Weather

Spring’s first warm breezes signal an end to winter’s chill across the northern United States. These comfortable breezes should be a signal to pilots to begin thinking about thunderstorms. Winter’s reluctance to give way to warmth helps account for the fact that the greatest number of severe thunderstorms occurs during spring. The National Weather Service defines a “severe” thunderstorm as one “that produces a tornado, winds of at least 58 mph (50 knots), or hail at least three-quarter-inch in diameter.” Some severe thunderstorms meet all of these criteria.

This definition applies to what people on the ground experience or could expect to experience if a severe thunderstorm is headed their way. As far as pilots are concerned, any thunderstorm should be considered severe.

By definition, a cumulus cloud becomes a thunderstorm as soon as it produces a lightning stroke, since lightning is needed to produce the sound of thunder. Up-and-down air motions (updrafts and downdrafts) inside a cloud that contains both water drops and ice crystals create the positive and negative electrical charges within the cloud needed to cause lightning.

This tells you that any storm or cloud close to becoming a thunderstorm has strong updrafts and downdrafts that create turbulence, which can be severe. Many water drops in such a thunderstorm are likely to be “supercooled,” which means they are colder than 32 degrees Fahrenheit but not yet frozen. These instantly turn into ice when they hit an airplane. In other words, even though the ground temperature might be above 90 degrees F at the surface, you must worry about icing in a thunderstorm.

Lightning is the least hazardous of a thunderstorm’s many dangers to aircraft occupants. Lightning regularly hits aircraft but almost always passes harmlessly through the conductive skin and airframe, causing minor damage. In fact, lightning’s biggest threat to a pilot is while she’s doing a preflight inspection on the ramp. This is why airlines stop refueling and loading baggage when lightning is striking near an airport (see “The Weather Never Sleeps: Lightning’s Hazards,” March 2008 Flight Training).

Thunderstorm ingredients. Weather forecasters (and pilots) look for possible thunderstorms when:

• The air is humid or humid air is moving in.
• The atmosphere is or will be unstable.
• Some mechanism will give air near the ground an upward shove.

The water vapor that makes air humid also fuels thunderstorms by releasing heat when the vapor in rising air cools and condenses into water drops or freezes into ice crystals. An unstable atmosphere is one in which air continues rising when an initial upward shove ends (see “Weather: Blowin’ in the Wind,” February 2012 Flight Training). Upward shoves include heated air rising from the ground, the arrival of a cold front, or wind flowing out along the ground from another thunderstorm.

During spring’s longer days, with the sun climbing higher into the sky, the ground heats up rapidly while the air aloft is much slower to warm up. Solar energy hardly warms the air it’s passing through on the way down; the Earth’s surface absorbs most of its heat. The big contrast between cold air aloft and much warmer air at the surface makes the atmosphere unstable (see “Weather: No Guarantees,” March 2012 Flight Training).

Spring usually brings regular cold fronts that give warm air an initial upward shove. As the northern parts of North America warm up during the summer, fewer and weaker cold fronts push south across the United States and the number of thunderstorms decreases—except in the Southeast, where very warm and humid air exists all summer, but severe storms aren’t as common as during the spring.

Varieties of thunderstorms. Many aviation training materials and FAA knowledge test questions refer to air mass and steady state thunderstorms. I learned these terms in the late 1970s as I was taking college meteorology courses and preparing for the private pilot knowledge test.

Today’s scientists talk about four kinds of thunderstorms:

• Single-cell storms generally last 20 to 30 minutes. A true single-cell storm is actually quite rare because cool air that blasts down from the thunderstorm travels across the ground to initiate other thunderstorm cells by pushing up warm air.
• Multicell cluster storms are the most common kind. They consist of cells initiated by air flowing out of other cells. Gentle winds around the cluster push it along as a unit, with each cell in a different phase of the thunderstorm life cycle. While each individual cell might last only 20 minutes or so, the cluster may last for several hours. The figure (below) is a simplified illustration with only three cells, each in a different stage of the thunderstorm life cycle. The smallest cell, on the left, is in the beginning or “towering cumulus” stage characterized by updrafts only. The mature stage, in the center, begins when rain or hail begins forming, which creates downdrafts. This is a thunderstorm’s most violent, turbulent stage. The dissipating stage on the right has weakening downdrafts, no updrafts, and decreasing rain.
• Multicell line storms, generally called squall lines, consist of a long line of storms with a continuous, well-developed gust front at the leading edge of the line. The line of storms can be solid, or there can be gaps and breaks in the line. Such a line can be extremely disruptive because it could stretch as far as from Illinois to Louisiana—with the tops of the storms more than 40,000 feet high. Even the highest-flying jets might have to divert hundreds of miles to avoid a squall line.
• Supercells live up to their name by being especially long-lasting and strong. Like a single-cell storm, they have only one updraft, but this updraft is extremely strong and the storm organizes itself in a way that makes it long lasting. Scientists estimate updrafts at 150 to 175 mph. The distinguishing characteristic of a supercell is its rotating updraft, which helps make the storm last for hours while possibly producing hail greater than two inches across; downdrafts of 80 mph or faster (imagine flying into a 175 mph updraft adjacent to an 80 mph downdraft); and the strongest and longest-lasing tornadoes.

Keep your distance. Flying into any thunderstorm is a bad idea. Flying too close to one can be as dangerous as flying into it. The Aeronautical Information Manual recommends keeping 20 miles from thunderstorms. This distance might seem a little conservative, but when it comes to thunderstorms, more than a little caution is a good idea. Thunderstorm downdrafts that spread out across the ground as “gust fronts” can cause sudden changes in wind speed and direction at airports miles away.

Even if you’re in an airplane that could climb high enough to get over the top of a thunderstorm in your path, don’t try it. This can be especially dangerous if you try to cross a squall line in what looks like a clear area between individual cells, or a cell that’s not as high as the others in the line. If you try this, a rapidly growing cell could quickly engulf your airplane, giving you a fast, turbulent ride in a 50-mph updraft.

High-altitude winds blowing over the tops of a thunderstorm can act like wind blowing over mountains to create violent mountain waves.

President Bill Clinton and about seven others were aboard Air Force One on June 12, 1996, as it cruised at 33,000 feet above the Texas Panhandle. The military version of a Boeing 747 zoomed up and sank down several times, tossing around the cabin the guacamole, tamales, frijoles, and salsa the crew were preparing for dinner. Fortunately no one was seriously injured. The lesson: Thunderstorms don’t care how big and well-maintained your aircraft is, or how skillful a pilot you are, or who you’re carrying on board.