The higher you go, the colder it gets

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

27 February 2005

The higher you go the colder it becomes; everybody knows that. But as you climb the closer you get to the sun – the source of all the heat on Earth. Therefore, shouldn’t we expect temperatures to increase with altitude?

First of all, it is not accurate to state that you get closer to the sun as you ascend higher in the sky. If you shot a rocket upwards at night, it would be getting further from the sun. Think about it – at any moment, some parts of the Earth face towards and others away from the sun. At night, your part of the globe is facing away thus when you go up, you are going further from the sun. This gives a new meaning to the phrase “up in heaven”

Secondly, the statement that the higher you go the cooler it becomes is also not accurate. It is only true up to about 11,000 metres above sea level. Between 11,000m and 20,000m, the temperature remains constant at about negative 60 degrees celcius.

From 20,000m to 45,000m above sea level, the temperature rises gradually and stabilises again at zero degrees.  But it then begins to decrease from 55,000m up to 80,000m where it stabilises again at minus 90 degrees celcius. From here onwards, there is a gradual increase in temperature.

To understand these changes in atmospheric temperature, we begin from the source of heat in the atmosphere – sunlight. When the sun’s rays strike the earth, they are absorbed and warm up the surface. This heat is radiated by the earth thus the air near the ground is hotter than that higher altitudes.

At altitudes above 11,000m, direct absorption of sunlight in the atmosphere causes heating thus the temperature begins to increase with height. This effect is responsible for the non-uniform variations in temperature.

Here is another way of understanding the phenomenon: If there were no atmosphere, the temperature of the earth would be about seven degrees centigrade near the equator. The air acts like a blanket to keep us warm. The higher you go the thinner the layer covering your body, so the colder you feel.

Now, it is true that the closer one gets to the sun the hotter is should get. But since the Earth is a great distance away from the sun (150 million kilometres), moving closer by, say, 100km does not have a discernible effect on the intensity of solar radiation.

Indeed, as our planet revolves around the sun, the distance varies by as much as five million km but the intensity of the radiation changes by only eight percent.