4.8 Earth's Geography and Climate

Weather is the condition of the atmosphere at a particular time and place: today's temperature, rain, wind and humidity. Climate is the average pattern of weather in a region over a long time, usually 30 years or more. One cold week doesn't change a climate.

Climate is shaped by latitude (which controls insolation), but also by elevation, mountains, distance from the ocean and ocean currents. Higher elevations are cooler, because temperature drops as you go up in the troposphere.

When wind pushes moist air from the ocean toward a mountain range, the air is forced up the slope. As it rises, it expands and cools, and its water vapor condenses into clouds and precipitation. So the side facing the wind, the windward side, is wet and often heavily forested.

By the time the air crosses the top, it has lost much of its moisture. As it descends the other side, the leeward side, it's compressed and warms, which lets it hold more moisture, so it rarely rains. This dry area is called a rain shadow.

In Washington State, the western slopes of the Cascade Mountains get heavy rain, while eastern Washington, in the rain shadow, is much drier. Likewise, the Sierra Nevada in California leaves a rain shadow over the Great Basin of Nevada, and the Himalayas block moisture from reaching the Tibetan Plateau.

Water has a high specific heat, which means it takes a lot of energy to change its temperature. Oceans warm up and cool down slowly, while land heats and cools quickly. So coastal places have milder climates, with cooler summers and warmer winters, while places far inland have hotter summers and colder winters.

This difference also causes daily sea breezes: during the day, land heats faster, warm air rises over it, and cooler air flows in from the sea.

Surface ocean currents, driven largely by prevailing winds, move warm water from the tropics toward the poles and cold water back toward the equator. The Gulf Stream carries warm water up the U.S. East Coast and across the Atlantic. Along with westerly winds blowing off the ocean, it helps give western Europe much milder winters than places at the same latitude in eastern Canada.

Cold currents have the opposite effect. The cold California Current cools the U.S. West Coast, which is why summers in San Francisco are often cool and foggy.

Deep ocean water moves too. In the North Atlantic, cold, salty water is dense enough to sink, driving a slow, worldwide loop of deep and surface currents called thermohaline circulation, or the global ocean conveyor belt. It redistributes heat around the planet over hundreds to a thousand years.