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2.6 Air Circulation 1 NASA Winds are created primarily because of uneven solar heating of the earth, which creates giant convection cells of warm air rising and cool air falling. Warm, moist air rises from the equator and cooler, dryer air falls near the poles. NASA 2 There are three different convection cell between the equator and each pole. NASA These are due to the rotation of the earth and the combined effect of gravitational pull and the Coriolis Effect. They deflect matter to the right in the northern hemisphere and to the left in the southern Hemisphere. 3 Cloud cover and rainfall are associated with the low pressure systems of the equator and at the subpolar region at 60o N and S latitude, whereas cooler, dry air is found at 30o N and S. These areas are often associated with desert biomes and are referred to at the 4 Horse Latitudes. Major wind patterns develop in association with the convection cells at approximately every 30 degrees. newmediastudio.org Image used with permission 5 ” NASA Identification Prevailing Westerlies Doldrums Easterlies a. Trade winds b. Polar Direction West to East Little/no wind East to West Latitude 30-60o 0 0-30o above 60o 6 A major high altitude westerly wind in the northern hemisphere that helps control weather patterns is called the jet stream. NASA It often brings cooler air from northern latitudes and pushes warm air in front of it. NASA 7 Weather fronts are associated with different temperature air masses. Where warm, moist air interacts with colder air, cloud cover is likely and precipitation will occur as a result of condensation. Bands of rain showers and snow are often observed at these weather interfaces. NASA Low pressure systems can also create more rain, as seen in tropical 8 cyclones or hurricanes. Tropical cyclones form over warm water and travel with the Trade Winds from east to west. In the Atlantic Ocean, they are called hurricanes; typhoons in the Pacific and cyclones in the Indian Ocean. NASA All of these storms are formed when warm moist air over the oceans rises and creates convection cells. The warmer the ocean water below, the stronger the convection becomes. This creates a “heat engine” for the developing storm and miles of clouds begin to form and winds are generated around the eye wall in a counterclockwise 9 direction. The upward flow of warm air and downward flow of cooler air produces the rain bands. The Trade Winds provide the force to push the storm towards the west. The storm’s direction is controlled by upper atmospheric conditions such as the prevailing westerlies and the jet stream as well as other stationary high and low pressure systems. 10 NASA Atlantic hurricanes form off the west coast of Africa and move towards the eastern US. The hurricane season is from June 1November 1, but hurricanes can develop whenever the ocean temperatures are above 80 degrees F. 11 These storms usually begin as a low pressure system, then a tropical depression and eventually become a tropical storm when winds reach 39-73 mph at which time the storm is named. As the barometric pressure continues to fall, the wind intensifies. The Saffir-Simpson scale is used to measure hurricane strength. US Army 12 In this image of Hurricane Katrina, a distinct eye-wall is surrounded by rain bands extending for hundreds of miles. The cyclonic flow is clearly counterclockwise. As a result, the most damaging winds are in the northeast quadrant of the storm where it approaches shore. NASA The area or distance of ocean covered by the storm is called the fetch. In 1989, Hurricane Hugo hit South Carolina with a fetch of over 1500 miles, making it the largest hurricane on record. 13 NASA Military and NOAA aircraft fly into hurricanes and other storms to determine the intensity of the winds and make projections about the storm track. Category of the hurricane is given by a number. 14