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Instruction 6-2

Weather and Climate | Climate and Topography | How Has Earth's Climate Changed Over Time? | Computer Models to Study Greenhouse Effect | Summary

Climate and Topography
CCSTD Earth Science  6.b.

Topography means "the lay of the land" -- the physical features of the surface of the Earth. These physical features include mountains, valleys, bodies of water (lakes, rivers, oceans) and even structures built by man (towns, cities, roads, etc.). An area's topography can have a major effect on its climate. This is especially true in the mountainous western United States.


High elevations such as mountains affect climate by changing the patterns of temperature, wind circulation and precipitation.
The most obvious effect is on temperature, which decreases with elevation. Air temperature decreases at a rate of approximately 6.5 degrees C. per kilometer.
Cold temperatures lead to year-round snow and a high albedo (reflection of radiation and heat from the Sun).
Mountains also play a direct role in changing the pattern of atmospheric circulation (wind).
Think of the wind as a stream and a mountain as a rock in the middle of the stream. Wind currents are deflected around the mountain and the current is changed downstream.
The orientation of a mountain affects precipitation. Windward slopes, those facing into the wind, are wetter. That's because as the air is forced up the slope, it gets cooler. And since cooler air can hold less moisture than warm air, it snows or rains.
On the other slope of the mountain (the leeward side), the opposite occurs. That's because as air is forced down the slope, it gets warmer. Since warm air can hold more moisture than cool air, it doesn't rain -- which makes for dry, almost desert-like terrain.
The wettest area in North America is along the Pacific coast from Oregon to Southern Alaska, where moisture-laden winds strike mountains along the shore. Average annual precipitation is more than 200 centimeters -- and in some places it's as much as 300 centimeters.
Air temperatures are also affected by a mountain's orientation. Slopes facing into the Sun are warmer than those facing away from it.
Land and Water

Climate is also affected by the relationship between land and water.
The ocean covers more than 70 % of the Earth's surface and stores vast amounts of energy in the form of heat.
The ocean also has a large temperature inertia (resistance to change). This means that oceans -- and other large bodies of water -- heat and cool more slowly than land. So areas near large bodies of water experience less extremes in temperature than other places.
This is called the maritime influence -- and here's how it works:
The prevailing wind systems move air from the ocean onto the shore, where it moderates summer and winter extremes. Although cold ocean currents can have a cooling effect, horizontal and vertical currents minimize seasonal variations in the surface temperature of the water. This, in turn, limits temperature extremes in the air mass above.
In other words, having a large body of water nearby makes winters warmer and summers cooler.
It can also make it wetter because of the amount of moisture in the air, although there are exceptions -- like the dry coast of Southern California and the Arctic coastline of Alaska.
Places far from water can exhibit great extremes in temperature. This is called continentality. Parts of the Great Plains, for example, have an annual temperature range of up to 100 C.


In addition to mountains, land and water, cities also affect weather and climate.
People know that it is hotter in the city than in the country, but only recently have scientists begun to study this phenomenon.
Beginning in 1996, NASA sponsored a study of Urban Heat Islands -- local urban weather sites that are regularly 5 to 8 degrees F. hotter than the outlying areas around them.
Atlanta was the first city studied; others include Baton Rouge, Sacramento and Los Angeles.
Urban areas generate heat because of population, manufacturing and transportation. In addition, asphalt roads and tar roofs absorb and hold heat -- and little reflective building material has been used on roofs and surfaces.
Another problem is the loss of trees.
Trees normally reduce the amount of heat and smog by absorbing heat during the day and carrying it away through evaporation (transpiration) at night.
But when cities are built, trees are cut down.
Between 1973 and 1992, almost 380,000 acres of forest were cleared just to accommodate the growth of Atlanta. So trees are no longer there to perform their cooling and cleansing functions.
As heat in the city builds, hot air rises. Colder air from outside the city rushes into the vacuum, creating wind. This cools the rising air and forms clouds, which result in frequent -- and sometimes dangerous -- local thunderstorms.
Is there anything that can be done about Urban Heat Islands?
Studies continue, but preliminary suggestions include planting trees (and other plants) and installing reflective roofing materials.



It can be difficult to observe the relationship between topography and climate from locations on Earth. But it's easy when one looks back at the Earth from space. In this "Earth Day" activity, students learn to become "satellite image sleuths." Click:

London, England is known for its thick, almost impenetrable fog. In this simple experiment, students learn to make "London fog" for themselves. Click: 

NOAA (the National Oceanic and Atmospheric Administration) watches over Earth's atmosphere, oceans and U.S. coasts. In these "Spuzzles," students try to unscramble pictures of various NOAA activities and sites (including Catalina Island off the Southern California coast). There are 8 Spuzzles with various levels of difficulty. Click:

Reading List
from the California Department of Education
  Arnold, Caroline: El Nino: Stormy Weather for People and Wildlife 
Sussman, Art: Dr. Art's Guide to Planet Earth: For Earthlings Ages 12 to 120

for Students, Parents and Teachers

Now let's do Practice Exercise 6-2 (top). Choose printer friendly or online exercises. Printer friendly version requires the Adobe Acrobat Reader 5. Click HERE to obtain a free copy.


Next Page:  How Earth's Climate has Changed Over Time? (top)