Download Precipitation Maps

PRECIPITATION MAPS
MAP:
http://water.weather.gov/precip/
QUESTIONS:
•What is precipitation?
•What are some examples of precipitation?
•Define the different types of precipitation on the map.
•What causes different types of precipitation?
BACKGROUND
In meteorology, precipitation is any product of the condensation of the atmospheric water vapor that
falls under gravity. The main forms of precipitation include drizzle, rain, sleet, snow, and hail.
Precipitation occurs when a portion of the atmosphere becomes saturated with water vapor so that
the water condenses and “precipitates”.
Precipitation: What makes clouds, rain, snow, hail and sleet
When warm, wet air rises, it cools, and water vapor condenses out to form clouds. A cloud is made up
of small drops of water or ice crystals, depending on its height and the temperature of the surrounding
air. Height and temperature also determine whether any "precipitation" which results will be rain or
the hail associated with thunderstorms, or the snow, sleet and freezing rain we associate with winter
weather.
To form rain, water vapor needs what's called a "condensation
nucleus", which can be tiny particles of dust, or pollen, swept up
high into the atmosphere. When the condensing droplets that
form the cloud get large and heavy enough to overcome the
upward pressure of convection, they begin to fall. If the
temperature all the way to the ground is above freezing, then--it's raining! When ice crystals form high
up in the cloud, and it's below the freezing point of water all the way down, then you get snow. But
when there are alternating layers of air above and below freezing, you get other types of precipitation.
For instance, if snow falls through the warm air, it melts or partially melts into raindrops. As the
melted snow falls through the cold layer of air, it re-freezes. It forms ice pellets, or sleet, before hitting
the ground.
If a snowflake falls through a region of the cloud where there's liquid water that coats the flake with
more and more layers of new ice, you begin to get hail. When the thunderstorm's updrafts are strong
enough, some of the young hailstones are swept back up and repeat their journey, getting coated with
more and more layers of ice. Eventually they grow so big that not even the strongest updraft can keep
them aloft, and so they fall to Earth, in sizes from that of a pea to a golf-ball, and up to the record
holder--6 inches long and 17 inches in circumference! (Kansas, 1970.)
As damaging as hail can be to houses and especially to agriculture, freezing rain can be even more
lethal, especially to travelers, as they bring "ice storms" like those of 1998. Freezing rain occurs when
earth and objects on the surface, such as roads, tree limbs and power cables, are at temperatures
below 0° Celsius, 32° F. Above the ground, however, falling snow first encounters a layer of somewhat
warmer air, which melts the flakes, and then, right above the surface, a very cold layer, which makes
the liquid water "super-cooled", ready to freeze up at the slightest provocation. The trigger is
encountering the freezing surfaces: what results is a thin, sometimes transparent film of ice.
The weight of the ice can cause tree limbs to fall across power lines, or sometimes just drag down the
lines themselves. Driving conditions are, of course, very dangerous. This is one of the factors that make
extreme heat and extreme cold the #1 weather killers in the United States--even more lethal than
lightning, floods, tornadoes and hurricanes.