The temperature at sea level, under standard conditions, is taken to be 15 degrees C. As we go up or our altitude increases, temperature falls. The rate of decrease of temperature is 6.5 degrees C for each 1 km altitude change. This can also be written as 3.6 degree F for every 1000 ft increase in altitude. That is why we find that mountaintops are always colder compared to plains and that some mountains are always covered with snow. As we keep on going higher and higher, temperature falls till we reach the top of the troposphere (except for the region known as tropopause), where the temperature is about -57 degrees C. Beyond that, temperature increases in the stratosphere, only to fall again in mesosphere. And at the top of our atmosphere, temperature increases with altitude in the thermosphere.
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