Thunderstorms — Page 249, Lesson 201

I want to walk you through some of the most dangerous phenomena associated with thunderstorms — the kind of conditions that demand real respect from any professional pilot. Let's start with the downdraught speeds we were just discussing. These downdraughts can reach between 50 and 90 knots. They are largely caused by descending raindrops which cool the surrounding air by evaporation. As that air cools, its density increases, and that higher density accelerates the downdraught still further — so it's a self-reinforcing process. Now look at Figure 14.15, which illustrates microbursts. A microburst is a concentrated burst of descending air that is up to 4 kilometres in horizontal length and has a lifetime of less than 5 minutes. So it's small in scale and very short-lived, but extremely intense. For comparison, a macroburst is a similar event but over a bigger area — the name tells you it's the larger version. Microbursts are most likely to occur in summer air mass thunderstorms in low latitude regions where surface conditions are dry. That might sound counterintuitive — dry ground producing a wet-weather hazard — but the key is the evaporation I just mentioned. When the air below the cloud is dry, the raindrops evaporate more readily, cooling the air more efficiently and driving that downdraught harder. These microbursts cause extreme turbulence and severe windshear conditions — two of the most critical threats to an aircraft on approach or departure. There is a warning sign you can look for: virga. Virga is streaks of precipitation falling from below the cloud that do not actually reach the ground. The rain evaporates before it hits the surface. If you see virga, especially in a dry, hot environment, you should be alert for a possible microburst beneath that cloud. Next, let's talk about water ingestion. If the updraught speed inside the thunderstorm approaches or exceeds the terminal velocity of the falling raindrops — that is, the maximum speed at which a raindrop can fall through still air — the raindrops get suspended or even carried back upward. This leads to very high concentrations of water in the cloud. Those concentrations can exceed the design limits for water ingestion in some turbine engines. The result can be engine flame-out — the engine stops burning — and/or engine structural failure. Water ingestion may also affect pitot heads, even though their heaters have been switched on. The pitot-static system can give false readings if water blocks the pitot tube, and that can be disastrous for your airspeed indication. Finally, tornadoes. Tornadoes are exclusively associated with cumulonimbus clouds and large cumulus clouds. They usually occur as a result of vertical windshear, with warm moist air at low altitude and cool dry air coming from a different direction at high altitude. That combination sets up the rotation. Tornadoes are very powerful whirlwinds with small horizontal extent and very low pressure in the centre — that low pressure is part of what makes them so destructive. The highest incidence of tornadoes is in the southern states of the USA, in a region known as tornado alley, during spring and early summer. The mechanism there is very warm air from the Gulf of Mexico moving north, meeting relatively cold air coming from the northwest. This gives massive instability and the windshear required to spin up a tornado. These tornadoes may have rotational speeds in excess of 200 knots and diameters up to 1 kilometre. That is an extraordinary amount of energy concentrated in a very small area.