Definition of clear air turbulence
Clear air turbulence is the term for medium- or high-level turbulence in regions with significant wind shear. Turbulent areas of this kind are mostly free of clouds. However, sometimes you can find cloud formations (mostly ice clouds) that are typical for turbulent areas, or a higher concentration of water vapor in the vicinity of CAT. Both of these hint strongly at the presence of CAT.
Often the designation "High Level Turbulence (HLT)" would be better than CAT, as it also includes the cloudy areas. However, in aeronautical meteorology and aviation, the term CAT is more common. This is why we shall use CAT in this module.
Figure 2.1. shows the four forces affecting an aircraft:
- Thrust
- Resistibility
- Lift
- Weight
When the aircraft is flying with a constant speed and a constant height, all fources are in balance. The amount of lift and weight are equal. The same applies to trust and resistibility. The lift force is a function from the tip velocity of the air (v2) and the air density (ρ).
This means that CAT can occur by the flight through a border of different air masses with significantly different densities. For instance where there is a temperature gradient and especially at the tropopause. A strong relationship exists between variations of wind speed from inflowing air and the CAT (wind speed with the square of 2).
Bigger airplanes have a greater wing spread and mass. Their vertical movements are less extreme than in smaller airplanes due to inertia. On the other hand, in bigger airplanes the structure is more loaded. In extreme cases of CAT the structure could be damaged or break up.
Fig. 2.1: Forces affecting an aircraft