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Science journalist Jack Williams has covered the weather for more than a decade. He provides succinct, clear answers to the “what and why” questions assembled here. What are ball lightning, Doppler radar, and black ice? Why do weather forecasts tend to be more accurate for the Eastern United States than for the Western part of the country? Why is the coast of North Carolina a storm-breeding area? Williams also offers safety advice for people concerned about lightning and hurricanes.
Q: Why are weather predictions for some areas more accurate than for others?
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A: All weather forecasts depend on observations of current weather conditions to figure out what the weather will be doing in the near future. To make a good forecast for tomorrow for the eastern United States, you have to know what’s going on over the rest of the country today, since weather generally moves from west to east in the middle latitudes. The farther west you go in the United States, the less “upstream” information forecasters will have since few weather observations are taken over the Pacific Ocean.
While weather satellites do give forecasters the big picture of what’s going on over all of the world’s oceans, they do not provide detailed information such as air pressure, temperature and humidity, and wind speeds and directions at the surface and above Earth. So while the satellites might show forecasters that a big Pacific storm is approaching the U.S. West Coast, the satellites supply little information that computer models can use to predict whether the storm will strengthen or weaken and what path it will follow for the next 24, 48, or 72 hours.
Even with a lot more data to work with, forecasters have a hard time determining exactly where snow will fall and where rain will fall when a winter storm hits the Middle Atlantic or Northeastern states. A change of a few degrees in temperature of the layer of air between Earth’s surface and a couple thousand feet up determines who gets rain and who gets snow. Forecasting models often can’t pin down the location of the dividing line between rain and snow to within 10 or 20 miles. Yet such a change in the rain-snow line can determine whether snow falls on big cities such as Philadelphia, Baltimore, or Washington, D.C., and whether the cities have rain while snow falls only on the suburbs to their north and west.
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Q: What is ball lightning? Does it really exist?
A: Ball lightning refers to glowing spheres that appear floating in the air; they move around slowly and then disappear, sometimes with the sound of a small explosion. These spheres seem to average about six inches in diameter and are usually seen before or during thunderstorms. They appear both outdoors and indoors and have even been seen inside aircraft in the air. They normally last less than a minute.
Scientists who study lightning agree that ball lightning is real because too many reliable reports of it have been made, going all the way back to ancient times. While the experts don’t dismiss ball lightning, no one has come up with an explanation of what it really is or how it occurs. There are several theories, however. The big problem for anyone who wants to study ball lightning is that it’s rare, lasts an extremely short time, and can’t be predicted.
Q: I’m 15 and have been studying storms, lightning, tornadoes, and storm-chasing. I hope to become a storm chaser. Do you have any advice for me? Are there courses I can take to help me?
A: Many people go storm chasing—looking for tornadoes—as a hobby, and a few people have jobs that include storm chasing. I assume you’re interested in eventually getting one of these jobs.
First, you should realize that only a few people have jobs that include storm chasing. Real storm chasers found many things wrong with the movie Twister. One of the most grievous errors was the idea that there is enough money in tornado-chasing to make it worthwhile for the bad guys in the black vans to get involved.
Even though no one gets rich from storm chasing, it can be part of a satisfying career. High school is the best place to begin preparing for one of these careers by learning as much math and science as possible. You can’t begin too early learning how weather works.
One of the best ways to begin learning about tornadoes and tornado chasing is the book The Tornado: Nature’s Ultimate Windstorm by Thomas P. Grazulis, published in 2001 by the University of Oklahoma Press.
Grazulis notes in his book that some chasers work for very competitive television stations in Midwestern or Great Plains states and are among the few who actually make any money storm chasing. Some research meteorologists specialize in tornadoes, and chasing is part of their jobs. But it involves more indoor work than chasing.
If you want a job that includes tornado chasing, you could aim for a television news career. But no matter how good a TV reporter or photographer you are, you would probably have little chance of getting one of those Midwestern or Great Plains jobs unless you knew a lot about thunderstorms and tornadoes and had some chasing experience. Enroll in a college meteorology program to get such experience, and learn how to be a television meteorologist. Try very hard to become one of the students who goes chasing with researchers.
If you are very interested in science, you could become one of the few scientists who studies tornadoes. Again, you should study meteorology in college. Since meteorology is a “hard” science based on physics and mathematics (with computers becoming increasingly important), you need to know a lot about science and math.
You should also realize that tornado-chasing can be frustrating. As Grazulis says in his book: “Even if you have studied the weather from textbooks for many years, it will still take thousands of miles of driving before you get a real-world sense of how storms develop and decay and how an outbreak of severe weather actually progresses. Even for the best chasers, perhaps nine of ten chases will be busts. You must be ready to appreciate a prairie sunset and a distant lightning show and to drive another 400 miles the next day.”
The American Meteorological Society’s Careers in Atmospheric Research and Applied Meteorology Web site has a lot of information on meteorological careers. The Tornado Project Online, which is Grazulis’s Web site, has good information on tornadoes and tornado-chasing.
Q: What is black ice?
A: Sometimes a layer of ice on a road will be thin enough to be almost completely clear. When this happens a driver can’t tell that a road is covered with ice. This is dangerous because the driver can be surprised when the car begins skidding on the ice. Often black ice will be in patches on a road, which can make it even more surprising to drivers.
Q: Why is weather over the ocean so changeable? I thought since the temperature of the ocean was more constant than land temperatures, weather would be more predictable. My colleague insists that moisture, and not temperature, drives weather conditions. Which is it? And does unpredictable ocean weather explain why areas such as Puget Sound in the Pacific Northwest have such erratic conditions?
A: Much of the changeable weather over the oceans of the middle latitudes—the parts of the Earth between the tropics and the polar regions—is caused by storm systems that are traveling across the oceans. From fall through spring one of these storms comes along every few days with changes from warm to cool, and with rain as part of the temperature shift.
These midlatitude storms, or extratropical storms (storms that form outside the tropics), draw most of their energy from temperature contrasts. These temperature contrasts extend high into the atmosphere, up to the bottom of the stratosphere. In other words, while the more-or-less uniform surface temperatures of the ocean can smooth out air temperatures in the lower part of the atmosphere, there are often strong contrasts only a few thousand feet up to supply plenty of storm energy.
Also, cold air moving over the warmer oceans—such as air from Siberia moving over the northern Pacific—creates temperature contrasts that stir up storms. For instance, the Atlantic Ocean off the coast of North Carolina is a key storm breeding ground because the warm Gulf Stream flows north along the East Coast from Florida and then turns sharply to the right to cross the Atlantic Ocean. At the same time, cooler water flows south along the East Coast. The warm and cool water meet off Cape Hatteras, North Carolina, to create an area of contrast in water temperatures. This area of contrast, in turn, sets up a zone with cool air to the north over the cool water, while warm, humid air to the south hovers over warmer water. Such areas—with warm air masses on one side and cool air masses on the other, both staying in pretty much the same place—are called stationary fronts and are a good place for storms to begin.
Moisture also plays a key role in storms. Without moisture in the air—humidity—midlatitude storms would be like storms on Mars, which doesn’t have moisture in the air. That is, they would be windy dust storms without clouds, rain, or snow. Humidity supplies the energy for thunderstorms and for tropical cyclones, such as hurricanes. Tropical cyclones, as the name tells us, form over tropical oceans where the temperatures are very uniform and where there are no strong contrasts in air temperatures at any particular altitude. Hurricanes and other tropical cyclones are powered mostly by the energy from the latent heat released as water vapor condenses into clouds and rain.
The ocean is only part of the story of the erratic weather around Puget Sound. The Olympic Mountains west of the sound help make the Seattle area’s weather unique. As low-level wind blows onto the Washington coast from over the Pacific, it hits the mountains, which split the winds into two steams: one going around the northern side of the mountains and the other around the southern side. These two streams meet, or converge, on the eastern side of the mountains, and the air is forced upward. The air cools as it rises, and its humidity condenses into clouds, which can produce rain or snow if the air is humid enough.
Small details of exactly how the winds are flowing determine whether the rain or snow is over the city or west of the city, over Puget Sound. The winds can shift back and forth during a single storm, pushing the rain or snow one way and then the other.
As you can imagine, the National Weather Service (NWS) office in Seattle has a hard time figuring out exactly what’s going to happen. Forecasters are getting better at it, however, because the Puget Sound convergence zone is an interesting scientific problem and both the NWS forecasters and university researchers are studying it. (The University of Washington Atmospheric Sciences Department has a Web site with information on the role of the convergence zone in the snowstorm of December 18, 1990, that dumped up to 36 cm [14 in] of snow on parts of Seattle.)
Q: Why are water droplets grouped together in clouds rather than being distributed more evenly throughout the atmosphere?
A: This is a more difficult question than it might seem at first glance. Gases that don’t react with other substances or that don’t change into liquid or solid form in the air do end up being mixed pretty evenly. The winds do a good job of stirring up the air. For example, some ozone-destroying gases that humans make are found in all parts of the atmosphere, including over the South Pole, which is about as far as you can get from where they leaked into the air. Scientists know these gases came from places where large numbers of people live because the gases have no natural sources.
Water is found in the air in all three of its forms: as a gas called water vapor, as ordinary liquid water, and as ice. It’s constantly changing from one form into another. Water becomes invisible water vapor when it evaporates into the air from oceans, lakes, rivers, streams, and even puddles. When a mass of air cools, water vapor condenses back into tiny drops of ordinary water, which make up clouds. If the air gets cold enough, its water vapor can turn directly into ice crystals.
Some clouds are made of ice crystals rather than of water drops. If conditions are right, a cloud’s water drops come together and grow big enough to fall as rain. Ice crystals in a cloud can grow big enough to fall as snow.
A lot of water evaporates into the air over warm oceans, while hardly any evaporates into the air over cold, dry land. Water is always falling somewhere on Earth as rain or snow. With so much water constantly evaporating into the air and falling from it, the water in the air never has a chance to be evenly mixed.
Q: I’ve heard that the Earth’s rotation causes water draining from sinks to spiral in a particular direction. Is this true?
A: The winds of big storms, such as hurricanes, spiral in a clockwise direction south of the equator and in a counterclockwise direction north of the equator. But, despite what you often hear, Earth’s rotation does not cause water to go a specific way down a sink. Earth’s rotation creates the Coriolis force, but the Earth’s rotation is slow: It takes Earth an entire day to turn full circle. The force created by that rotation is small, and only affects movements of wind or water that extend over long periods of time. Thus the Coriolis force is too small to affect water going down a drain.
Q: What is Doppler radar and why do meteorologists use it?
A: Christian Doppler, an Austrian scientist, died in 1853, a half century before radio, much less radar, was invented. We hear Doppler’s name on television almost every night because the principle he discovered explains why a train whistle changes pitch as it moves toward and then away from someone. When the train is coming toward you, the sound waves are, in effect, “squeezed,” which increases their frequency. When the train is moving away, the whistle’s sound waves are “stretched,” which decreases the frequency and lowers the pitch.
Radio waves work the same way that sound waves do. The radio waves from a radar antenna that bounce off of something and return to the antenna increase or decrease in frequency if the object is moving toward or away from the antenna. Radar can tell the speed of the object by measuring how much the frequency changes. This is how a police officer’s radar can tell if you are going faster than the speed limit.
Weather radars bounce radar waves off of raindrops, snowflakes, hailstones, and even insects in the air. Today’s Doppler weather radars can therefore tell which way and how fast raindrops, snowflakes, and other things are moving.
In addition to having a “Doppler” ability, today’s weather radars are more sensitive than those used in the past, and they are connected to powerful computers that help make sense of all of the complicated air movements and patterns of precipitation the radar detects. All of this additional information helps forecasters “see” details of what’s going on in thunderstorms and snowstorms. As a result, they can spot thunderstorms that are likely to spin out a tornado and they can spot the parts of a snowstorm that will bring the heaviest snow.
Q: How do I find the weather for a particular time and place? I’m researching Tokyo, Japan, and I need the weather for December 2000 to January 2001. Where would I find older weather data (from the last 100 or 200 years)?
A: In general, the place to begin looking for information about weather of the past for anywhere in the world is the U.S. National Climatic Data Center (NCDC) in Asheville, North Carolina. It’s part of the National Oceanic and Atmospheric Administration (NOAA) and is the official repository for weather data from the United States. The NCDC collection also includes a huge amount of data from around the world. Data from recent years is available in digital form via the NCDC Web site.
The best place to begin looking for measurements from weather stations outside the United States is the part of the NCDC Web site containing foreign data.
You should be aware that many different kinds of data are available—the NCDC’s Web site is complex and will take a while to master. One hint: The weather information that most people seek is “surface data,” which includes the measurements of temperatures, rainfall, snow, humidity, and more from ordinary weather stations—most of which are at airports.
It takes about a month or two for U.S. data to become available on the NCDC site. If you are looking for more recent U.S. data, try the Web site of the National Weather Service (NWS) office that forecasts for the place you are interested in. You can find the site you want by going to a NWS Web page with a map and clickable links to all NWS local offices.
In general, you will find U.S. weather data is more easily available than data from most other nations. The U.S. government has a policy of making its data freely available, whereas many other nations charge for even basic weather data. The basic U.S. and other data on the NCDC Web site can be obtained at no cost, but you will have to pay for most complex data sets, both in electronic and print forms.
Sometimes it’s worth going to the Web site of a particular nation to find weather data. The World Meteorological Organization has a Web page with links to the sites of most of the organization’s members.
Q: How and when does frost form?
A: Frost, which is made of ice crystals, forms when the temperature is below freezing (0°C/32°F). Water vapor (the invisible, gas form of water) in the air turns directly into ice without ever condensing into water. Meteorologists in the United States refer to the process of water vapor turning directly into ice as sublimation. Meteorologists in other English-speaking parts of the world often call it deposition.
Frost forms when temperatures fall below freezing during the night. It’s most common when the sky is clear because clouds tend to keep heat from radiating away from Earth overnight. Frost isn’t likely to form if the wind is blowing. The wind stirs up the air, keeping the air near the ground from becoming as cold as it otherwise would.
Frost also forms on the insides of windows, because the air inside a building is usually more humid—that is, it has more water vapor in it—than the air outside on a cold day. Of course, the inside air has to come into contact with a cold window. Frost forms on the inside of a car’s windshield or windows when people get into the car because we exhale water vapor, which makes the air inside the car more humid. When frost forms on a window it’s often easy to see the six-sided shape of ice crystals.
As the sun comes up, frost on grass and car windows often melts or turns directly back into water vapor even though the air temperature might stay below 0°C. This is because the sunlight warms the grass or glass more than it warms the air.
Q: Since Earth is due for another Ice Age soon, would the global cooling that accompanies it balance out the effects of our industrially caused global warming?
A: You are correct that Earth is “due for another ice age soon” if you are thinking in terms of thousands of years. In geological terms, Earth is now in an interglacial period of an ice age that began around 100 million years ago. Scientists who study past climates of Earth say that we’ve been in an ice age for about 100 million years because during this period varying amounts of permanent ice have covered the polar regions. Before then, the evidence indicates there was no polar ice for millions of years. Further back in time, other ice ages occurred. Climate scientists consider the current period an “interglacial period” because during the last 100 million years there was far more permanent ice than there is now.
In terms of human history—not geological history—the last ice age began maybe 100,000 years ago, reached a peak about 20,000 years ago, and ended around 8,000 years ago when Earth’s permanent ice shrunk to about what it is now. While the causes of these changes aren’t completely understood, scientists who study past climates generally agree that changes in Earth’s orbit around the Sun play a major role.
Certain factors indicate that we should begin moving back into a glacial period sometime soon. However, “soon” is in geological terms, not in the lifetimes of humans. These factors include the record of changes from glacial times when ice covered more of the Earth than it does now, the record of interglacial times such as the current one, and the prediction of changes that will occur in Earth’s orbit.
Scientists expect the natural trend to be a slow, 90,000-year cooling into the depths of a new ice age, according to Richard B. Alley in his book The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future. But, he writes, “the globally averaged rate of cooling over that time would be something around 0.01 degree (Fahrenheit) per century, and maybe three to four times bigger in the polar regions, where changes are largest. Human-induced changes are likely to be one hundred or more times faster, so the next natural ice age won’t save us from ourselves.”
Q: Why does high humidity make the temperature feel warmer than it really is?
A: When we begin getting too hot, our bodies automatically release water through the sweat glands to expel water through the skin. This water on our skin then evaporates into the air; that is, the water turns into invisible water vapor, which is a gas. Evaporation requires heat because the molecules of water on our skin need to speed up to turn into water vapor. Body heat supplies the energy needed to speed up the water molecules. In other words, the water evaporating from our skin carries heat from our skin into the air.
However, as the air becomes more and more humid, less and less water vapor can evaporate into it. This means that the evaporation of sweat from our bodies slows down and less heat is carried away, so the temperature feels warmer than it actually is. High humidity doesn’t increase the air’s temperature, but since it slows down evaporation, it means people don’t cool off as quickly as they do in dry air.
If the air isn’t too humid, a fan can help you feel cooler because it blows dry air past you to replace the air that’s been made more humid by evaporation of sweat from your body. But a fan doesn’t help if it’s very hot and humid. This is why many communities make special efforts to find air-conditioned places for elderly people to go during heat waves. Air conditioning not only cools the air, but also reduces its humidity.
The National Weather Service uses a heat index chart to measure the danger of combinations of heat and humidity. This chart combines the air temperature and the humidity to produce an “apparent temperature,” which measures the danger of the heat making people ill. This heat index chart is part of a National Oceanic and Atmospheric Administration (NOAA) Web site, which has a great deal of information about the dangers of heat and how to avoid them.
Q: Why do hurricanes occur in the Southern and Eastern United States but not in the West?
A: Winds blowing from east to west carry most hurricanes that form off Mexico’s Pacific Coast out over the ocean, far from any land. A few storms, however, turn to the north and then back to the east to hit Mexico. Some head toward California, but they die over the cold water off the West Coast (hurricanes need warm water). Even after a hurricane’s winds die down, it can still bring heavy rain to the U.S., in the Southwest and California.
Q: How can I avoid getting hit by lightning?
A: The only safe place in a thunderstorm is in a sturdy building—not a picnic shelter or a shed. A vehicle (unless it’s a convertible) is a good shelter if you keep the windows rolled up. You should take shelter when you see a lightning bolt or hear thunder. Five seconds between seeing lightning and hearing thunder means that the lightning is about a mile away—way too close for safety. Rubber shoes or the tires of a bicycle will not insulate you from lightning.
Q: Who, other than the National Oceanic and Atmospheric Administration, gathers live weather data? Do you know of any companies that specialize in setting up data-reporting weather stations?
A: Live weather data is collected by individuals, schools, highway departments, utilities, universities, television weather broadcasters, states, airports, and others. The only national network for gathering and disseminating weather data is the U.S. National Weather Service (NWS), which is part of the National Oceanic and Atmospheric Administration (NOAA). NOAA, in turn, is part of the U.S. Department of Commerce. The NWS network is part of a global network that is overseen by the World Meteorological Organization and includes weather services of governments around the world. Among other things, it enables pilots flying anywhere in the world to obtain weather data.
To ensure that observations are consistent, there are standards a weather station’s instruments and those who read the instruments must meet in order to be a part of the NWS or the global network. A reading of “20 degrees Celsius,” or “light rain” has to mean the same thing whether it is reported by Berlin, Germany; Butte, Montana; or Beijing, China. In the United States, stations that are part of this network are operated by NWS employees, the military services, or contractors or others that meet NWS standards. You can find current reports from the global network on the U.S. NWS Internet Weather Source site.
In addition to the network of stations that meet global standards, the NWS also has an even larger network of voluntary observers. All these observers make regular temperature measurements; many measure rain and snow; and some measure things such as stream levels. Many of these observations are sent to the nearest NWS office, where much of the data is put on the NWS Web sites.
Volunteers also send their data to the National Climatic Data Center (NCDC), where the information can be used for climate studies. Some of the volunteer stations have been reporting for more than 100 years, with generations of the same family taking and reporting observations. Many stations are in rural areas and are a good source of data from places that haven’t been affected by city growth. This information can be obtained through the NCDC Web site, although it often takes a couple months to process data, so the information is not current.
Since weather is so important to pilots, weather observations are collected at many airports. In fact, the majority of reports collected by the NWS and global systems are from airports. In recent years the NWS, the Federal Aviation Administration (FAA), and the U.S. military have installed Automated Surface Observing System (ASOS) instruments at most weather stations. These automated observations are supplemented by human observations. ASOS stations have been installed at many airports that previously did not have current weather data available.
In addition, the FAA has set up Automated Weather Observing System (AWOS) stations at other airports. These stations don’t meet the standards of the NWS network, but they do provide reliable data at airports that formerly lacked such information. Data from some of these stations is put on local NWS Web sites and is generally available to pilots via automated telephone or radio systems. The FAA’s Automated Weather Sensors Web site has information on both systems, including how to establish a station and maps showing stations in operation, with phone numbers and radio frequencies for each station.
Many television stations around the United States set up local networks, often with the stations at schools. Many of these are part of the WeatherNet System of Automated Weather Source, Inc., which makes the reports from around the U.S. available on its Web site. These are not part of the NWS network.
It is difficult to find data from the multitude of smaller networks—such as those used by highway departments to know when to send out salt trucks and snow plows, or those used by farmers in some areas to know when frost threatens their crops—unless you are connected with the organization that operates a specific network.
If you are interested in setting up a weather station at an airport, you should begin with the FAA’s Automated Weather Sensors Web site and contact the nearest NWS office for advice. It is costly and time-consuming to set up a station to become part of the NWS network. On a smaller scale, several companies manufacture home weather stations (many advertise in Weatherwise Magazine).
Q: I’m retiring this year to the Clearwater-Dunedin area in western Florida, but am concerned about hurricanes. I hear most of the large storms hit southern and eastern Florida and work their way up the coast. Am I wrong? Where can I obtain information and history of the area I’m retiring to?
A: Don’t be fooled by the fact that major hurricanes are rare in western Florida. Although the last major hurricane (as of July 23, 2001) to hit the Tampa Bay area was in October 1921, another one could hit any year. There is no way of knowing whether it will be this year or 80 years from now.
Hurricane Watch: Forecasting the World’s Deadliest Storms, a book I coauthored with Bob Sheets, a former director of the National Hurricane Center, lists the probabilities of a storm hitting various places any year. The odds of any hurricane hitting the Tampa Bay area are 17.5 percent in any year. The odds that a major hurricane—a hurricane with winds faster than 177 km (110 mph)—will hit are 4.8 percent in any year.
The only places in the United States with higher odds of a major hurricane are all in southern Florida, beginning around Fort Myers and extending south and east across the Keys and north along the Atlantic Coast to Fort Pierce. Miami has the highest odds of a major hurricane—11 percent in any year.
In other words, the odds of a hurricane are slightly higher south of Tampa Bay on Florida’s Gulf Coast and even higher in the Keys and southeastern Florida. Still, anyone who lives around Tampa Bay should be concerned about hurricanes. Emergency managers in the area take hurricanes very seriously.
To get an idea of what could happen, you might want to take a look at the article “What a Hurricane Could Do” on Tampa Bay Online. It describes what is likely to happen if a storm similar to the 1921 hurricane hit now. After reading this you’ll see why it is a good idea to begin learning all you can about hurricanes. The National Hurricane Center’s Hurricane Awareness page is a good place to start.
I wouldn’t advise someone to avoid moving to an area they really like because of hurricane danger. However, I do think it’s a bad idea to buy waterfront property because the first couple rows of houses along the shoreline, whether along the ocean or a bay that opens into the ocean, usually suffer the most damage from wind and storm surge. Also, if you live right on the water you will have to evacuate every time a hurricane threatens. Before buying a house, you can check with the emergency management department of the city or county to learn which areas could be flooded by storm surge.
The safest place to live is away from the danger of storm surge or of flooding from inland streams that are pushed out of their banks by the heavy rain that accompanies most hurricanes. Your house should be built to stand up to a hurricane and have a “safe room,” a small room in the center of a house that’s designed to protect those inside it from a hurricane or tornado.
You can get good information from emergency management departments about flood dangers, evacuations, and public shelters. Don’t trust local building codes or building inspectors to ensure that a house you buy or build will stand up to even a weak hurricane. A good source of information on what’s needed to give a house a fighting chance against a hurricane is the Florida Alliance for Safe Homes Web site.
Q: Why does the coldest part of the year fall in January and February even though the shortest day (when the Northern Hemisphere receives the least amount of energy from the Sun) falls in December?
A: Earth is always losing heat to space, but solar energy replaces the lost heat. We experience heat imbalances at certain times of the year as the seasons. During summer, temperatures increase because more energy is arriving than is leaving. During winter, temperatures decrease because not enough solar energy is arriving to make up for the heat loss.
We can look at the energy arriving at and leaving Earth as a sort of bank account. Each day we deposit energy (instead of money) into the account, and each day we also withdraw energy. To keep things simple, we can imagine that we withdraw the same amount of energy each day all year. The amount that we deposit on any day depends on how high the Sun rises in the sky that day and how long it stays in the sky. When our daily energy balance goes down, temperatures fall. When it goes up, temperatures rise.
Let’s look at how the energy bank account works in the Northern Hemisphere during a year. At some time in the fall, as days grow shorter, less energy is arriving each day than is leaving, and temperatures begin to fall. Around December 21, at the time of the winter solstice in the Northern Hemisphere, the days are shortest and the least amount of energy is arriving each day. The energy bank account balance is falling quickly. Then the days slowly begin to grow longer, but much more energy is still leaving than is arriving each day. Although the energy bank account balance is not decreasing as quickly as it was around the solstice, it does continue to decrease.
Temperatures continue to fall until late January or early February, when the Sun finally begins to replace all of the energy the Northern Hemisphere is losing each day. The energy balance stops decreasing and temperatures no longer fall.
Around June 21, at the time of the summer solstice, the energy balance is growing the fastest. As the days begin growing shorter, the balance isn’t growing as fast as it was on the solstice, but it’s still growing. This is why summer’s hottest days usually arrive in July.
Q: Why is the sky blue?
A: (a) The air has water in it and light shining through water turns blue.
(b) The molecules of the gases that make up air scatter blue light in all directions.
(c) The molecules that make up the air are blue and transparent--like blue glass.
Answer (b)--Molecules of nitrogen and oxygen, which account for more than 98 percent of the gases that make up air, are just the right size to scatter blue light. The light from the sun is “white;” that is, it has all of the colors. But the blue colors “bounce” off of molecules of air, going in all directions. No matter what direction you look in during the day, blue light is coming toward you from the sky, unless clouds hide the sky.
Q: What is the difference between sleet and hail?
A: Sleet always falls from winter storms, while hail falls from thunderstorms. Sleet is created when the air high above the Earth is warmer than the freezing air near the ground. Snow that falls from high clouds melts to become rain in the warm air. Then, as it falls into the freezing air closer to the ground it begins to turn to ice. If the raindrops freeze completely before hitting the ground, they are called sleet. If the layer of freezing air isn’t thick enough for the rain to turn completely to ice, it falls as freezing rain, which turns to ice when it hits the ground. Such weather is called an ice storm. The National Weather Service uses the term “ice pellets” to report that sleet is falling. In forecasts, sleet is often simply called “ice.”
Hail forms in thunderstorms, which are most common in the spring and summer. While sleet is always small, hailstones can sometimes be as big as softballs.
Q: Is it true that no two snowflakes are alike?
A: That depends on what you mean by “snowflake” and “alike.” When you say “snowflake,” most people think of elaborate, six-sided shapes called dendrites. The shapes of dendrites are complex, and you’re unlikely to find two that look alike. Other snow crystal shapes are much simpler, such as hexagon plates that look much alike. But if by “alike” you mean that molecules in two snowflakes are arranged exactly the same, then no two flakes are alike. Some molecules are always leaving a crystal, and other molecules are constantly sticking to crystals. Even a single snow crystal isn’t alike from minute to minute.
Q: Why is the South Pole colder than the North Pole?
A: The South Pole is in the middle of the continent of Antarctica, atop about 2,700 m (9,000 ft) of very cold ice. The North Pole is in the middle of the ice-covered Arctic Ocean. Even though 10 to 20 or more feet of ice might cover the ocean, the water is about –1°C (30°F). (Salt water freezes at a lower temperature than fresh water.) Some heat from this water escapes through the ice to warm the air over the North Pole. At times leads (cracks) open in the ice even at the North Pole, allowing ocean heat to escape into the air.
An ocean current that completely circles Antarctica turns aside water that’s moving south, keeping it from warming the continent. Both poles are the same distance from the sun, and both have six months of light and six months of darkness. While the South Pole is about 2,700 m (9,000 ft) higher than the North Pole, this elevation does little to make it colder.
Q: What do the rankings of a tornado, F-0 through F-5, mean, and how are the rankings determined?
A: After a tornado hits, meteorologists from the National Weather Service inspect the damage and assign a rating from a low of F-0 to a high of F-5 based on the damage the tornado has done. While each ranking lists wind speeds, the National Weather Service notes that these “are untested, unknown, and purely hypothetical.” Airplanes fly into hurricanes to measure winds, but they do not fly into tornadoes because tornadoes’ higher wind speeds are too dangerous. While Doppler radars can do a good job of detecting thunderstorms that might produce tornadoes, they can’t measure the wind speeds in a twister unless the twister comes very close to the radar.
The ranking scale was developed by Tetsuya Theodore “Ted” Fujita, a University of Chicago tornado scientist. He died in 1998.
Q: Can tornadoes cross water?
A: Tornadoes definitely can cross water; in fact, tornadoes can form over the water and move onto land. The idea that tornadoes don’t cross water is one of those myths you often hear, but it’s hard to understand how it got started.
Tornadoes over water even have their own name: waterspouts. A waterspout can be a tornado that forms over land and happens to cross a pond, a river, or a lake. It can also be a tornado that forms over the water and either stays over the water or moves to hit land.
All tornadoes are formed by thunderstorms, and they go where the parent thunderstorm goes. The thunderstorm, in turn, goes where winds in the upper atmosphere push it. Nothing on the ground, whether it’s a lake or a hill, is going to stop the thunderstorm and its tornado from going where they are pushed by the winds.
Q: What factors classify a tropical storm as a typhoon? Please explain in detail the difference between a typhoon, a hurricane, and a cyclone.
A: A typhoon is a tropical cyclone with winds of 120 km/h (74 mph) or faster that forms over the Pacific Ocean west of the International Date Line and north of the equator. A hurricane is a tropical cyclone with winds of 120 km/h or faster that forms over the Atlantic Ocean, Caribbean Sea, Gulf of Mexico, or Pacific Ocean east of the International Date Line and north of the equator.
Elsewhere, these storms are just called tropical cyclones or cyclones. They form in the southern and northern Indian Ocean, north of Australia, and over the southern Pacific Ocean. The term “tropical storm” is used in many parts of the world—including the “hurricane” regions—for storms with sustained winds between 63 and 117 km/h (39 and 73 mph). Many but not all of these grow into hurricanes or typhoons.
As you can see, “tropical cyclone” is the general name for a kind of storm, while “typhoon” and “hurricane” are the names applied to these storms in particular locations. All tropical cyclones form over warm oceans and begin losing strength when they move over cool water or land. Meteorologists call them “warm core” storms because their centers are warmer than the surrounding air. “Extratropical cyclones” form over land or oceans away from the tropics. While tropical cyclones contain only warm, humid air at the Earth’s surface, extratropical cyclones have both warm and cool, or even cold, air. Winter storms are extratropical cyclones.
Q: Will future meteorologists be able to accurately predict the weather a week or more in advance?
A: Forecasts of day-to-day weather are made by computer models using data on the air’s temperature, pressure, and humidity and on wind speed and wind direction. These models run on the largest and fastest supercomputers and use various mathematical laws to make predictions. The biggest problem with current models is that weather is chaotic; tiny changes now can lead to big changes in the near future. Information about what the weather is doing in between the places where it’s measured can’t be fed to the computers. Yet slight changes that occur between measurements could turn rain into snow or start a storm moving in a different direction.
In 1999 a committee of National Research Council experts published a report entitled A Vision for the National Weather Service: Road Map for the Future. This very readable report looked forward to weather forecasts in the year 2025. The report includes many examples of the practical effects of improved weather forecasts that we might expect by 2025. For example, “Hundreds of thousands of commuters in San Francisco leave their umbrellas and rain gear at home because the approaching rain is guaranteed not to arrive before 7 PM that evening.” A better network of weather observations using new technology, scientific advances in our understanding of the weather, and much more powerful computers will provide these vastly improved forecasts. The report reads like science fiction, yet it was written by men and women who understand today’s weather science and forecasting and have a realistic idea of what can happen during the next 25 years.
Even with all of the advances this group foresees, the report says that by 2025, “Predictions in the five to seven day time frame are about as accurate as two to three day forecasts were in 2000.” In other words, all of the advances that the experts can imagine over the next 25 years will not provide perfect forecasts a week ahead of time.
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Meteorology; Weather
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