1. Chapter 2: Weather Factors
Section 5:
Precipitation

2. Goals/Objectives
After completing the lesson, students will be able to . . .
Identify the main types of precipitation
Describe how precipitation is measured and ways that it might be controlled

3. Introduction Rainfall varies greatly around the world.
Some areas receive rainfall regularly, while others rarely get any.
Precipitation—Any form of water that falls from clouds and reaches Earth’s surface.
Precipitation always comes from the clouds, but not all clouds produce precipitation.

4. How Precipitation Occurs
For precipitation to occur, cloud droplets or ice crystals must grow heavy enough to fall through the air.
One way that cloud droplets grow is by colliding and combining with other cloud droplets.
As the droplets grow larger, they fall faster and collect more and more small droplets.
Finally, the droplets become heavy enough to fall out of the cloud as raindrops.

5. Types of Precipitation
Common types of precipitation include:
Rain
Sleet
Freezing Rain
Hail
Snow

6. Rain Rain is the most common type of precipitation.
Drops of water are called rain if they are at least 0.5 millimeter in diameter.
Precipitation made up of smaller drops are referred to as mist or drizzle
Mist and drizzle usually fall from nimbostratus clouds

7. Sleet Sometimes rain will fall through a layer of air below 0˚C.
As it falls, the raindrops freeze into solid particles of ice.
Sleet is categorized as ice particles smaller than 5 millimeters in diameter.

8. Freezing Rain
Sometimes, raindrops do not freeze when the fall through the air; rather they freeze upon hitting a cold surface.
A thick layer of ice can build up through this process.
Effects:
The weight can break tree limbs, which can cause damage to buildings or break power lines.
Sidewalks and roads become to slippery for safe travels.

9. Hail
Round pellets of ice larger than 5 millimeters in diameter are called hailstones.
Hail only forms inside cumulonimbus clouds during thunderstorms.
They start off as an ice pellet, but constant updrafts carry it in the colder region many times resulting in new layers of ice each time.
Eventually, it becomes too heavy and falls to the ground.
Due to its size, hail can cause tremendous damage to crops, buildings, and vehicles.

10. Snow
Often water vapor in a cloud is converted directly into ice crystals called snowflakes.
Snowflakes have an endless number of different shapes and patterns, all with six sides or branches.
Snowflakes often join together into large clumps of snow in which the six-sided crystals are hard to see.

11. Measuring Precipitation
Meteorologists measure rainfall with a rain gauge.
Rain Gauge—An open-ended can or tube that collects rainfall.
Some rain gauges have a funnel at the top in order to get an accurate measurement.
Rain gauges are usually marked for measurements, but ruler can be used.
Snowfall is measured using a ruler or by melting collected snow and measuring the depth of water it produces.

12. Controlling Precipitation
Droughts—Long periods of unusually low precipitation.
Effects:
Can cause crops to fail
Farmers suffer from lost income
Consumers suffer from high food prices
In less-developed countries, droughts can cause hunger, or famine.
Scientists have been trying to produce rain during droughts.
The most common method is called cloud seeding.
Tiny crystals of dry ice and silver iodide are sprinkled into clouds from airplanes.
The dry ice can cool particles down even further to help form precipitation, and the silver iodide allows water particles to condense on it.
Cloud seeding has also been used with some success to clear fog from airports.
Dry ice is sprinkled into the fog, which causes ice crystals to form, and allows pilots to see the runway.