How do convection currents form downdrafts?
1 Answer
They don't.
Explanation:
Convection causes air masses to lift due to that air mass being warmer than the surrounding air. That air will cool as it rises eventually cooling to the dew point and cloud will form.
The change of state from vapor to liquid releases heat into the atmosphere causes the air to cool at a slower rate as it rises causing a more enhanced convective effect. The air will keep rising until it is cooler than the surrounding air (in the stratosphere).
In order for downdrafts to occur the air needs to become colder than the surrounding air. This occurs during precipitation formation and evaporation. Precipitation usually forms at a high enough altitude and cool enough temperatures that it forms as solid. The solid precipitation is held up in the atmosphere from the updrafts of convection. As the precipitation particles grow they become heavier until the updrafts can no longer support them, and they fall.
As the precipitation falls the air around it gets warmer, and eventually the precipitation melts. The process of changing from solid to liquid absorbs heat from the surrounding air. This part of the atmosphere was in equilibrium but now that the air temperature is dropping as the ice absorbs the heat and changes to liquid water, the air around it becomes colder and beings to fall.
So that is a downdraft. The more precipitation the more intense the downdraft becomes. If it is intense enough we call it a microburst. If the precipitation falls through a dry layer below the cloud it might evaporate into the dry air, absorbing more heat from that air. As this occurs in dry air we call this a dry microburst. Both are very dangerous to aircraft.
I hope that answers your question.
