How and Why Do Rain & Drizzle Form?
Below is a diagram that meterologists use to show the vertical structure of the atmosphere. This is very helpful in determining what form precipitation will take on as it falls through a layer with specific temperature and wind profiles. This profile was made by a balloon that carries sensors up into the atmosphere. This diagram is called a Skew T Log P Diagram and is also known as a "Sounding".
sounding below is a typical rain sounding. The air is moist, and
this is indicated by the dashed black line showing that the dewpoint
temperature and actual air temperature are
almost the same throughout the lower levels of the atmosphere*. [The
temperature is the temperature at which water
vapor will condense and the air will be saturated, that is to say it will
hold all the water vapor that it can at that particular temperature.
Any extra water vapor will be condensed into liquid water and thus, we
have clouds & precipitation!] Notice that the maroontemperature
line is above freezing at levels beneath the
cloud layer, this will cause any preciptation to melt and fall as rain.
It may help to picture the precipation following the maroon line all the
way to the surface, thus encountering a changing environmental temperature
as it falls.
Skew T/Log P base diagram taken from PSC website.
*If the layers beneath the cloud were drier, (if the black dashed line were farther to the left of the maroon line), then the falling precipitation may evaporate totally before hitting the surface. When this happens it is called Virga, sometimes virga can be seen as a shaft or darker area of precipitation that seems to be hanging overhead.
OR Click here for the Main Precipitation Page!