Winter's last gasp agitates April's air
| Photo: NOAA |
When April rolls around, winter is in full retreat across almost all of the United States, but it doesn't give up without a fight. And when the weather is in a fighting mood, pilots need to stay alert.
In addition to new leaves on trees, blooming flowers, and pilots no longer freezing their fingers during preflight inspections, spring also heralds the arrival of thunderstorms.
While most places east of the Rockies and in the desert Southwest see more thunderstorms in the summer than in April, the spring storms tend to be more violent because winter's lingering effects add more energy to the atmosphere. This makes early spring a good time for pilots to review what they know about thunderstorms and begin thinking about avoiding them.
The very basic things you ought to know about thunderstorms begin with the ingredients they need:
- Unstable air that will continue to rise if something shoves it up.
- Plenty of humidity in the rising air to condense into clouds and rain, releasing the latent heat the helps power thunderstorms.
- Something to give humid air near the ground the needed upward shove.
The atmosphere is unstable when temperatures drop relatively rapidly with altitude. Rising air cools at the rate of 5.4 degrees Fahrenheit for each 1,000 feet gained, and at a slower rate when water vapor in the air begins condensing into cloud droplets. When weather balloons find that the temperature of the air that's in place-it's not rising-falls more quickly with altitude than rising air would, meteorologists know that the air is unstable and thunderstorms are possible, if other ingredients are available.
Winter turns to spring because the days are growing longer and the sun rises a little higher in the sky each day from late December until late June. Each day's added sunlight warms the Earth, but-this is an important point-sunlight warms the atmosphere hardly at all on its way to the ground. This means most of Earth's warming comes from most of the increasing sunlight heating the Earth's surface, which in turn warms the air next to the surface while the air aloft retains much of winter's chill.
In other words, spring's warming tends to make the air unstable.
You need to know, however, that this general picture can change from day to day at any particular place as cold or warm air moves in either near the surface or aloft. Still, spring on the whole tends to be a season of unstable air. Fortunately, we don't have thunderstorms each day of spring because at least one of the other two needed ingredients is missing.
But, spring's warmer air can carry more moisture than winter's chilly air. When winds are blowing from the Gulf of Mexico or the Atlantic Ocean, they are more likely than winter's chilly winds to be humid enough to feed thunderstorms.
By the way, one reason the westernmost United States have fewer thunderstorms than the central and eastern states is because the general flow of air is coming from over the Pacific Ocean, which is cooler than the Gulf of Mexico and the Atlantic and therefore doesn't evaporate as much water into the air.
When the air is unstable and humid enough, the upward shove needed to kick off thunderstorms can be as simple as air beginning to rise as the sun warms the ground, especially bare fields, and the ground warms the air next to it, causing the air to begin rising.
For the strongest thunderstorms, however, the upward shove is likely to come from an advancing cold front-a warm/cool air boundary with the cool air replacing the warm air. Sometimes the shove occurs when the heavier cool air pushes under the lighter warm air, pushing the warm air upward. In such a case, thunderstorms form along the frontal boundary.
Often, however, an advancing cold front disturbs the warm air for maybe 100 or more miles ahead of it in ways that cause the air to begin rising to create a prefrontal squall line of thunderstorms.
Before the 1980s, scientists couldn't always pin down the source of the upward shove that got thunderstorms going. But when they began using more sensitive weather radars and other new instruments, they found that the key to storm formulation often could be found in boundaries between air masses that aren't sharp enough to be recognized as a front, which older weather-observing systems missed. Such boundaries can be residual cold air that came down from a thunderstorm which is long dead and was maybe 100 miles away. Since today's Doppler radar detects these boundaries, forecasters can do a better job of saying where storms are likely to begin.
Once all of the ingredients have come together and a thunderstorm gets going, it will go through a three-stage life cycle:
- The cumulus stage, characterized by updrafts of rising air. The cloud is growing taller.
- The mature stage that begins when precipitation commences falling. The storm now has both updrafts and downdrafts and is more turbulent than at the other stages.
- The dissipating stage as updrafts end, leaving only downdrafts, and the rain is easing up.
The three stages of a thunderstorm are nice to know, but of little practical use to a pilot. Don't even consider flying into a thunderstorm because you think it hasn't reached the most violent, mature state, or that the worst is over because it's dissipating. A meteorologist, watching the storm on a weather radar, might be able to say what stage it's in, but a pilot looking at a thunderstorm from the air isn't very likely to make the correct guess about what's going on inside. And what's going on inside is what makes thunderstorms so dangerous for airplanes.
A really powerful, Great Plains thunderstorm can tower well over 40,000 feet into the sky. Think of how much energy goes into pumping the needed water-as water vapor or the water drops that make up the cloud-that high. Now think about flying your small airplane into the middle of this. In relation to a large thunderstorm, a Boeing 747 is a small airplane.
In addition to updrafts of maybe 50 mph or more and downdrafts maybe half as fast-think of how a 50-mph upward wind and a 25-mph downward wind adjacent to each other would shake your airplane. And the inside of a thunderstorm is likely to contain extremely heavy rain, maybe with the drops supercooled-still liquid even through they are colder than 32 degrees Fahrenheit-which can freeze on contact with your airplane and coat it with ice.
Strong thunderstorms are likely to contain hail-balls of ice sometimes as big as baseballs (see "The Weather Never Sleeps: The Hailstone from Hell," March AOPA Flight Training).
And any thunderstorm has lightning. Since lightning is needed to create thunder, a thunderstorm, by definition there is lightning flashing in and from a thunderstorm.
Over the years various names have been applied to different types of thunderstorms, but the names that meteorologists most frequently use today are:
- Single-cell storms with one updraft.
- Multi-cell cluster storms usually with one larger updraft but with other updrafts growing as the storm's downdrafts hit the ground and flow out to force air up, creating new updrafts. As some updrafts are weakening, others are growing.
- Multi-cell line or squall line storms with the individual cells spread out in a line, which can be more than a couple of hundred miles long.
- Supercell storms, which contain a rotating mesocyclone, maybe 10 miles across, rising into the storm. The mesocyclone is part of a complex pattern of air movements that helps keep supercells going for hours and also helps them to produce the strongest tornadoes, along with other kinds of very nasty weather. (If you see a weather radar display with the word "meso" near the storm, you know the storm is a supercell.)
As warmer weather makes flying more attractive, the main lesson for pilots is to keep an eye out for days when all of the ingredients could be coming together for thunderstorms.
A quick way to do this is to access the opinions of the National Weather Service's thunderstorm experts by going to the NWS Storm Prediction Center's convective outlooks on the Web. Convection refers to the up-and-down air motions needed for thunderstorms.
The Center focuses on severe thunderstorms-storms that could produce surface winds of 55 knots or faster, hail at least three-quarter-inch in diameter, or tornadoes.
If the Center is not forecasting severe storms, you still want to obtain a good preflight weather briefing to ensure that you aren't likely to run into garden-variety storms. Even a "weak" thunderstorm represents a lot of power that you don't want to challenge with an airplane.
Unless you are very knowledgeable about the weather, don't count on being able to be able to look at a cloud and tell whether it's dangerous. You might think that all dangerous storms look dark, even menacing, but that's not the case. If you are on the same side of even the most dangerous supercell as the sun, especially in an airplane, the storm's cloud is going to be white.
Unless your airplane has onboard weather radar that you know how to use, or you're in contact with someone on the ground who's looking at a weather radar (air traffic controllers' radars generally do not show weather in detail), you have no way of knowing what's hidden in a white cloud.
Your best defense is knowing that thunder-storms are around, and avoiding them.
Jack Williams is the weather editor of usatoday.com. An instrument-rated private pilot, he is the author of The USA Today Weather Book and The Complete Idiot's Guide to the Arctic and Antarctic, and co-author with Bob Sheets of Hurricane Watch: Forecasting the Deadliest Storms on Earth.
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