In this section, you can find information about variables and indices that are used in the aeronautical meteorology and aviation, such as CAT or in-flight icing for instance.

Clear air turbulence (CAT)

Clear-air turbulence (CAT) is a measure of high level turbulences in the atmosphere that are not associated with clouds. It describes the turbulent movement of air masses in the absence of any visual clues such as clouds, and appears when bodies of air moving at widely different speeds meet. Thus, CAT shows the regions in the atmosphere where significant wind shear appears with no visual signs. Clear air turbulence (CAT) is often used in the aeronautical meteorology and aviation.

The intensity of CAT is divided into the following three classes: light, moderate and severe. Clear air turbulences occur in every level of the atmosphere. Therefore, every altitude show different intensities of CAT.

turbulence intensity aircraft reactions
Light slight, erratic changes in altitude and/or attitude (pitch, roll, yaw)
Moderate changes in altitude and/or attitude with more intensity, aircraft remains in control
Severe large and abrupt changes in altitude and/or attitude, larges variations in airspeed

Heavy turbulences may be dangerous for aircrafts and passengers. Therefore, the possibility of visualising the invisible clear air turbulence on weather maps is very helpful to be prepared for regions where heavy turbulences could appear. It allows to plan a calm flight path by avoiding regions with severe CAT for example.

Soaring index

soaring index soaring conditions
< -10 poor
-10 to 5 moderate
5 to 20 good
> 20 excellent

The soaring-index (dimensionless) is very similar to the k-index and is used for an estimation of the thermal situation. It is useful for paraglider and other sport pilots. The soaring-index is defined as follows:

soaring index = 850 hPa Temperature – 500 hPa Temperature + 500 hPa dew point temperature – 700 hPa dew point deficit

A higher soaring-index also shows a greater chance of showers and thunderstorms, as it is a measure of the lift caused by convective clouds. The soaring index is independent of altitude, as it is computed with variables in different altitudes. Therefore, there is one value shown per atmospheric column in maps.

In-flight icing

In-flight icing is used in the aviation and describes the freezing of water on the external structure of the aircraft during the flight. That occurs because atmospheric water is often supercooled (< 0°C) and does not freeze until there are freezing nucleuses. Because an airplane's surface serves as a freezing nucleus, the supercooled moisture can freeze instantly on it. The risk of in-flight icing is often expressed as a percentage (%).