1. PRECIPITATION "RAIN DROPS KEEP FALLING ON MY HEAD" B.J. THOMPSON (1969)

2. INTRODUCTION How much rain do you get in your area? How about snow? This amount varies around the world. For example, people living in Spokane, Washington get much more rain than those in Phoenix, Arizona.

3. PRECIPITATION Precipitation is any form of water that falls from the atmosphere to Earth's surface. In order for precipitation to occur the clouds must become heavy enough with water droplets or ice crystals. If the droplets or crystals are too small then they stay suspended within the clouds. The most common types of precipitation are rain, sleet, freezing rain, snow, and hail.

4. DROPLET SIZE

5. RAIN Rain is the most common type of precipitation The droplet sizes are.5 millimeters or bigger. Droplets smaller than that are called drizzle These usually fall from stratus clouds that are higher up in the atmosphere.

6. SLEET Sleet occurs in two ways. The first way is that is that it starts off as crystals and they begin to fall. The crystals hit a band of warm air and melt. They hit another band of cold air and refreeze. The second way is that the droplets fall from the atmosphere and freeze on the way down before hitting the surface.

7. FREEZING RAIN Many people get freezing rain and sleet mixed up. Sleet occurs when the water droplets freeze in the air while freezing rain occurs when the droplets hit the ground. The droplets pass through the atmosphere at a low temperature, but not enough to freeze them. When they touch the ground they freeze instantly because of the temperature of the surface. Freezing rain can cause very dangerous conditions.

9. SNOW Snow occurs when water vapor in the clouds is turned into crystals within the cloud. The shape that these crystals create are endless. Typically the colder, dryer air creates very small crystals while the more moist, warmer air creates large plate-like crystals.

11. HAIL Hail is one of the more rare types of precipitation and for a good reason. If hail becomes big enough it can be deadly. Hail can only form in cumulonimbus clouds during thunderstorms. Strong winds carry water droplets upward over and over again into cooler air. This causes them to recrystallize again and again until they are heavy enough the fall. The stronger the winds are in the storm the larger the hail.

13. DROUGHTS Some regions go through periods of dry spells where they get much less rain than usual. This is known as a drought. Droughts can be very dangerous for an area and they can even deplete water tables deep underground. Droughts can also alter the soil and make it less fertile.

14. CLOUD SEEDING Scientists have been studying ways to alter precipitation and to help areas affected by droughts. One method of altering precipitation is known as cloud seeding. Cloud seeding is when an aircraft is taken above the clouds. These airplanes drop iodide and dry ice crystals onto the clouds. The idea is that the water vapor will condense due to cold dry ice and form around the iodide crystal. Experiments have been occurring since the 1940s, but it is not very efficient at this time.

16. MEASURING PRECIPITATION Measuring precipitation is very important for people such as farmers, ranchers, and forecasters. Measuring precipitation can include various methods including basic rulers to digital rain gauges.

17. SNOWFALL MEASUREMENT The two main ways of measuring snowfall are using a ruler or melting an amount of snow and see how much water is collect. For every inch of rain there are thirteen inches of snow, but this number can vary depending on how wet the air is and how big the snowflakes are. The lighter the snow the less water, but it could be much higher if more fell.

18. RAINFALL MEASUREMENT A rain gauge is an open-ended tube that collects rainfall. This can be measured by a rule that is printed on the tube or by dipping a ruler into it. Some gauges are made to catch ten times the amount of rainfall that the tube can on its own. It is necessary to divide by ten. Some agencies use advanced, digital rain gauges that do the measuring for them.

20. REVIEW QUESTIONS What is the most common form of precipitation? How is sleet different from freezing rain? How can you accurately measure snowfall? Critical thinking: How does the funnel in a rain gauge increase the accuracy of the measurement?