When you throw the ball to your friend, it will again to appear to land to the right of him. Now let’s pretend you’re standing at the North Pole. If you throw the ball in a straight line, it will appear to land to the right of your friend because he’s moving slower and has not caught up. Let’s pretend you’re standing at the Equator and you want to throw a ball to your friend in the middle of North America. Near the poles, Earth rotates at a sluggish 0.00008 kilometers (0.00005 miles) per hour. Earth is wider at the Equator, so to make a rotation in one 24-hour period, equatorial regions race nearly 1,600 kilometers (1,000 miles) per hour. Specifically, Earth rotates faster at the Equator than it does at the poles. The key to the Coriolis effect lies in Earth’s rotation. The Coriolis effect is responsible for many large-scale weather patterns. They meet extremely cold air flowing toward the equator from the poles and form the polar easterlies.The Coriolis effect describes the pattern of deflection taken by objects not firmly connected to the ground as they travel long distances around Earth.
Polar Easterlies – In both hemispheres, the westerlies start rising and cooling between 50° and 60° latitude as they approach the poles. These winds are called the westerlies and are located between 40°to 60° latitude in both hemispheres. Prevailing Westerlies – Some of the cool, sinking air continues to move toward the North and South. The rising air at the equatorial regions and the sinking air at about 30°N and 30°S form huge convection current, known as a Hadley cell for the English meteorologist who first proposed their existence to explain the trade winds. The air moving back toward the equator forms warm, steady winds, known as the trade winds. At the horse latitudes some of the sinking air travels back toward the equator. Deserts, such as the Sahara in Africa, are also common at 30°N and 30°S. These are called the horse latitudes, because when food ran out, sailors had to throw horses overboard. There are few clouds and little rainfall. Trade Winds – About 30° north and south of the equator, the warm, moist air that rose vertically cools and begins to sink. Also at the equator, warmer, moist air rises and produces a low-pressure area extending many kilometers north and south of the equator. As you learned the Northern Hemisphere has more landmass and is relatively warmer than the Southern Hemisphere. The latitude where Earth’s mean annual surface temperature is highest is located at 10°N. This area is known as the intertropical convergence zone (ITCZ), and is the area with the most active weather. This belt of air around the equator receives much of the sun’s radiant energy. The area is known as the doldrums because there are light winds. Near the equator, the trade winds converge into a broad east to west area of light winds.
This is known as the Coriolis effect, which is the apparent shift in the path of any fluid or object moving about the surface of the Earth due to the rotation of the Earth. In the southern hemisphere, winds appear to curve to the left. All winds in the Northern Hemisphere appear to curve to right as they move. Global winds do not move directly from north to south or south to north because the Earth rotates. At the poles, the cooler air sinks and moves back toward the equator. Warm air rises at the equator and moves toward the poles. In area near the equator, the sun is almost directly overhead for most of the year.
Unequal heating of the Earth’s surface also forms large global wind patterns. Also as elevation or altitude increases, air becomes less dense. For example, air with a large amount of water vapor is less dense than dry air because the water molecule has less mass than either an individual nitrogen or oxygen molecule. Air pressure therefore depends on elevation or altitude (higher up means less air above), the average temperature of the air above the particular location (hot air is lighter than cold air), and what the air's composition is.
Pressure is force per unit area, and air pressure is simply the weight (force) of the column of air above a particular location, per unit area. To understand how global winds form and drive the major ocean currents, you need to know that wind is the basically the movement of air from an area of high pressure to an area of low pressure. These global wind systems, in turn, drive the oceans’ surface currents. Large global wind systems are created by the uneven heating of the Earth’s surface.
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