COMPILED BY THE RISK MANAGEMENT STAFF

The welcome relief from winter’s bitter cold and snow brings with it another significant hazard — the spring and summer severe weather conditions. It is crucial all aviators are familiar with the inherent dangers of severe weather.


Introduction to thunderstorms
Almost every second, on average, a lightning strike between the ground and a cloud occurs in the United States. More than 100 lightning strikes take place every second above Earth where over 44,000 thunderstorms are occurring at any given moment, which presents a significant hazard to aviation and ground operations. Therefore, there is a good chance you'll encounter a thunderstorm within the next few months. During that encounter, you will face the thunderstorm’s many powerful hazards, including strong winds and wind shears, heavy precipitation, lightning, hail and tornadoes. Are you ready?

The weather forecaster’s definition of a thunderstorm is pretty basic, yet misunderstood by many. A thunderstorm is any local storm with lightning and thunder produced by a cumulonimbus cloud, usually producing gusty winds, heavy rain and, sometimes, hail. However, the only official criterion a weather observer uses to identify a thunderstorm is thunder. That's all — just thunder, according to the handbook published for observers. Cumulonimbus clouds are vertical columns of cloud mass with rain descending from them, which could potentially be thunderstorms. But technically, until the first thunderclap is heard, it is not a thunderstorm.

Avoiding the thunderstorm in flight
Thunderstorms are laden with myriad unacceptable environmental hazards to aviation. In simpler terms, avoid thunderstorms while flying your aircraft. But how do you do that? The first technique is the old see-and-avoid concept. Look out of the cockpit for signs of convective activity. The following is a short list of things to look for that give evidence of convective turbulence, lightning, hail, downbursts, microbursts and severe wind shears:

• Anvil cloud form approaching


• Darkened color to clouds


• Churning vertical clouds


• Vertical clouds that are growing

The next step is to use the weather radar while airborne. Not every hazard in a thunderstorm is visible on weather radar, including small cloud droplets, fog, ice crystals, or small, dry hail or graupel (granular snow pellets). This list is significant for three reasons. First, if you are using your weather radar to scan your flight path for weather that is out of visual range (150 to 200 nautical miles), you may paint a group of individual cells and conclude you could visually circumnavigate them. In reality, you may be facing a wall of clouds with embedded thunderstorms. Second, the low reflectivity of the surrounding clouds may not show up on the radar, creating the false impression that there is a “hole” in the clouds. Finally, the anvil portion of a thunderstorm does not appear on radar since it consists primarily of ice crystals.

Depending on the precipitation type and its movement, recognizable thunderstorm patterns will show where the hazards are. It's important to know what to avoid on our radar screens (click the graphic at the bottom of the page).

• Avoid a target with a dry intrusion (drier air being sucked into the thunderstorm) giving it a V or U shape. There are several reasons for this. Severe thunderstorms have dry air mixing in the middle altitudes which can create an intrusion. Hail rising and descending in a thunderstorm would also appear as a missing area cut out from the storm.


• Avoid a target with a hook or bow shape. Hook shapes are indicative of rotations taking place within severe thunderstorms. This is a strong clue to ground weather observers that hail and tornadoes are possible.


• Avoid a target with protruding “fingers.” Like a hook, a finger shows strong possibilities for tornadoes and hail.


• Avoid a target with an asymmetric coloring and shape. Remember, severe storms created by wind shears aloft will tilt to one side. This gives shapes and colorings that are not even or concentric.


• Avoid a target with an arrow shape. Again, this is indicative of a storm with tilt and the possibility of severe hazardous weather.


• Avoid a target with scalloped edges. Scalloped edges show turbulent motions taking place within the cloud. There is also a good chance for hail here.


• Avoid a target with changing shapes. Rapidly growing shapes show rapid motions taking place within the cloud. Turbulence will almost always take place under these conditions.


• Avoid a target with a few VIP Level 1 dots showing nearby. Many times, hail falls outside of the thunderstorm. Checking the winds at altitude and correlating it to the side of the storm that hail will fall should help identify that potential hazard.

Flying techniques to remember
Publications from the FAA and USAF give aviators numerous tips and techniques to help with the occasional encounter with a thunderstorm — some of which are important enough to repeat again.

• Don't fly over thunderstorms. Storms can grow rapidly through your altitude, producing severe turbulence. Also, hail can shoot through the top of the thunderstorm in clear air above and fall downwind.


• Don't fly under the anvil, where hail damage and lightning can occur.


• Don't fly into virga, where turbulence is likely.


• Avoid all thunderstorms by 20 miles or more since lightning and hail have been known to extend that far from the clouds.


• Weather warnings are for thunderstorms defined as “severe.” These storms produce three-quarter-inch hail, tornadoes or 50-knot wind gusts. A lot of damage can occur in thunderstorms that are not flagged by warnings or a SIGMET (significant meteorological report).

If you have to penetrate:

• Go straight. Don't turn around.


• Avoid the altitudes with temperatures of plus/minus 8 C.


• Don't chase altitude. Hold your attitude and watch airspeed.


• Use all anti-icing equipment.


• Turn all lights in the cockpit on full and lock shoulder harnesses.

Conclusion
Thunderstorms are one of nature’s most hazardous phenomena. They can impact aviation from wind shears, lightning, heavy precipitation, tornadoes and severe turbulence. Knowing how to recognize and avoid thunderstorms and their hazards is one of the most important lessons of aviation weather training. Think safety and fly safe.