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

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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

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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.

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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.

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Tornado

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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

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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.

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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

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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.

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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.

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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.

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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.