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Atmospheric Pressure: (remember to takes notes and observations in your ISN) High vs. Low Pressure System Explained An important characteristic of the Earth's atmosphere is its air pressure as it often determines wind and weather patterns across the globe. By definition, atmospheric or air pressure is the force per unit of area exerted on the Earth’s surface by the weight of the air above the surface. The force exerted by an air mass is created by the molecules that make it up and their size, motion, and number present in the air. These are important factors because they determine the temperature and density of the air and thus its pressure. The number of air molecules above a surface determines air pressure. As the number of molecules increases, they exert more pressure on a surface and the total atmospheric pressure increases. By contrast, if the number of molecules decreases, so too does the air pressure. Today, air pressure is often measured with a mercury or aneroid barometer. A mercury barometer measures the height of a mercury column in a vertical glass tube. What is a High Pressure System? A high pressure system is a whirling mass of cool, dry air that generally brings fair weather and light winds. When viewed from above, winds spiral out of a highpressure center in a clockwise rotation in the Northern Hemisphere. These bring sunny skies. A high pressure system is represented as a big, blue H. H What is a Low Pressure System? A low pressure system is a whirling mass of warm, moist air that generally brings stormy weather with strong winds. When viewed from above, winds spiral into a low-pressure center in a counterclockwise rotation in the Northern Hemisphere. A low pressure system is represented as a big, red L. L Visit: The Highs and Lows of Air Pressure Answer the following questions: 1) What is air pressure the measure of? 2) If you are on top of a mountain, is there more or less air pressure than at sea level? Why? 3) What causes air pressure to change from day-to-day? 4) When an area is warmed, what happens to the air and the air pressure? What kind of pressure system is formed? 5) What kind of air pressure system is known for good weather? Why does this pressure cause good weather? What direction do the winds swirl in this system? How is this system indicated on a map? 6) What tools are used to measure air pressure? What units are used to describe air pressure?