Winds — Reading Comprehension Lesson

Reading Passage

Winds

Wind is the horizontal movement of air from areas of high atmospheric pressure to areas of low atmospheric pressure. The greater the pressure difference between two areas, the stronger the wind. Wind forms because Earth’s surface is heated unevenly by the sun. Land heats up and cools down faster than water, and different surfaces absorb different amounts of solar energy. These temperature differences create pressure differences that set air in motion. Wind speed is measured with an anemometer and direction is described by where the wind is coming from — a north wind blows from the north toward the south.

On a local scale, temperature differences between land and water create predictable daily wind patterns. During the day, land heats up faster than nearby water, causing air over the land to rise and cool air from the water to flow in as a sea breeze. At night, the land cools faster than the water, reversing the cycle: now air over the warmer water rises and cool air from the land flows out as a land breeze. Valley breezes blow upslope during the day as sunlight heats mountain slopes; mountain breezes blow downslope at night as the slopes cool rapidly and dense cold air drains into valleys.

On a global scale, unequal heating between the equator and the poles drives large, persistent wind patterns called prevailing winds. Near the equator, intense heating causes air to rise, creating a band of low pressure and light, variable winds called the doldrums. This rising air flows toward the poles at high altitude, cools, and sinks at about 30° latitude, creating subtropical high pressure belts. From these highs, surface winds flow back toward the equator as the trade winds (deflected west by the Coriolis effect) and toward the poles as the westerlies (deflected east). The westerlies dominate the weather of the mid-latitudes, including most of the United States.

The jet streams are fast-moving rivers of air in the upper troposphere and lower stratosphere, typically found at altitudes of 6 to 9 miles. The polar jet stream, which affects weather in the mid-latitudes, can reach speeds of over 200 miles per hour. Jet streams follow the boundary between cold polar air and warmer mid-latitude air, and their position shifts with the seasons — moving south in winter and north in summer. Disturbances in the jet stream create the high and low pressure systems that bring changing weather to the mid-latitudes. Airline pilots take advantage of jet streams when flying east (gaining speed) and try to avoid them when flying west.

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