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Earth's atmosphere is pressing against each square inch of you with a force of 1 kilogram per square centimeter (14.7 pounds per square inch). The force on 1,000 square centimeters (a little larger than a square foot) is about a ton! 1 Why doesn't all that pressure squash me? Remember that you have air inside your body too, that air balances out the pressure outside so you stay nice and firm and not squishy. 2 Atmospheric Pressure: is the pressure the air exerts as gravity pulls it towards the centre of Earth. At ground level, the average atmospheric pressure is approximately 100 kPa. The pressure decreases at higher altitudes. Pressure varies from the average of each altitude. 3 Weather forecasters measure air pressure with a barometer. Barometers are used to measure the current air pressure at a particular location in "inches of mercury" or in "millibars" (mb). A measurement of 29.92 inches of mercury is equivalent to 1013.25 millibars. 4 Air pressure can tell us about what kind of weather to expect as well. If a high pressure system is on its way, often you can expect cooler temperatures and clear skies. 5 If a low pressure system is coming, then look for warmer weather, storms and rain. 6 In this first diagram there is no horizontal temperature or pressure gradient and therefore no wind. Atmospheric pressure decreases with altitude as depicted by the drawn isobars (1000 to 980 millibars). http://www.physicalgeography.net/fundamentals/7o.html 7 In the second the potential for solar heating is added which creates contrasting surface areas of temperature and atmospheric pressure . The area to the right receives more solar radiation and the air begins to warm from heat energy transferred from the ground. The vertical distance between the isobars becomes greater as the air rises. To the far left, less radiation is received because of the presence of cloud, and this area becomes relatively cooler than the area to the right. In the upper atmosphere, a pressure gradient begins to form because of the rising air and upward spreading of the isobars. The air then begins to flow in the upper atmosphere from high pressure to low pressure. 8 The above diagram shows the full circulation system in action. Beneath the upper atmosphere high is a thermal low pressure center created from the heating of the ground surface. Below the upper atmosphere low is a thermal high created by the relatively cooler air temperatures and enhanced by the descending air from above. Surface air temperatures are cooler here because of the obstruction of shortwave radiation absorption at the Earth's surface by the cloud. At the surface, the wind blows from the high to the low pressure. Once at the low, the wind rises up to the upper air high pressure system because of thermal buoyancy and outflow in the upper atmosphere. From the upper high, the air then travels to the upper air low, and then back down to the surface high to complete the circulation cell. The circulation cell is a closed system that redistributes air in an equitable manner. It is driven by the greater heating of the surface air in the right of the diagram. 9 The straw works because we use our mouth to reduce the pressure over the fluid inside the straw. The pressure over the rest of the liquid in the glass continues to be exerted and, because it is higher than the pressure over the liquid inside the straw, the net effect is for the liquid to be forced from the area of higher pressure to the area of lower pressure. 10 Bernoulli's Principle states that: Where the speed of a fluid is high, the pressure is low, and where the speed of a fluid is low, the pressure is high. 11 12 13 14 15 16 17 18 19 20 newspaper 21