Seasons and Weather

Main Ideas

Seasons and weather occur because of the earth's changing position to the sun.
Weather extremes are related to location on Earth.

Places & Terms solstice

hurricane
equinox
typhoon
weather
tornado
climate precipitation rain shadow
blizzard drought

Seasons

Hurricanes occur frequently in the southern and eastern United States during summer and fall. Storm systems with strong winds form over warm ocean water during these seasons.

EARTH'S TILT

Seasons have an enormous impact on us, affecting the conditions in the atmosphere and earth that create our weather. As the earth revolves around the sun, it is tilted at a 23.5° angle about the sun. Because of the earth's revolution and tilt, different parts of the earth receive the sun's direct rays for more hours of the day at certain times of the year. This causes the changing seasons on the earth.

Notice in the diagram to the right that the northern half of the earth tilts toward the sun in summer and away from the sun in winter.

Two lines of latitude the Tropic of Cancer and the Tropic of Capricorn-mark the points farthest north and south that the sun's rays shine directly overhead at noon. The day on which this occurs is called a solstice. In the Northern Hemisphere, the summer solstice, or the beginning of summer, is the longest day of the year. Winter solstice, the beginning of winter, is the shortest.

Another signal of seasonal change is the equinox-es. Twice a year on the equinox, the days and nights all over the world are equal in length. The equinoxes mark the beginning of spring and autumn.

Weather

Weather and climate are often confused. Weather is the condition of the atmosphere at a particular location and time. Climate is the term for weather conditions at a particular location over a long period.

Northern Russia, for example, has a cold climate. A

WHAT CAUSES THE WEATHER? Daily weather is the complex result of several conditions. For example, the amount of solar energy received by a location varies according to the earth's position to the sun.

Large masses of air absorb and distribute this solar energy, which in turn affects the weather. Other factors include:

water vapor This determines whether there will be precipitation-falling water droplets in the form of rain, sleet, snow, or hail.
cloud cover Clouds may hold water vapor.
landforms and bodies of water Water heats slowly but also loses heat slowly. Land heats rapidly but loses heat quickly as well.
elevation As elevation above sea level increases, the air becomes thinner and loses its ability to hold moisture.
air movement Winds move the air and the solar energy and moisture that it holds. As a result, weather can change very rapidly.

PRECIPITATION

Precipitation depends on the amount of water vapor in the air and the movement of that air. As warm air rises, it cools and loses its ability to hold water vapor. The water vapor condenses, and the water droplets form into clouds. When the amount of water in a cloud is too heavy for the air to hold, rain or snow falls from the cloud. Geographers classify precipitation as convectional, orographic, or frontal, as illustrated in the diagram

(insert photo here)

Convectional precipitation occurs in hot, moist climates where the sun quickly heats the air. The heated air rises, and by afternoon clouds form and rain falls. Orographic precipitation falls on the windward side of hills or mountains that block moist air and force it upward. The air cools and rain or snow falls. The land on the leeward side is called a rain shadow because it gets little rain from the descending dry air. Frontal movement causes most precipitation in the middle latitudes. A front is a boundary between two air masses of different temperatures or densities. Rain or snow occurs when lighter, warmer air is pushed upward by the colder, denser air. The rising air cools, water vapor condenses, and precipitation falls.

Weather Extremes

As air masses warm and cool and move across the earth's surface, they create weather. Sometimes the clashes between air masses cause storms, which can be severe. They disrupt the usual patterns of life and often cause major property damage and loss of human life. Hurricanes, tornadoes, blizzards, droughts, and floods are examples of extreme weather.

HURRICANES

Storms that form over warm, tropical ocean waters are called hurricanes-
- also known as typhoons in Asia. These storms are
called different names around the globe: tropical cyclones, willy-willies (Australia), Baguio (Philippines), and chubascos (Mexico). Hurricanes are one-way heat from the tropics is moved out of the region. Air flowing over an ocean with a water temperature of 80°F or higher picks up huge amounts of moisture and heat energy. As these water-laden winds flow into a low-pressure core, they tighten to form an
"eye." The eye is usually 10 to 20 miles across and has clear, calm skies. But the winds moving around the eye may be as strong as 200 miles per hour.
The clouds and winds stretch over a vast area, sometimes as wide as 500 miles.
Upper air currents blowing from the east steer the hurricanes in a westerly direction. As the hurricane hits land, it pounds the area with howling winds and very heavy rains. It may also cause a storm surge along coastal regions. This wall of seawater, pushed ashore by the winds, may rise to 16 feet or more. The low-lying coastal regions of Bangladesh in South Asia are especially vulnerable to storm surges from tropical cyclones.
TORNADOES Unlike hurricanes, which take days to develop, tornadoes form quickly and sometimes without warning. A tornado, or twister, is a powerful funnel-like column of spiraling air

THEMES

Canada

REGION

Tornado Alley

When cold, dry air collides with /arm, moist air, a tornado can rew. In the United States, these old funnel clouds occur frequently between May and tober in a region known as Mado Alley."

The flat plains stretching from <as through Nebraska present an ideal staging ground for tornadoes. Cold, dry air from nada rushes south and collides

1 warm, moist air moving in from the Gulf of Mexico. veen 200 and 300 major ms erupt there each year, vning hundreds of tornadoes.

United States

Doppler radar image shows possible tornado-causing weather for Texas or Oklahoma.

Average number of tornadoes per 10,000

sq. mi. per year

More than 9

7 to 9

5 to 7

1 to 5

Less than 1

Gulf of Mexico

Azimuthal Equal-Area Projection

TORY

VIDEO

tornado

Wister

Born from strong thunderstorms, tornadoes are capable of immense damage. In a tornado, winds swirl counter-clockwise around a low-pressure center. These winds may reach speeds of 300 miles per hour, blasting apart buildings and lifting objects as large as cars and mobile homes. Generally, tornadoes have small diameters (about 300 feet), travel about a mile, and last only a few minutes. However, the largest and most forceful can reach a mile across and stay on the ground for hours, hopscotching from one location to another. The largest outbreak of tornadoes in the United States occurred during 16 hours, on April 3 and 4, 1974. A total of 148 tornadoes ripped through the Ohio and Tennessee valleys, killing 330 people. The largest share of tornadoes, about 3 of every 4, hit in the United States. On average, the U.S. National Weather Service counts 700 tornadoes each year. C

BLIZZARDS A blizzard is a heavy snowstorm with winds of more than 35 miles per hour and reduced visibility. These weather conditions snarl traffic, endanger livestock, and trap travelers. The greatest snowfall for a

The 24-hour period was 76 inches (6 feet 4 inches) in Silver Lake, Colorado, in 1921. A snowstorm that lasted from February 13 to 19, 1959, dumped 189 inches (almost 16 feet) of snow on Mt. Shasta, California.

Because of their location, some areas of the country are frequently hit with snowstorms that produce huge amounts of snow. For example, the eastern and southern shores of the Great Lakes are snowbelts that experience days and days of heavy snow resulting in enormous snow depths. Around the Lake Erie and Lake Ontario areas, the annual snowfall can be as much as 450 inches (37.5 feet).

Atlantic Ocean

Geographic

Thinking+

Making

Comparisons

C How are tornadoes different from hurricanes?

BACKGROUND

A series of droughts in Texas between 1996 and 2000 caused $5.3

billion in damages.

DROUGHTS

A drought is a long period without rain or with very minimal rainfall. This lack of rain results in crop failures and drastically reduced levels in water storage facilities. In the early 1930s, a drought hit the Great Plains in the United States. Dust storms damaged farms across a 150,000-square-mile region that became known as the

"Dust Bowl." Suf-

feeling the effects of a harsh climate, thousands of families were forced

to leave their land to find work elsewhere. (See the Dust Bowl Disaster feature on pages 150-151.) In 2000, a large portion of the southern United States was struck with a long drought. Northern Texas was particularly hard hit, with 84 straight days of no rain and extremely high temperatures.

FLOODS When water spreads over land not normally covered with water, it is called a flood. Melting snow or rainwater fills streams or rivers until they reach flood stage, the point at which the banks can no longer contain the water. The water then flows into the surrounding area, called a floodplain.

Floods take lives every year, especially in low, flat places like Bangladesh, where millions of people live on the flood plains and the delta. In 1993, flooding along the Mississippi and Missouri rivers claimed 50 lives and caused about $15 billion in damage. Nearly 150 rivers and their tributaries were involved. It was the largest flood ever to hit the United States.

In the next section, you will learn about how climate affects people's lives and how humans adapt to changes in climate.

REGION

Before the drought in Texas, this boat floated on the waters of a lake now barely visible in the background.

Main Ideas

How do the earth's revolution and tilt affect the seasons?

What is the difference between weather and climate?

What are some examples of extreme weather?

Geographic Thinking

Determining Cause and

Effect What must be present for any type of precipitation to occur? Think about:

the cause of precipitation

the types of precipitation See Skillbuilder Handbook, page R9.

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Seasons and Weather Geography notes Chapter 3:

Seasons and weather occur because of the earth's changing position about the sun.

The earth is tilted at a 23.5° angle on its axis, which causes different parts of the earth to receive more or less direct sunlight at different times of the year.
This difference in sunlight causes the seasons.
The Earth's orbit around the sun is also elliptical, this means that the Earth is closer to the sun at some times of the year than at others.
This difference in distance from the sun also affects the seasons.

Weather is the condition of the atmosphere at a particular location and time.

Weather is caused by a combination of factors, including the amount of solar energy received by a location, the movement of air masses, and the presence of water vapor in the atmosphere.
Climate is the term for weather conditions at a particular location over a long period.
Climate is affected by a mixture of factors, including latitude, altitude, and proximity to bodies of water.

Extreme weather events, such as hurricanes, tornadoes, and floods, can have a big impact on human life and property.

Hurricanes are powerful storms that form over warm ocean waters.
Tornadoes are violent, rotating columns of air that can cause severe damage.
Floods occur when water spreads over land that isn’t normally covered by water.

The earth's position and tilt have an impact on weather conditions because they affect the amount of solar energy received by different parts of the Earth.

The earth's *revolution around the sun** causes the seasons. As the earth revolves, different parts of the planet receive more or less direct sunlight, which impacts the temperature and weather patterns in those areas.
The earth's *axial tilt** also affects the amount of sunlight received by different parts of the planet. The tilt of the earth's axis causes the Northern and Southern Hemispheres to experience different seasons at the same time.

Ocean currents are like rivers flowing in the ocean.

They move in large circular systems.
Warm waters flow away from the equator toward the poles.
Cold water flows back toward the equator.
Winds blowing over ocean currents affect the climate of the lands that the winds cross.
Cold ocean currents can lead to deserts by chilling the air and preventing rain from falling.

The climate varies based on latitude, elevation, and topography.

Latitude:

Polar zones: Cold year-round with a summer high of 50°F.
Temperate zones: Varies greatly due to higher summer solar heating.

Elevation:

The temperature drops 3.5°F for every 1,000 feet above sea level.
Climates above 12,000 feet resemble Arctic conditions.

Topography:

Mountains affect rainfall and temperature.
Winds cool and release precipitation on the windward side.
Winds are warm and dry on the leeward side.

El Niño

A natural climate change involving the warming of waters off the west coast of South America.
Occurs every 2-7 years when easterly winds slow or reverse direction.
Causes a shift in ocean temperatures and affects weather worldwide.
During El Niño, easterly winds push warm water toward the Americas, leading to floods and mudslides.
Asia and Australia experience drought conditions during El Niño.

La Niña

The opposite of El Niño.
Winds push warmer water toward the lands on the western Pacific rim.
Causes increased precipitation in places like India.
Causes increased dryness along the Pacific coasts of the Americas.

The greenhouse effect is causing global warming. The burning of coal and oil releases gases that trap solar energy, leading to higher temperatures.

Since the late 1800s, the earth's temperature has increased by one degree Fahrenheit.
Estimates suggest that the temperature will increase from 3.2 to 7.2 degrees in the next century.
More nations industrializing will likely increase the greenhouse effect and global warming.
Many of the world's glaciers have already shrunk, and there is less ice in the Arctic Ocean.
If the trend continues, ice caps will melt, flooding some coastal areas and covering low-lying islands.
Many scientists predict that weather patterns will become more erratic, with droughts becoming more severe.