Weather

Meteorological spring—March, April, and May—and early summer are the times for pilots to bone up on thunderstorms in all of their forms and resolve to stay away from them. Lightning and the resulting thunder give thunderstorms their name.

As it turns out, lightning poses a greater hazard to pilots performing preflight inspections than in the air.

In addition to lightning, thunderstorms produce strong downdrafts—downward-moving winds that can spread out when they hit the ground to cause downbursts, the quick changes in wind speed and direction, or even stronger microbursts. Downdrafts and updrafts create extreme turbulence inside a thunderstorm.

Any thunderstorm can be more than the most skilled pilot in the strongest airplane can safely manage (see “Weather: Get Ready for the Boom,” April 2015 Flight Training). FAA Advisory Circular 00-24C, issued in February 2013, sums up thunderstorm hazards: “Penetration of any thunderstorm can lead to an aircraft accident and fatalities to those on board.”

This advisory updates some of the information in earlier, now-outdated FAA weather materials. It’s online (www.faa.gov/documentlibrary/media/advisory_circular/ac%2000-24c.pdf).

One important piece of advice this advisory circular gives is to stay at least 20 miles away from any thunderstorm that’s listed as being “severe.” Your best bet is to avoid all thunderstorms by at least 20 miles. You can’t always tell just by looking whether a thunderstorm meets the National Weather Service criteria for a severe thunderstorm: hail, at least three-quarters of an inch in diameter, damaging winds at least 58 mph, or producing at least one tornado.

STAY AWAY. If you hear about a “squall line” during a preflight briefing or see a weather chart, like the one of current conditions in the chart above for the region you’re planning to fly across, your best bet would be to think about practicing takeoffs and landings at your home airport that day.

The orange and red colors are strong thunderstorms, while the other colors represent weaker thunderstorms and showers. The squall line stretches from Illinois and Wisconsin south to the Gulf of Mexico, with the strongest thunderstorms creating a pretty solid line from around St. Louis to the Gulf.

In such a case, with a squall line headed toward your airport, you should plan to make the last landing of the day in plenty of time to ensure the airplane is securely tied down or in a sturdy hangar before the squall line arrives.

MIND THE GAP. Signs in London’s Underground subway system say “Mind the gap,” telling riders to be careful of the space between the cars and the platform as they board or leave trains.

A good slogan for a pilot facing a squall line would be “Fear the gap” between any two thunderstorms in the line. Even if you’re pretty sure the gap is at least 40 miles wide, which would keep you the suggested 20 miles from each thunderstorm, you shouldn’t even think about trying to squeeze through a squall line.

Individual storms in a squall line are always weakening and dying as new thunderstorms form and grow. Those puffy clouds below as you fly through a gap could grow into a new thunderstorm that catches your airplane in minutes.

WHERE SQUALL LINES FORM. Squall lines form most often in the warm sector of middle-latitude storm systems, south of a warm front and to the east or southeast of an advancing cold front. While the entire area south of the warm front and east of the cold front is warm, additional warm, humid air moving in from the south adds extra heat and humidity to feed thunderstorms.

The dashed line on the diagram represents an upper air trough, which encourages warm, humid air to begin rising to trigger squall line thunderstorms.

As the squall line moves, generally toward the southeast or east, some of its storms die out as new ones sprout and grow.

SQUALL LINES CAN FORM OUT OF SEASON. A squall line is most likely to get in the way of your flying plans during the spring or early summer, but it should be noted that the weather doesn’t always follow our calendars.

While the squall line shown on the chart would be more typical for April or May, this one occurred on October 13, 2014, causing widespread damage from both straight-line winds and tornadoes along its length.

Both the timing and the tornadoes were unusual. Not only are squall lines most likely in the spring and summer, few of them cause many tornadoes.

Squall lines in the United States most often form east of an advancing cold front and south of a warm front.