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 the aviators and possible evasive action was not carried out leading to disastrous results.

We thought that it was useful to dedicate some room to document and summarize information regarding thunderstorms and the effects on aviation and pilots. We explain some of the characteristics of the phenomena and possible actions avoid the storms or penetration procedures when caught inside.

Some background

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 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:

• Heating of an air mass from the earths surface upward
• Advection of cold air in higher levels of the atmosphere
• Lifting of potentially stable air due to terrain etc

As moist air becomes warmer than the surrounding air when lifted, 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 a factor.
In a well developed high pressure area subsidence inversions will inhibit 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 low 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 reflected against these water droplets.

A developing thunderstorm goes through three stages: cumulus, mature and dissipating.

Classification

Typical Thunderstorm

T-storms can be classified by they way the are formed, click on the image to the right to see how the wind and turbulence flows in a Cb.

• By heating, thermal
• Frontal, squall line
• Orographic, mountain areas
• Coastal trough
• Cold trough

Thermal
During summer time these storms develop over warm land. 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 advected due to warming by the earth.

Frontal
At mid-latitudes 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.

Orographic
In mountainous areas the inclination angle of the sun is more vertical on the sun side slopes where convective activity will be more pronounced.

Coastal trough
During night or early morning, over areas where seawater temperature is higher that the air temperature, storms (Cb) 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 undeep 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 Cb's develop.

Another form of coastal Cb's develops over land during daytime when the earth's 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.

Cold trough
When a certain amount of cold and ascending air in and ahead of a trough is advected over warmer land or sea, an unstable situation may arise causing Cb's to form.