Earth Science
Chapter 16
Section 5
A. Precipitation:
Precipitation is any form of water that falls from clouds and
reaches the Earth’s surface.
Precipitation always comes from clouds.
In order for precipitation to occur, cloud droplets or ice
crystals must grow heavy enough to fall away from the cloud
and reach the ground.
B. Types of precipitation:
1. Rain: Falls mostly from nimbostratus and cumulonimbus
clouds.
- The bulk of rain originates as snowflakes or hailstones, which
melt on the way down as they descend through air that is above
32 F.
- Most commonly, raindrop diameters are in the range of 1 to 6
mm. At larger diameters the drops become unstable and break
up.
2. Freezing Rain: forms a coating of ice that sometimes grows
thick and heavy enough to bring down limbs, snap power
lines and disrupt traffic.
- Freezing rain develops when rain falls from a relatively
mild air layer aloft into a shallow layer of subfreezing air
at ground level . The drops become super cooled and then
freeze immediately on contact with cold surfaces
3. Snow: An assemblage of ice crystals in the form of flakes.
The form of snowflake varies with water vapor concentration
and temperature and may consist of plates, stars, columns or
needles.
4. Sleet, are actually frozen raindrops. They develop in much
the same way as freezing- rain with one difference:
1. The surface layer of cold air is so deep that raindrops
freeze prior to hitting the ground.
** We can readily distinguish ice pellets from freezing rain,
because ice pellets bounce when striking the ground and
freezing rain does not.
5. Hail: Consists of rounded or jagged lumps of ice often
characterized by concentric layers resembling the internal
structure of an onion.
- Hail develops within intense thunderstorms where strong
convection currents carry ice pellets up into the middle of
the cumulonimbus cloud.
- Along the way, ice pellets grow larger by collecting
super cooled water droplets and eventually becoming
heavy enough to fall to earth.
C. Measuring Precipitation:
Meteorologists measure rainfall with a rain gauge.
A rain gauge is an open-ended can or tube that collects
rainfall.
The amount of rainfall is measured by dipping a ruler into the
water or by reading a marked scale.
D. Controlling Precipitation:
Cloud seeding = an attempt to stimulate natural precipitation
processes by injecting nucleating agents into clouds.
The objective of seeding cold clouds is to stimulate the
Bergeron process in clouds that are deficient in ice crystals.
The seeding agent is either Silver Iodide (AgI), which has
crystal properties similar to those of ice, or Dry ice which is
solid Carbon Dioxide (CO) which has a temperature of about
-80 C ( -112 F ).
Silver Iodide crystals are freezing nuclei that are active at
-4 C (25 F) and below.
Dry ice pellets are so cold that within a cloud they cause
surrounding super cooled water droplets to freeze.
Dry ice is also used to disperse fog around airports. Dry ice
cause ice crystals to form increasing visibility.