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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.