% increase of winds by the end of the 21st century due to global warming, this "6th" category would not likely get utilized more than once every decade or two. To get the point across that a catastrophic hurricane is threatening, a "Cat 5" designation will certainly still suffice. (Answered by Chris Landsea, science and operations officer at the National Hurricane Center, Oct. 3.
2007)
Q: When rainfall is reported in inches, what does this mean? A: Usually, reported rainfall is the amount of water captured by a rain gauge at an official observing station during a 24-hour period. Since these gauges report rainfall every hour, rainfall can be tallied over shorter periods of time and reported during a rain event. Sometimes, rainfall rate is mentioned during a heavy rainstorm -- for example, two inches per hour. This data is typically gathered by Doppler radar, as the radar beam's reflected energy can be translated to give meteorologists an idea of how much rain a given storm might produce if it were to stall. Learn more about rain on this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Oct. 1, 2007) Q: How does moisture in the air fuel the development of hurricanes? A: Water vapor is required for hurricanes to develop, since it’s the release of energy when water vapor condenses that's the fuel that keeps a hurricane's heat engine running at peak efficiency. As rising water vapor condenses and latent heat is released, surrounding air is warmed and made less dense, causing the air to rise. The thunderstorms that make up the hurricane’s core thus grow taller and stronger. As air rises within the storms, pressure at the surface decreases and moister, tropical air is drawn to the center of the circulation, providing even more water vapor to fuel the hurricane. Check out this interesting article about how warm ocean water fuels hurricanes. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Sept. 30, 2007) Q: Why do the tropics have more precipitation than other locations? A: The tropics include all locations between the Tropic of Capricorn at 23.5 degrees of latitude south of the equator and the Tropic of Cancer at 23.5 degrees north of the equator. As the Earth revolves around the sun through the year, the sun's direct rays range from the Tropic of Capricorn on the winter solstice to the Tropic of Cancer on the summer solstice. Since the tropics receive so much direct solar energy, this heating produces more evaporation over the tropics than at higher latitudes. This warm, moist air rises, condenses into clouds and thunderstorms and returns to earth as precipitation. The greater the evaporation, the greater the precipitation. Learn more about the global energy balance at this USA TODAY resource page. (Answered by meteorologist Bob Swanson, USA TODAY's assistant weather editor, Sept. 26, 2007)
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