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The atmosphere is divided into four layers--troposphere, stratosphere, mesosphere and thermosphere. These vary in altitude as well as temperature. The troposphere is the layer closest to Earth's surface and extends to 12 km (7.5miles) above Earth. As one travels up through this layer, the temperature decreases by -6 degrees Celsius(-11 degrees Fahrenheit) for every kilometer. Turbulent wind occurs in the troposphere along with clouds and storms. The uneven heating of the parts of the troposphere by solar radiation causes convection currents.
The air at the equator receives more solar energy than the air at the poles. This creates winds that move heat as well as water vapor around the globe. Air rises at the equator and sinks at the poles. Due to the rotation of Earth, air is deflected on its path creating surface winds. These move from east to west (easterlies) in the tropics and polar areas. Winds in the middle latitudes moves from west to east and are called westerlies. One type of wind called trade winds from the southern and northern latitudes converge on latitudes of 10 degrees north and south which causes the ITCZ or Intertropical Convergence Zone. This area is characterized by low pressure and calm winds. In the upper troposphere, due to the convergence of wind, a descending motion in the air column leads to an increase in surface pressure and a clear sky. Likewise, divergence can cause ascending motion in the air column which leads to a decrease in surface pressure and cloudy weather. In an upper air chart, contour lines can be viewed with a trough and ridges. Winds blow faster in the ridge( high pressure) than the trough(low pressure) and as it approaches the slower moving air in the trough, the air piles up producing convergence. The slower moving air, when it approaches the ridge, spreads out or diverges. The strongest divergence occurs on the northeast side of the trough, where a surface low develops and the strongest convergence occurs on the northwest of the trough where a surface high develops. Therefore, in the upper troposphere, the wind can converge as well as diverge.
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