|HABYTIME MINI LECTURE 15: PRECIPITATION
METEOROLOGIST JEFF HABY
Precipitation is water or frozen water that falls from clouds and reaches the earth’s surface. Some common forms of precipitation
are rain, snow, sleet, hail, and ice pellets. There are just two ingredients needed for precipitation and they are saturated air
and lifting of the saturated air. The closer the air is initially to saturation, then the less the lifting needs to be in order
to produce precipitation. Air that is very dry will require substantial lifting while air that is already saturated only requires
a little lifting in order to produce precipitation.
There are two categories of lifting (rising air) and they are dynamic lifting and convective lifting. Dynamic lifting is forced lifting. This
is air that has to have a mechanism always in place to continue the lifting of the air. Examples of these mechanisms are low level warm
air advection and differential positive vorticity advection. The other category of lifting is convective lifting. When convective
lifting occurs, air will rise on its own without no longer needing to be force lifted. Dynamic lifting is generally much slower
and thus the precipitation it produces trends toward being light. Convective lifting is generally much faster and thus the precipitation
it produces trends toward being heavy. Both dynamic and convective precipitation often occur together. In fact, in order for
convection to be initiated there typically has to be some forced lifting before the air will begin to rise on its own.
There are mechanisms that will tend to turn precipitation events into severe weather events. Two of these storm enhancers are strong
instability and wind shear. The faster air rises convectively then the stronger the downdraft tends to be (heavier rain, strong
convective wind gusts). Faster convective motion is also more likely to produce large hail. Strong thunderstorm winds and hail
are associated with strong convective currents. Wind shear is strong upper level winds interacting with a thunderstorm and it
is also a significant change in wind direction with height. This turning of the air and enhanced wind speeds at the top of a
storm create an environment that is more conducive to longer lived storms. These storms are also more likely to have tornadoes, especially
if low level shear is significant.
Convective motion is very quick and can develop a storm in the matter of minutes. Dynamic motion is slower and it can take hours
for the air to lift enough in order to produce precipitation. Convective motion produces thunderstorms while dynamic lifting
produces stratiform rain and snow. Both convective and dynamic lifting can occur together and this can produce localized very
heavy rain and snow events.