Rain Gauge Records at Oakbend
1Oakbend is a farm town in the Central Valley of California, at about 38°N. The town and its numbers are made up, but they are based on real records from the region. Kaveh Morrison's aunt, Mrs. Beatriz Lindqvist, grows almonds there and has kept a rain gauge for 30 years. Last winter was one of the driest she has seen, so she had to pump much more water from her well. Kaveh, a sixth grader, wanted to know why the rain at Oakbend comes when it does, and why some winters bring so little.
2His science teacher shared a model of how air moves around Earth. Sunlight strikes Earth most directly near the equator, so the air there is heated the most. The warm air rises, cools and drops heavy rain. High above the ground, this air spreads out toward the poles. Around 30° north and south of the equator, much of it sinks. Sinking air warms and dries, so few clouds form there, and many of the world's large deserts lie under these belts.
3Near the ground, some of the sinking air flows back toward the equator and some flows toward the poles. Because Earth rotates, air that moves a long way across its surface curves. In the Northern Hemisphere, it curves to the right; in the Southern Hemisphere, it curves to the left. Air flowing from about 30°N toward the North Pole curves toward the east, so between about 30°N and 60°N, winds near the ground usually blow from the west. These westerlies carry storms from the Pacific Ocean onto the west coast of North America.
4The belts are not fixed. During the year they shift partway north and south, following the latitude where the Sun's noon rays are most direct. That latitude is north of the equator from late March to late September and south of it for the rest of the year. In some dry winters, a large area of sinking air and high pressure stays over the ocean west of California for weeks, and the storms carried by the westerlies pass farther north.