1. Unit 4-3: Precipitation

2. Cooling of Air Adiabatic Change: Heating or cooling without transferring heat to/from surroundings. As a parcel of air rises and expands, it undergoes an adiabatic change and cools. As a parcel of air sinks and compresses, it undergoes an adiabatic change and heats. Adiabatic Change: Heating or cooling without transferring heat to/from surroundings. As a parcel of air rises and expands, it undergoes an adiabatic change and cools. As a parcel of air sinks and compresses, it undergoes an adiabatic change and heats.

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4. Cooling of air Dry air cools at a rate of about 1dC for every 100m of height. Wet air cools at a rate of about 0.8dC for every 100m of height. The wet air cooling rate is very useful for determining the cloud base height. Dry air cools at a rate of about 1dC for every 100m of height. Wet air cools at a rate of about 0.8dC for every 100m of height. The wet air cooling rate is very useful for determining the cloud base height.

5. Determining Condensation Level Condensation level is where in the sky a cloud’s base can form. If the dew point and temperature at the surface are known, then the condensation level can be determined: Con.Level = Surface Temperature – Dew Pt * 100m 0.8  C Condensation level is where in the sky a cloud’s base can form. If the dew point and temperature at the surface are known, then the condensation level can be determined: Con.Level = Surface Temperature – Dew Pt * 100m 0.8  C

6. How does precipitation form? Precipitation – any form of water that falls to the earth. Precipitation forms as water droplets in clouds continually grow and get heavier until they can no longer be held aloft by air currents in the cloud. Condensation nuclei allow small amounts of water vapor to attract to each other and grow. Precipitation – any form of water that falls to the earth. Precipitation forms as water droplets in clouds continually grow and get heavier until they can no longer be held aloft by air currents in the cloud. Condensation nuclei allow small amounts of water vapor to attract to each other and grow.

7. Types of Precipitation Drizzle: Very small drops that are very close together and fall very slowly. Also described as mist. Rain: Water droplets about 0.25cm in diameter that fall very fast. Amount of rain depends on the storm. Drizzle: Very small drops that are very close together and fall very slowly. Also described as mist. Rain: Water droplets about 0.25cm in diameter that fall very fast. Amount of rain depends on the storm.

9. Types of Precipitation Snow Water vapor that has frozen directly from its gaseous form without becoming water first. Snow is composed of six-sided crystals that generally drift slowly to earth. If the temperature of the air as the snow falls is slightly above freezing, the snow will melt slightly and clump together. Snow Water vapor that has frozen directly from its gaseous form without becoming water first. Snow is composed of six-sided crystals that generally drift slowly to earth. If the temperature of the air as the snow falls is slightly above freezing, the snow will melt slightly and clump together.

11. Types of Precipitation Sometimes, there is a temperature inversion during wintertime. This will cause warm rain clouds to be above a layer of freezing air. When rain falls through this freezing air, the rain freezes as well This is called sleet. Sleet is frozen rain, or pellets of clear ice Sometimes, there is a temperature inversion during wintertime. This will cause warm rain clouds to be above a layer of freezing air. When rain falls through this freezing air, the rain freezes as well This is called sleet. Sleet is frozen rain, or pellets of clear ice

13. Types of Precipitation Winter Ice Storms: When rain is below freezing while still being liquid, it is called being super cooled. When the super cooled rain strikes a below freezing surface, it instantly freezes This creates a glaze of ice on the surface. This is dangerous for trees, power lines, and buildings because the ice can become very heavy. Winter Ice Storms: When rain is below freezing while still being liquid, it is called being super cooled. When the super cooled rain strikes a below freezing surface, it instantly freezes This creates a glaze of ice on the surface. This is dangerous for trees, power lines, and buildings because the ice can become very heavy.

15. Types of Precipitation Hail: -An onion-like ball of ice Forms only in thunderclouds Hailstone Formation: Begins as a small frozen particle It grows as it falls by collecting liquid water droplets in a cloud. Updrafts in the thundercloud pull the hailstone upwards When it gets high enough, the liquid water freezes The hailstone falls, rises, falls, rises, over and over Hail: -An onion-like ball of ice Forms only in thunderclouds Hailstone Formation: Begins as a small frozen particle It grows as it falls by collecting liquid water droplets in a cloud. Updrafts in the thundercloud pull the hailstone upwards When it gets high enough, the liquid water freezes The hailstone falls, rises, falls, rises, over and over Hail stinks? NO! They both have layers, like ogres!

16. The first piece of ice gather a little water. Then updrafts pull it up so it freezes. The process repeats over and over

17. Measuring Precipitation The National Weather Service measures rainfall to the hundredth of an inch. They use a tool called a rain gauge. A rain gauge is a tube that collects precipitation that also has measurements printed on it. Snow is measured to the tenth of an inch. Simply measured with a ruler The National Weather Service measures rainfall to the hundredth of an inch. They use a tool called a rain gauge. A rain gauge is a tube that collects precipitation that also has measurements printed on it. Snow is measured to the tenth of an inch. Simply measured with a ruler

18. I love it when it rains!

19. Measuring Precipitation Snow takes up more space than rain. The rain equivalent of the snowfall is determined by melting a definite area of the snow. Dry snow will take up more space than wet snow On average, ten inches of snow will equate to one inch of rain. Snow takes up more space than rain. The rain equivalent of the snowfall is determined by melting a definite area of the snow. Dry snow will take up more space than wet snow On average, ten inches of snow will equate to one inch of rain.

20. Where does it rain? Precipitation, regardless of form, occurs in every part of the world. Some areas however, it may not precipitate for years at a time Other areas will get precipitation on a daily basis. What accounts for such differences? Precipitation, regardless of form, occurs in every part of the world. Some areas however, it may not precipitate for years at a time Other areas will get precipitation on a daily basis. What accounts for such differences?

21. Where does it rain? Precipitation will occur whenever the air rises high enough and in a large enough quantity. The warmer the air, the more moisture it could contain The higher the air can rise, the more water it can drop as precipitation. Precipitation will occur whenever the air rises high enough and in a large enough quantity. The warmer the air, the more moisture it could contain The higher the air can rise, the more water it can drop as precipitation.

22. Where does it rain? The most precipitation will be where the air rises in great quantities: Doldrums: Warm humid rising air. Almost daily thunderstorms, especially over land. Includes dense tropical forests of the Amazon, Congo, and Indonesia. The most precipitation will be where the air rises in great quantities: Doldrums: Warm humid rising air. Almost daily thunderstorms, especially over land. Includes dense tropical forests of the Amazon, Congo, and Indonesia.

23. Where does it rain? The most precipitation will be where the air rises in great quantities: The windward side of mountains: The side of a mountain where the prevailing winds are forced to climb towards the top of the mountain. Example: The Cascade Mountains in the northwestern US. Because the air is forced so high, it creates rain on the windward side, and very little rain on the other. The most precipitation will be where the air rises in great quantities: The windward side of mountains: The side of a mountain where the prevailing winds are forced to climb towards the top of the mountain. Example: The Cascade Mountains in the northwestern US. Because the air is forced so high, it creates rain on the windward side, and very little rain on the other.

25. Where does it rain? The most precipitation will be where the air rises in great quantities: Storm areas: Areas where hurricanes, cyclones, typhoons, etc. All these areas are where great masses of moist air is rising. After the air drops its water load, it won’t precipitate until it sinks back down and gets more water. The most precipitation will be where the air rises in great quantities: Storm areas: Areas where hurricanes, cyclones, typhoons, etc. All these areas are where great masses of moist air is rising. After the air drops its water load, it won’t precipitate until it sinks back down and gets more water.

26. Where does it not rain? The areas of least precipitation are where air is sinking and being warmed by compression. The air can hold more and more water as it sinks, so it will not give up its water to precipitation. The air can become so hot and dry that it can create desert conditions. The areas of least precipitation are where air is sinking and being warmed by compression. The air can hold more and more water as it sinks, so it will not give up its water to precipitation. The air can become so hot and dry that it can create desert conditions.

27. Where does it not rain? Examples: The horse-latitude belts The leeward side of a mountain (the side opposite to the windward side) The polar highs Anywhere else there is sinking air masses. Examples: The horse-latitude belts The leeward side of a mountain (the side opposite to the windward side) The polar highs Anywhere else there is sinking air masses.

28. Precipitation in the Tropics Precipitation follows a simple pattern in the tropics: The air in the tropics is moist The air rises in the tropics because of the doldrums. This rising moist air results in almost daily thunderstorms. Precipitation follows a simple pattern in the tropics: The air in the tropics is moist The air rises in the tropics because of the doldrums. This rising moist air results in almost daily thunderstorms.

29. Precipitation in the Tropics The opposite occurs in the Horse Latitudes Sinking air brings desert conditions However, areas of land between the horse latitudes and the doldrums have seasonal shifts in rainy and dry seasons The opposite occurs in the Horse Latitudes Sinking air brings desert conditions However, areas of land between the horse latitudes and the doldrums have seasonal shifts in rainy and dry seasons

30. Precipitation in the Tropics Some places in the tropics get rain because they are on the eastern side of the mountain. This is because the winds are easterly in between the doldrums and horse latitudes. As we saw previously, air rising upwards against a mountain drops its water-load. Some places in the tropics get rain because they are on the eastern side of the mountain. This is because the winds are easterly in between the doldrums and horse latitudes. As we saw previously, air rising upwards against a mountain drops its water-load.

31. Precipitation in Middle Latitudes Middle Latitudes are between 35 degrees and 65 degrees. These areas have prevailing westerly winds. There are no major world-circling belts such as the all-wet doldrums or the very dry horse latitudes. Middle Latitudes are between 35 degrees and 65 degrees. These areas have prevailing westerly winds. There are no major world-circling belts such as the all-wet doldrums or the very dry horse latitudes.

32. Precipitation in Middle Latitudes The chief factors influencing rainfall are: Mountain Ranges Distance from an ocean Storms The rainiest places are the windward side of high coastal mountains. The driest areas the the leeward sides of these mountains. The chief factors influencing rainfall are: Mountain Ranges Distance from an ocean Storms The rainiest places are the windward side of high coastal mountains. The driest areas the the leeward sides of these mountains.

33. Precipitation in Middle Latitudes The interior areas are dry for two reasons: Distance from ocean High mountains keep out moist ocean air. As the east and south coasts are approached, the rainfall increases: Large number of storms in these areas The air coming in from the oceans has a high degree of moisture. The interior areas are dry for two reasons: Distance from ocean High mountains keep out moist ocean air. As the east and south coasts are approached, the rainfall increases: Large number of storms in these areas The air coming in from the oceans has a high degree of moisture.

34. Precipitation in High Latitudes High latitudes reach from 65 degrees to 90 degrees. These areas are covered by the polar easterlies or the polar highs. Extremely cold air cannot hold much water whatsoever. Because the air is cold here, there is very little precipitation even when the air rises. High latitudes reach from 65 degrees to 90 degrees. These areas are covered by the polar easterlies or the polar highs. Extremely cold air cannot hold much water whatsoever. Because the air is cold here, there is very little precipitation even when the air rises.

35. Precipitation in High Latitudes The only precipitation in this area is generally from traveling storms. Yearly precipitation is only about 10 to 15 inches. Because of the low temperatures, there is very little water that evaporates from the surface. The only precipitation in this area is generally from traveling storms. Yearly precipitation is only about 10 to 15 inches. Because of the low temperatures, there is very little water that evaporates from the surface.