For a PPL, CPL & ATPL aspirants who wants to become a good pilot, during flying prior to departure or enroute, earth atmosphere plays a vital role for safety of the aircraft. To ensure this, pilot has to be well versant with earth atmosphere & other topics related to aviation meteorology. Study Kwik eLearning module of Aviation Meteorology helps the PPL, CPL & ATPL aspirants to understand & apply the concept of winds when flying.
1. If the wind is blowing along curved isobars of radius r with a velocity V it has an acceleration towards the centre, called centripetal force (V2/r). The centripetal force(C) acting on a unit volume of air=ρV2/r. If the Coriolis force is negligible as compared to the forces P & C, then
ρV2/r=P therefore V= (Pr/ρ)1/2 . Such a motion is called cyclostrophic. Near the centre of a tropical revolving storm or in a circular tornado, the equation gives a good approximation to the actual wind.
2. Cyclostrophic wind is defined as wind that blows as a result of balance between PG Force & Cyclostrophic Force.
It is the wind that blows parallel to the curved isobars under the balance of Pr Grad Force, Coriolis Force & Centripetal Force. In mid latitudes this wind is closer approximation than geostrophic wind.
When the pressure changes rapidly, the geostrophic and gradient rules do not apply. In such a case another force called isallobaric force comes into play. This force is directed from higher isallobar to lower isallobar. This deflects the wind towards the falling pressure. The wind under the influence of P, f and isallobaric force is called the Isallobaric wind.
Effect of Surface Friction
The rough terrain causes friction. Depending upon wind speed, lapse rate and roughness of the surface, effect of friction may extend to about 1 km. This layer is called friction layer. Thickness of the layer is variable. Within the friction layer wind slows down and the Coriolis force reduces proportionately and is insufficient to balance Pressure Gradient Force. The wind is therefore, deflected towards the low pressure and the flow become cross isobaric.
As a rough rule, over sea where friction is small, surface wind blows at about 15o to isobars. Over land it is 30o to the isobars with its speed about 1/3 to 1/2 of geostrophic value.