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Weather Changes Chapter 12, Section 2 I. Weather Constantly Changes! A. Air masses – large body of air that has properties similar to the part of the Earth’s surface over which it develops 1. Can be over water or land from tropical or polar regions a. If air mass forms over land, the air mass is dry b. If air mass forms over water, the air mass is moist c. If air mass forms in polar regions, air mass is cold d. If air mass forms in tropical regions, air mass is warm 2. Temperature and humidity conditions in air masses are important to weather when these air masses move 3. Can cover thousands of square kilometers I. Weather Constantly Changes! B. High Pressure 1. Winds blow from high pressure to low pressure A. High-pressure areas are associated with fair weather B. Rotation of Earth causes these winds to spiral in a clockwise direction in northern hemisphere I. Weather Constantly Changes! C. Low Pressure 1. Winds blow from high pressure to low pressure A. Low-pressure areas are associated with cloudy weather II. Fronts Front is a boundary between 2 air masses of different density, moisture, or temperature B. Cloudiness, precipitation, and storms occur at frontal boundaries C. 4 types of fronts A. 1. 2. 3. 4. Cold front Warm front Stationary front Occluded front Cold Front Dense, cold air mass pushes into warmer air mass and lifts it, causing the warm air to rise, cool, and condense & form clouds When the temperature difference is large between cold front and warm air, thunderstorms & tornadoes may form. Snowstorms & blizzards can occur if temperature is low enough Warm Front Lighter, warmer air advances and slides over denser, colder air Light, steady precipitation, stratus clouds Stationary Front Stalemate between air masses Both air masses can remain in same place for several days Can produce precipitation and light wind Occluded Front Occurs with 3 air masses: cold, cool, and warm air Cold air mass moves toward cool air mass, warm air between them (this is a rare front) Cold air pushes warm air up away from Earth’s surface Can create violent weather Front Summary Presentation Express animations! III. Severe Weather a. Thunderstorm 1. storm with thunder, lightning, heavy rains and strong winds; form within large cumulonimbus clouds; usually form along a cold front but can form within an air mass. b. Tornado 1. rapidly whirling, funnel-shaped cloud that extends down from a storm cloud; the very low pressure and strong winds can cause great damage to people and property; are likely to form within the frontal regions where strong thunderstorms are also present. III. Severe Weather A. Hurricane -- low pressure tropical storm that forms over warm ocean water; winds form a spinning circular pattern around the center, or eye, of the storm; the lower the air pressure at the center, the faster the winds blow toward the center of the storm. IV. Severe Weather Safety A. Watch 1. B. When conditions are favorable for severe thunderstorms, tornadoes, hurricanes, floods, and blizzards Warning 2. Severe weather conditions already exist and action should be taken Severe Weather Caught Ya Being Brilliant! According to the map, the most likely areas of heavy precipitation are: a. in the Western United States. b. in the Eastern United States. c. across the entire country. d. nowhere--there is no precipitation anywhere in the country. Caught Ya Being Brilliant! According to the map, which type of weather situation is more likely in the Western part of the United States than in the Eastern part? a. high pressure systems b. low visibility c. heavy precipitation d. low pressure systems Caught Ya Being Brilliant! A meteorologist who is looking at the above map might be concerned that: a. the Western part of the United States is going to see flooding in certain areas. b. the Eastern part of the United States is going to be struck by a hurricane. c. the Western part of the United States is going to have heavy snow storms. d. the Eastern part of the United States is going to suffer from a drought.