... boundary (18k jpeg) of the layer ranges in height from 8 km in high latitudes, to 18 km above the equator. Its height also varies with the seasons; highest in the summer and lowest in the winter. A narrow zone called the tropopause separates the troposphere from the next highest layer called the stratosphere. Air temperature within the tropopause remains constant with increasing ... in the stratosphere is essential for life as we know it. This ozone layer prevents much of the sun's harmful ultraviolet light reaching us.) Haze appearing over large cities in summer is often due to photoch...

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