The descent is the upward movement of an object. This term is used in meteorology to refer to the move to a higher altitude of a parcel of air either thermodynamically or mechanically. The formation of clouds occurs by the rise of the air. Gliders and birds use this effect to stay in flight. The ancestry of the air will be constant if the air is stable but will accelerate if the air is unstable. His reverse is subsidence .
Ancestry occurs thermodynamically when the temperature of a parcel of air at a given level is warmer than the environment and must rise according to Archimedes’ thrust . This is the case of an updraft in a thunderstorm while air, locally hotter and wetter than the environment, undergoes a vertical acceleration.
This also occurs when two soil zones are reheated differently. For example, in atmospheric circulation near the equator, in sea breezes or differential warming between valley and mountain. In these cases, the air in the warmer region rises and is replaced by air from the colder sector.
The movement of the air can make it meet an obstacle and force it to rise. Thus one speaks of orographic descent if the air has to go up the slope of a mountain. The same thing happens when two air masses with a different temperature structure meet, the hottest being raised by the colder one, that is the description of fronts in meteorology.
The air will change temperature by rising to a different rate depending on whether it is saturated or not. It follows a variation according to the dry adiabatic gradient as long as it is not saturated, then the change is done according to the rate of the wet adiabatic gradient (left part of the image). In both cases, if the rate of change in the temperature of the raised air mass becomes larger than the environment, the raised air becomes unstable. Thermal ancestry will then add to the mechanical effect which will give a stronger ancestry.
The air can be raised over large areas, what is called the synoptic scale , locally, mesoscale or very locally, micro scale :
- on a large scale, we will have clouds forming around depressions . The rate of climb is low but continuous:
- on a small scale, we will have the formation of waves in the air downstream from the ancestral zone. For example, downstream of an orographic uprising, the air oscillates to find its point of equilibrium and gives orographic waves . In the same way, the rising in a thunderstorm will create waves of turbulence in altitude around this one.
Model: Related Article: Mountain Meteorology
Clouds and Precipitation
The rising air, due to the progressive decrease in pressure, relaxes and cools so much that the dew point eventually reaches the temperature of the environment and then forms a cloud. In the case of uplift by relief , one speaks of orographic cloud , styling the summit of the mountain or the hill. In the case of thermal or mechanical lifting in clear air, we will have formation of different types of clouds.
If the vertical thrust continues, precipitation is formed. In the case where the air is stable, there is rain or continuous snow, so-called stratiform . If the air is or becomes unstable because of the rising, then showers or thunderstorms are formed . Precipitation is often more intense at the top of mountains in orographic uplift because the uprising makes the air unstable.
The gliders and birds use ancestry to micro-scale for their flight. Indeed, the vertical air current found there is important enough to increase their potential energy. Synoptic movements are usually too weak to serve in these cases.