We all know that we are surrounded by air, we feel it in the form of wind. But have you ever thought why it gets cold as we go up? Or how is it that our earth never heats up even when the sun supplies it with so much energy? Or how is the temperature at night maintained even when there is no sun? Or how is fog formed? And why is it formed mostly in winters (or in areas having low temperature)?

The troposphere is the bottom-most layer of the Earth’s atmosphere (contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapor and aerosols, source-Wikipedia). It is the densest layer of the atmosphere (owing to the highest gravitational force experienced in this layer). Most types of clouds are found here. All the weather and climate activity take place in this layer. The troposphere has the highest amount of water vapor and the temperature decreases as we go up.

The troposphere extends 10 km up above the sea level. The height of the top of the troposphere varies with latitudes, lowest at the poles and highest at the equator: the poles have low temperature and the equator has high temperature (low density) and by season, lower in winter and higher in summer, again a function of density. It is approximately 20 km near the equator, and about 7 km over the poles in winter. Air is warmest at the bottom of the troposphere near ground level, because of terrestrial radiation of earth. Air gets colder as one rises through the troposphere, as the molecule density decreases and inter-molecular space increases and so less radiation is absorbed. That’s why the peaks of mountains are many-a-times snow covered throughout the year. Air pressure and the density of the air also decrease with altitude, all a function of the number of molecules.

The layer immediately above the troposphere is called the stratosphere separated from the troposphere by tropopause. The tropopause is the area between the two atmospheric layers where the temperature is constant.

The sun emits short wavelength radiations that are absorbed by the earth’s surface (note, surface not atmosphere). The earth’s surface later emits long wavelength radiations that are absorbed by the molecules of air (the Green house gases) which is responsible for maintaining the earth’s temperature. The oceanic currents and the wind movements, due to change in pressures is what is responsible for distributing the earth’s heat so that heat is distributed and no region experiences any extreme. The radiations absorbed by the gas molecules are reflected back towards the earth as well as towards the space. This heat, thus, absorbed by the earth goes back to the atmosphere. The terrestrial radiation is what maintains the earth’s temperature when there is no sun.

Now, can you guess the formation of fog. It is formed in areas where cold and warm air or ocean currents meet. It is the condensation of water molecules, around nuclei (molecules to act as bases) near to the earth’s surface. Now you can guess why it happens in winters. The temperature difference increases making condensation easier. The phenomenon, where the temperature of earth increases with height is known as temperature inversion, like what happens when terrestrial radiation heats air upwards or when hot air rises due to convection. It is the condition of instability.