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Explanation of the 500 mb Flow
The sun is the primary source of earth's weather, by causing differential heating between the tropics and polar regions. This sets up a state of motion in which the atmosphere is always trying to balance itself: the warm air moves poleward in patterns called ridges, and the cooler air moves equatorward in patterns called troughs. In the mid-latitudes (30 to 60 degrees North and South) the rotation of the earth generally causes weather systems to move eastward.
This dynamic process is best seen on the 500-millibar chart. This chart shows the circulation of the atmosphere at roughly 18,000 feet (5486 meters) and is based on soundings taken by weather balloon on a twice-daily basis. These soundings are then plotted on a map and the lines of equal pressure are connected. Ridges extend toward the pole, are usually associated with warm, dry weather, and have the general shape of an upside down "U" in the Northern Hemisphere. Troughs extend toward the equator, are usually associated with cool, wet weather, and have the general shape of a "U" in the Northern Hemisphere. The area of greatest surface instability (thunderstorms) is usually immediately ahead of (to the right of) the 500 mb trough.