Precipitation is nothing but cloud droplets falling to the ground. Most precipitation starts off as snow as the temperature at which the cloud exists is usually below freezing. There are different types of precipitation depending on what the vertical temperature profile is. The following is a list of precipitation types.
Rain occurs when
tiny cloud droplets collide to form bigger droplets. This keeps happening
until the droplet is two heavy for the air to support it. The droplet then
begins to fall, colliding with more cloud droplets as it gains in size. If
the liquid water does not encounter a deep layer of sub-freezing air, it will
remain liquid and fall to the ground as rain. Sometimes rain will show up
on the RADAR but there is no rain reaching the ground. This phenomenon is
is a product of very intense thunderstorms. People commonly mistake hail
for sleet but they each form under different conditions. Hail is rarely
seen when the surface air temperature is below freezing. It forms as a
byproduct of strong updrafts that exist in thunderstorms. The cumulonimbus
clouds that are associated with thunderstorms can grow to heights where the
temperature is below freezing. Drops of water will rise up with the upward
directed wind as they collide with other droplets and grow larger. This
will eventually result in the droplet freezing into a hailstone.
The hailstone will "cycle" through the cloud. The updrafts will cause it to rise and gravity will cause it to fall. While this is occurring the hailstone is picking up more moisture and growing in size. Eventually the hailstone will grow too large for the updrafts to keep it suspended and it will then fall to the ground.
The size of hailstones varies with the intensity of the thunderstorm. The stronger the thunderstorm, the stronger the updraft and the longer the hailstone will have to grow in size. Hail size is often described using common spherical objects. Sizes can range from pea-sized to golf-ball sized, but have even been seen as big as a softball or even larger.
Freezing rain is
probably the most dangerous type of precipitation. There is no noticeable
difference between freezing rain and rain so people ignore the fact that is can
cause such things as black ice on roadways.
Freezing rain occurs when there is a shallow layer of air at the surface that is below freezing followed by a layer of above freezing air above it. The precipitation will fall through the warmer layer so it will not freeze over. When it hits the sub-freezing layer it will cool but not freeze. Since the surface temperature is below freezing, objects in contact with the air are also below freezing. The super-cooled water droplets will freeze on contact to these surfaces. This can cause problems with ice forming on roadways while it is raining. The following diagram shows the atmospheric temperature profile that can cause freezing rain.
Sleet is nothing more than frozen raindrops. Sleet occurs when there is a warm layer of air above a relatively deep sub-freezing layer at the surface. The layer above freezing will allow for liquid precipitation but as the drops hit the cold layer, they will freeze and hit the ground as frozen water droplets. Sleet usually doesn't last long and mainly occurs ahead of warm fronts during winter months. The following chart will show the temperature profile typically associated with sleet.
Snow occurs when the layer of the atmosphere from the surface of the earth through the cloud is entirely below freezing. The precipitation falls from the cloud as snow and does not melt at all while falling to the ground. The following chart will show the vertical temperature profile associated with snow.
super-cooled water - Water that exists in liquid form but has a temperature below freezing. This water will freeze on contact with any surface.
virga - Raindrops that evaporate before hitting the ground. This is usually associated with rain falling through a dry layer of the atmosphere.
This page was last updated: December 11, 2003
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