L’utilisation des ascendances thermiques et dynamiques par les oiseaux

Imagine you are watching birds in a famous spot for the migrations of raptors (Eilat, Gibraltar, …). You may see a kettle of large birds circling in the air above you, their wings outstretched and motionless, their bodies slightly tilted. What you are seeing is the act of gaining altitude using rising currents of air called thermals. The act of “thermalling” refers to the use of columns of warm rising air to gain altitude. Thermals are the result of the sun hitting the ground then warming the surrounding air creating a concentrated or “hot” spot. Since warm air rises, raptors and sailplanes utilize these circular rising currents to go upwards. The cumulus clouds that tend to form at the top of the thermal can often be used to identify their location.
For raptors, the challenge in soaring is finding air currents that are rising faster than they are descending. Fundamentally, raptors use thermal lift to attain altitude then soar for as far as possible while descending, then riding the next available thermal to gain altitude again.
Besides using thermals, raptors also use updrafts made by wind that is deflected off ridges. The longer the ridge and the steadier the wind, the farther a raptor can fly with little use of its own energy.
The same type of phenomenon also exists on the sea.