Thunderstorms: Managing The Risk
Day or night, how do you fly responsibly?
If flying beneath a CB involves a roll of the dice, climbing above a storm often isn’t even possible, unless you happen to be flying a U2. I fly the Rockies and Sierra Nevada on a regular basis, and it’s not unusual to hear briefers warning of tops above 50,000 feet, and updrafts and downdrafts in excess of 3,000 fpm. Tops typically reach 30,000 to 35,000 feet on even a small storm, and you wouldn’t believe how fast a CB can build when temperature and humidity are just right. Once, in the summer of 1979, on the last leg of a delivery flight from Santa Maria, Calif., to Lakeland, Fla., in a new Aerostar, I was out over the Gulf of Mexico at FL250 watching a CB cloud develop, and I decided to take a closer look.
As I approached the storm from perhaps 30 miles away, I could see the horizon on the other side of the cloud tops, so I knew I was above them. When I was within two or three miles, however, the building cumulus was already at my level and climbing higher. As I sidestepped by the clouds, now climbing slightly above me, I could see the muscular cumulus boiling upward at probably 800 to 1,000 fpm. Fifteen minutes later, when I was perhaps 60 miles past the clouds, I looked back and saw the tops passing through at least 35,000 feet.
The only viable (read safe) method of negotiating a thunderstorm is to fly around it. Traditional wisdom regarding circumnavigating a CB (in the Northern Hemisphere) is to always deviate to the right, the tailwind side of any low-pressure system. (Just remember the rule “low counter”—air always flows around a low-pressure system counterclockwise.) That may not work if turning right will put you on the downwind side of the storm.
Question is, how wide a berth should you allow between you and a fully developed thunderstorm? It may be a little pointless to lay down a strict numerical rule, since even many pilots don’t know the difference between 10 and 20 miles. Twenty miles is often considered a reasonable buffer. Even if the storm is moving in the direction of your diversion at a typical 20 to 30 mph, you’ll have plenty of time to widen out as necessary.
Most folks with experience around thunderstorms suggest the safest method of circumnavigating a storm is on the upwind side, regardless of whether that will provide the best winds. The downwind side may be beneath the anvil, and that location is infamous for rain, hail, turbulence and other forms of meteorological misery.