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Understand what air pressure measures. Learn the air pressure symbols. Learn how to interpret a Low Pressure System (Cyclone). Learn how to interpret a High Pressure System.

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This is the weight or pressure the air exerts on the ground and is measured in millibars. It is important to be able to read air pressure because pressure systems are associated with certain weather patterns.  The average air pressure system measures 1013 mb (29.92 inches of mercury).  A typical strong high pressure system measures around 1030 mb (30.42 inches of mercury). A typical low pressure system measures around 1000 mb (29.54 inches of mercury. To read air pressure on a surface analysis weather map, check for isobars (iso = equal, bar = pressure) – plain, curved lines that indicate areas of equal air pressure. Isobars play a major role in determining the speed and direction of wind.  When the isobars form concentric closed (but not always round) circles, the smallest circle in the center indicates a pressure center. This can be either a high-pressure system (depicted by an "H" in English, "A" in Spanish) or a low pressure system (depicted by an "L" in English, "B" in Spanish).  Air does not flow "down" pressure gradients; it flows "around" them due to the Coriolis effect (Earth spinning).  Hence, wind direction is indicated by the isobars, counterclockwise around lows (cyclonic flow) and clockwise around highs (anticyclonic) in the northern hemisphere, thus creating wind. The closer the isobars are to one another, the stronger the winds. These storms are characterized by increased cloudiness, winds, temperatures, and chance of precipitation. They are represented on a weather map by isobars that are close together with arrows traveling clockwise (Southern Hemisphere) or counter-clockwise (Northern Hemisphere), usually with a "T" in the middle isobar, which forms a round circle (the letter can vary, however, depending on the language the weather report is presented in).  Radar imagery can show low-pressure systems. Tropical cyclones (South Pacific) are also named hurricanes around America or typhoons in coastal Asia. These conditions indicate clear, calm weather with reduced chance of precipitation. Drier air usually results in a greater range of high and low temperatures. They are represented on a weather map as isobars with an "H" in the middle isobar and arrows showing which direction the wind is flowing (clockwise in Northern Hemisphere, counterclockwise in the Southern Hemisphere). Like cyclones, they can also be shown with radar imagery.