15.4: Local topography influences the weather.

Why is it so windy on the sidewalk around tall buildings? Why does it rain all the time on one side of some mountain ranges, while deserts form right on the other side? And is it actually warmer in the city than in the country? The answers reflect how the physical features of land, its topography—including features created by humans—can have dramatic but predictable effects on temperature, precipitation, and wind.

High altitudes have lower temperatures. With increasing elevation, the air pressure drops, because the weight of the atmosphere decreases as altitude increases. And when pressure is lower, the temperature drops. For each 1,000 meters above sea level, the temperature drops by about 6° C. This is why the changes in weather and vegetation that you see while climbing a mountain are similar to those you would see as you moved farther and farther away from the equator—it gets colder in both cases.

Rain shadows create deserts. Air rises to get over the top of a mountain. But because the air cools as it rises, it can’t hold much moisture. So clouds form and rain falls. As the air eventually passes over the top of the mountain, it falls and warms. And because warm air holds onto moisture, there is rarely any rain on the back side of the mountain. In fact, often the air will pull moisture from the ground, intensifying the already dry conditions, creating rain shadow deserts (FIGURE 15-8). Along the west coast of the United States, the Sierra Nevada and the Cascade mountain range are responsible for the Mojave Desert in California and the Great Sandy Desert in Oregon.

Asphalt, cement, and tops of buildings absorb heat, raising the temperature. Modern landscapes also influence the weather, creating “urban heat islands.” When energy from the sun hits concrete, pavement, or the dark roof of a building, some of it is reflected, heating the air around it, and most of the rest is absorbed by these man-made surfaces and held until night. In the darkness, the surfaces lose heat to the sky, which is colder by comparison, through radiation. In contrast, when sunlight hits trees or other plant life, the solar energy evaporates water in the leaves. The ground surface doesn’t get much hotter, and neither does the air. It’s not surprising, then, that cities tend to be 1° to 6° C warmer than surrounding rural areas. And not only is it hotter in cities, but the rising warm air also alters rainfall patterns both in and around cities.

A variety of steps are being taken to create “greener” cities that absorb and release less heat from the sun, and require less energy for air conditioning. These methods include the creation of buildings with lighter-colored rooftops, the planting of trees around buildings and along roads, and the development of rooftop gardens rich with vegetation.

Tall buildings channel wind downward. Tall buildings are responsible for the perpetual winds you feel when walking on a city sidewalk. Here’s why. Winds blow more strongly when they are higher above the earth, freed from the earth’s frictional drag (from plants, dirt, rocks, and water) that slows wind as it gets near the surface. When these strong, elevated winds suddenly encounter tall buildings, they are deflected. Some of the wind goes up and over the building, but much of it is pushed downward, reaching double or even triple its initial speed by the time it reaches street level (FIGURE 15-9).