Unit 2: Atmosphere // Section 7: Climate, Weather, and Storms
Climate and weather are tightly connected, but it is important to distinguish between them. Climate refers to long-term weather trends and the range of variations that can be detected over decades in a specific region. Specific weather trends like annual snowfall may vary widely from one year to another (as a famous saying puts it, "Climate is what you expect, weather is what you get"), but forecasters can predict that these trends will fall within certain ranges over time based on long-term climate records. For example, southern Arizona has a hot, dry climate but its weather patterns include heavy rainstorms in July and August.
The global circulation patterns described in the preceding sections create predictable regional climate zones (Fig. 13). Low-pressure belts at the equator and at 50° to 60° north and south latitudes generate abundant precipitation. At latitudes around 30° north and south, dry descending air from high pressure belts—like the descending flow in a sea breeze circulation—produces arid zones that include Earth's major deserts.
Figure 13. Global circulation and climate
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Because the Coriolis effect prevents mass and heat from moving readily to polar latitudes, temperatures decline and pressures increase sharply between middle latitudes and the polar regions. This sharp pressure gradient creates powerful jet stream winds flowing from west to east at the boundary area. Jet stream winds meander and transport heat as they shift northward and southward. In the process they bring much of the weather system activity in the middle latitudes. When the mid-latitude jet stream dips down from Canada into the United States during the winter, it can carry arctic air and winter storms into the southeastern states.
To see how local climatic conditions create specific weather patterns, consider two types of storms: hurricanes and mid-latitude cyclones. Hurricanes form over tropical waters (between 8° and 20° latitude) in areas with high humidity, light winds, and warm sea surface temperatures, typically above 26.5°C (80°F). The most active area is the western Pacific, which contains a wide expanse of very warm ocean water. More hurricanes occur annually in the Pacific than in the Atlantic, which is a smaller area and therefore provides a smaller expanse of warm ocean water.
The first sign of a potential hurricane is the appearance of a tropical disturbance (cluster of thunderstorms). At the ocean's surface a feedback loop sometimes develops: falling pressure pulls in more air at the surface, which makes more warm air rise and release latent heat, which further reduces surface pressure. The Coriolis force will lead the converging winds into a counterclockwise circulation around the storm's lowest-pressure area.
Meanwhile, air pressure near the top of the storm starts to rise in response to latent heat warming. This high-pressure zone makes air diverge (flow outward) around the top of the center of the system. It then drops to the ground, forming powerful winds (Fig. 14). This upper-level area of high pressure acts like a chimney to vent the tropical system and keeps the air converging at the surface from piling up around the center. If air were to pile up at the center, surface pressure would rise inside the storm and ultimately weaken or destroy it.
Figure 14. Hurricane wind patterns
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Source: © National Aeronautics and Space Administration, Goddard Space Flight Center.
At the surface hurricanes can diminish quickly if they move over cooler water or land and lose their supplies of warm, moist tropical air, or if they move into an area where the large-scale flow aloft is not favorable for continued development or maintenance of the circulation.
Mid-latitude cyclones cause most of the stormy weather in the United States, especially during the winter season. They occur when warm tropical and cold polar air masses meet at the polar front (coincident with the jet stream). Typically, warm air is lifted over the colder air and the system starts to move into a spiral. Because mid-latitude systems create buoyancy through lifting, their strongest wind velocities are at high altitudes (Fig. 15). In contrast, hurricanes generate buoyancy from rising warm air, so their highest velocities are at the surface where pressure differences are greatest.
Figure 15. Mid-latitude cyclones along the polar front
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Source: © Dr. Michael Pidwimy, University of British Columbia Okanagan.
In many parts of the globe, atmospheric dynamics and ocean circulation patterns interact to create other distinct climate cycles that occur over longer periods than a single storm. Examples include seasonal monsoon rainstorms in Asia and the American southwest and multi-year patterns such as the El Niño Southern Oscillation (ENSO). These climate cycles are discussed in detail in Unit 3, "Oceans."