Understanding Climate vs. Weather: A Comprehensive Guide

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Introduction to Climate

Climate, the average condition of the weather in an area for a period of years. Weather, on the other hand, is the condition of the atmosphere at a particular time and place. In order to describe the climate of a given region, monthly and seasonal averages are made of rainfall and such properties of the atmosphere as temperature, humidity, pressure, and wind velocity.

Climate has a major effect on human activities. What people eat and wear, as well as how they earn their livelihoods, all depend to some extent on the local climate. A drastic change in a region's climate has sometimes contributed to the migration of large groups of people.

What Influences Climate

Latitude

The latitude of an area—that is, its distance from the Equator—determines the amount of heat it receives from the sun. Close to the Equator, the sun's rays fall nearly vertically on the earth's surface most of the year. North or south of the tropics, the rays always reach the earth at a slant. Where the rays fall at a slant, they provide less warmth because they are spread out over a larger area.

Altitude

As the altitude, or height above sea level, increases, the temperature decreases at a rate of about 1 F. for each 300 feet, or 1 C. for every 165 m. The thinner air at higher altitudes does not hold the heat radiated from the earth; this heat escapes into space. Even near the Equator, the climate at high altitudes may be extremely cold, as on Africa's snowcapped Mount Kenya.

Winds

Prevailing winds carry the heat and moisture content of the areas where they originate to other areas. The winters of Sitka, in southern Alaska, because of southwesterly winds off the warm Pacific Ocean, are as warm as those of Philadelphia, 1,000 miles (1,600 km) closer to the Equator.

Large Bodies of Water

A body of water heats and cools much less rapidly than land does. A large body of water, therefore, provides a cooling effect in the summer and a warming effect in the winter, particularly where the prevailing winds blow onshore. In contrast, areas of land far from any large body of water tend to have larger temperature variations with the seasons. For example, the difference between the average temperatures of the warmest and coldest months in San Francisco, on the Pacific Ocean, is only 11 F. (6 C.). In Kansas City, Missouri, in the interior of North America, the difference is 51 F. (28 C.).

Ocean currents also play an important part in determining climate. They carry heat from the tropics toward the poles. For example, the warm waters of the Gulf Stream help keep the British Isles and northwestern Europe much warmer during the winter than they would otherwise be, giving them mild seasons.

Mountains and Rainfall

As winds travel up the slope of a mountain range, moisture in the air tends to condense and fall to earth as rain or snow. The air that passes to the other side of the mountain range is drier and warmer than it would be otherwise. For example, the prevailing winds in southern Chile blow from west to east, leaving the western slopes of the southern Andes with plentiful rainfall and snow. East of the mountains, in the region of Argentina called Patagonia, the climate is dry.

Changes In Climate

Although climates are generally stable, they do change over time. One method scientists use to determine the nature of past climates is to sample the atmospheric gases trapped long ago, deep inside the ice sheets of Antarctica and Greenland. Scientists who study climate believe there may be several causes for climatic change, including (1) the shifting of the continents and the formation of new mountain ranges, (2) variations in the earth's orbit and in the tilt of the earth's axis, (3) the emission of large amounts of dust into the air from volcanic eruptions or the impact of large meteorites, and (4) changes in the amount of radiation emitted by the sun. It also appears that the climate is affected by the emission of gases into the atmosphere through such human-related activities as the burning of fossil fuels.

Kinds of Climates

Many systems have been developed for classifying climates. The simplest is based on latitude, dividing the world into five zones—the north polar, north temperate, torrid (or tropical), south temperate, and south polar. Because it ignores such factors as altitude and the effect of the oceans, this system is not very accurate. Another system classifies climate as continental (cold winters and warm summers) or marine (mild seasons). A more complete classification is the one devised by Wladimir Kppen (1846-1940), a Russian-German climatologist. He defined five major types of climate:

Tropical Rainy Climates

The average temperature of the coldest month is above 64 F. (18 C.). During the two rainy seasons 6 to 15 inches (150 to 380 mm) of rainfall a month are usual. These climates are found in much of northern South America, central Africa, and Southeast Asia.

Dry Climates: Steppes and Deserts

There is less than 20 inches (500 mm) of rainfall a year, as in the western plains of North America and deserts throughout the world.

Humid, Warm Climates

The average temperature of the coldest month is between 64 and 27 F. (18 and -3 C.). Yearly rainfall ranges from 20 to 40 inches (500 to 1,000 mm). These climates prevail in the southeastern United States and western Europe and on the coasts of the Mediterranean Sea and Indian Ocean.

Humid, Cold Climates

The average temperature of the coldest month is below 27 F. (-3 C.) and the average temperature of the warmest month is above 50 F. (10 C.). The yearly rainfall averages under 20 inches (500 mm). This is the climate of Siberia, Alaska, and northern Canada.

Polar Climates

The average temperature of the warmest month is below 50 F. (10 C.). Although precipitation is slight, low temperatures allow what falls to accumulate in permanent snow and ice fields. The polar zones are the Arctic and Antarctic regions. However, polar climates are also found on high mountain peaks, even those near the Equator.