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Composition of the Earth's Atmosphere


The Tropopause: At the very top of the troposphere is the tropopause which separates the troposphere from the next layer. Some scientists call the tropopause a "thermal layer" or "cold trap" because this is a point where rising water vapour cannot go higher because it changes into ice and is trapped. If there is no cold trap, Earth would lose all its water!.
             Most of what we call weather occurs in the troposphere. The uneven heating of the regions of the troposphere by the Sun causes convection currents and winds. Warm air from Earth's surface rises and cold air above it rushes in to replace it. When warm air reaches the tropopause, it cannot go higher as the air above it (in the stratosphere) is warmer and lighter preventing much air convection beyond the tropopause. The tropopause acts like an invisible barrier and is the reason why most clouds form and weather phenomena occur within the troposphere.
             STRATOSPHERE.
             This layer lies directly above the troposphere and is about 35 km deep. It extends from about to 50 km above the Earth's surface. Compared to the troposphere this part of the atmosphere is dry and less dense. The stratosphere is warmer at the top than the bottom. The lower portion has a nearly constant temperature with height but in the upper portion the temperature increases with altitude because of absorption of sunlight by ozone. This temperature increase with altitude is the opposite of the situation in the troposphere. The stratopause separates the stratosphere from the next layer.
             THE OZONE LAYER .
             The stratosphere contains a thin layer of ozone molecules (with three oxygen atoms) which forms a protective layer shielding life on Earth from the Sun's harmful ultraviolet radiation. Ninety-nine percent of "air" is located in the troposphere and stratosphere. .
             MESOSPHERE.
             Directly above the stratosphere, extending from 85 km above the Earth's surface, the mesosphere is a cold layer where the temperature generally decreases with increasing altitude.


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