One of the greatest dangers to aviation are thunderstorms. When evaluating accidents related with thunderstorms it became apparent that the dangers associated were not always recognized by aviators and possible evasive action was not carried out leading to disastrous results.
We thought that it was useful to dedicate some space to document and summarize information regarding thunderstorms and the effects on aviation and pilots. We are going to explain some of the characteristics of the phenomena and possible actions on how to avoid these storms and penetration procedures when caught inside.
The next few pages will explain the background of these thunderstorms, highlight the dangers with tips to avoid them. We also see how they can be detected and the main reasons to avoid these powerful phenomena.
For more in-depth theory behind these storms we kindly refer you to meteorological sites specialized in the subject, there are a number of official sources to be found on the Internet. On these pages we assume some basic meteorological knowledge on this subject.
As thunderstorms belong to the cumulative cloud family, their development is based on processes which will cause an air mass to rise. That will happen by either thermal instability or by an upward force along some frontal system in a convergence zone, trough or even mountainous terrain. There are three processes causing instability:
As moist air becomes warmer than the surrounding air when lifted, and due to saturated adiabatic processes, a high amount of water vapor at lower levels contributes to the formation of thunderstorms. They develop easier in warm air because warm air can contain more water vapor than cold air. As such temperature is an important factor.
In a well developed high pressure area subsidence inversion will inhibit most of the vertical growth of cumuliform clouds. In warm air with a high tropopause, thunderstorms will grow higher and be more intense than in cool (northern and southern) areas with a lower tropopause.
As moist air lifts more easily than dry air, rain showers which have developed in dryer areas, have much more energy (violent) in respect to turbulence since there is more energy to form the cloud. Storm detection is also difficult as water content is less and radar pulses are only reflected against these water droplets.
A developing thunderstorm goes through three stages: cumulus, mature and dissipating.
T-storms can be classified by they way the are formed, move the mouse over the image to the right to see how the wind and turbulence develops in a Cb.
Thunderstorm formation processes:
During summer time these storms develop over land warmed up by the sun. Thermal lows intensify the storms above which is a high tropopause. In coastal areas the storms occur inland when the vapor loaded maritime air is moving horizontally due to warming by the Earth.
At mid latitude in the northern and southern hemisphere these storms are found in westerly circulations connected with a polar front. The intensity depends on: frontal activity, origin of the air mass, season, Orographic effect and pressure changes. Squall lines may develop ahead, on the cold front as well as several hundreds miles behind in a trough.
In mountain areas the inclination angle of the sun is more vertical on the sun side slopes where convective activity will be more pronounced.
During night or early morning, over areas where sea water temperature is higher that the air temperature, these storms may develop if the lapse rate is potentially unstable. The air mass over the surface is lower (stable) than the air mass over sea in the lower parts of the atmosphere.
This phenomenon is more prevalent in areas of shallow water and high amount of solar radiation, thus the seawater heats up easily. The Indian Ocean is a good example where during the late night early morning Cbs develop.
Another form of coastal Cbs develops over land during daytime when the Earths temperature is higher than the seawater. The sun forms a thermal low in several days, the sea breeze advects moist air in the lower levels whereas the air is relatively dry in the higher levels
Thus an ideal potentially unstable situation develops for Cb formation, especially when the tropopause is at a high level.
When a certain amount of cold and ascending air in and ahead of a trough is moved horizontally (advection) over warmer land or sea, an unstable situation may arise causing Cbs to form.