5/2/2011 Understanding Clouds Clouds form as warm air is forced upward As the air is forced upward, it expands and cools 1

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1 Understanding Clouds Clouds form as warm air is forced upward As the air is forced upward, it expands and cools 1

2 Understanding Clouds As the air cools, the relative humidity reaches 100% Water vapor begins to condense in tiny drops around nuclei. Nuclei are small particles of dust, salt, and smoke in the atmosphere 2

3 Forms of Precipitation Precipitation is any form of water that falls to the Earth's surface. Types of Precipitation The type of precipitation that falls to the ground depends upon the formation process and the temperatures of the environment between the cloud and the surface 3

4 Can you name the different types of precipitation? Rain Snow Hail Sleet Freezing Rain Rain Rain develops when growing cloud droplets become too heavy to remain in the cloud and as a result, fall toward the surface as rain 4

5 Rain can also begin as ice crystals that collect each other to form large snowflakes As the falling snow passes through the freezing level into warmer air, the flakes melt Rain from snow! 5

6 Snow Snow is formed when ice crystals form from water vapor that is in the clouds directly above your heads! This process is called sublimation 6

7 Hail is formed when updrafts carry raindrops upwards into extremely cold areas of the atmosphere Hail There the raindrops merge and freeze. When the frozen clumps get to heavy they fall to earth Hail 7

8 Hail can vary in size, from the size of a small stone to that of a baseball! So be careful Hail Sleet Sleet is frozen raindrops. Sleet begins as rain or snow and falls through a deep layer of cold air that contains temperatures below freezing that exist near the surface. 8

9 Sleet Rain that falls through this extremely cold layer has time to freeze into small pieces of ice Freezing Rain Freezing rain is falling rain that cools below 0 C, but does not turn to ice in the air The water is supercooled 9

10 When the drops hit anything they instantly turn to ice! Weather Patterns and Forecasts 10

11 Air masses An air mass is a large body of air that has properties similar to the part of the Earth s surface over which it develops. 11

12 The atmosphere has pressure. If you imagine a column of air that is 1 inch square and goes from the Earth's surface all the way up through the atmosphere, the pressure of the air in that column is 14.7 pounds per square inch. Air Pressure Highs and lows Atmospheric pressure varies over Earth s surface. Winds blow from areas of high pressure to areas of low pressure. 12

13 Winds blow toward a center of low pressure Low pressure areas are associated with stormy weather and are called cyclones. Low Pressure High Pressure Winds blow away from a center of high pressure. High pressure areas are associated with fair weather and are called anticyclones. 13

14 Measuring Air Pressure Barometer Greek for weight and measure 14

15 Front A boundary between two air masses of different density, moisture, or temperature. Cloudiness, precipitation, and storms sometimes occur at frontal boundaries. There are four types of fronts: cold, warm, occluded, and stationary. Our weather fronts move from West to East 15

16 Cold Front Occurs when colder air advances toward warm air. The cold air wedges under the warm air and lifts it. The air cools and the water vapor condenses, forming clouds. When the temperature difference between the cold and warm air is large, thunderstorms and even tornadoes can form! 16

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18 Warm front Forms when warmer air advances over colder air Warm air slides over the colder air with a gentle slope, unlike in a cold front where it is suddenly lifted. This gentle sloping upwards can leads to hours, if not days, of wet weather. 18

19 Isotherms Lines on a weather map that connect points of equal temperature. Iso means same and therm means temperature 19

20 Isobars A line drawn to connect points of equal atmospheric pressure. If they are close together, it shows a large pressure difference over a small area. A large pressure difference causes strong winds. If they are spread apart, it means a small difference in pressure (gentle winds) Can also indicate high and low pressure areas. 20

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