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The Atmosphere Section 3 Fronts and Severe Weather 〉How do fronts affect the weather? 〉When fronts move through an area, the result is usually precipitation and a change in wind direction and temperature. • air mass: a large body of air where temperature and moisture content are similar throughout • front: the boundary between air masses of different densities and usually different temperatures The Atmosphere Section 3 Fronts and Severe Weather, continued • Interactions between air masses have predictable effects on the weather in a given location. • Clouds, rain, and sometimes snow can occur at fronts. • Front types include • warm fronts • cold fronts • stationary fronts The Atmosphere Section 3 Fronts and Severe Weather, continued • Lightning is a discharge of atmospheric electrical energy. – Water droplets and ice crystals in thunderclouds build up electrical charges. – Lightning is a big spark that jumps between clouds or between clouds and Earth to equalize the charge. • Lightning heats the air so fast that the air expands faster than the speed of sound. – The shockwave created is thunder. The Atmosphere Section 3 Fronts and Severe Weather, continued • Tornadoes are funnels of high-speed wind. – Tornado winds are the most violent winds on Earth. • Wind speeds may reach up to 500 km/h. – Tornadoes typically form along a front between cool, dry air and warm, humid air. – funnel cloud: a column of water droplets • Tornadoes are fast moving and unpredictable. The Atmosphere Tornado Section 3 The Atmosphere Section 3 Fronts and Severe Weather, continued • Hurricanes are large, rotating tropical storm systems. • These storms are called – hurricanes in North America and the Caribbean – cyclones in the Indian Ocean – typhoons in the western Pacific • tropical depressions: intense low-pressure areas that can become hurricanes The Atmosphere Section 3 Fronts and Severe Weather, continued • Hurricanes are powered by energy released as water vapor condenses to form clouds. • The center of a hurricane is called the eye. • The eye is usually calm. • Hurricane winds can reach speeds greater than 250 km/h. The Atmosphere Section 3 Climate 〉How is climate different from weather? 〉Weather changes from day to day, but climate does not change as quickly. • climate: the average weather conditions in an area over a long period of time The Atmosphere Section 3 Climate, continued • Temperatures tend to be higher close to the equator. – Sunlight strikes the earth more directly close to the equator. – The sun’s rays are less concentrated at the poles, and do not warm the atmosphere as much. The Atmosphere Section 3 Climate, continued • Earth’s tilt and rotation account for our seasons. – When the North Pole is tilted toward the sun, the Northern Hemisphere experiences summer. • There is more daylight, and the temperature increases. • Earth is farthest from the sun on July 4. • When the South Pole is tilted toward the sun, the Southern Hemisphere experiences summer. The Atmosphere Section 3 Climate, continued • Earth’s surface features affect climate. • topography: the size and shape of the land surface features of a region, including its relief – Variations in topography affect the climate of a region. • Mountains can trap moisture on one side. – Deserts may form on the dry side of a mountain. • Broad flat surfaces allow winds to merge on the plains. – Thunderstorms and tornados may form. The Atmosphere Section 3 Climate, continued • Global climate changes over long periods of time. • Many factors produce changes in Earth’s climate, such as: • eruptions of volcanoes • shift of the continents • changes in Earth’s tilt • Many scientists think that increases in human-made greenhouse gases have caused the recent increase in global average temperature.