Snow is one of several forms of water based precipitation, the most common of which are rain, snow and hail.
Some theories of precipitation are based on the fact that all rain begins as snow, but that the snow melts into rain before it reaches the ground. However this occurrence will depend on the height of the cloud, and therefore the temperature at which the raindrops or snow crystals form.
Snow is white or translucent and can be distinguished easily from both rain and hail, the latter being a solid mass of ice formed in an entirely different way. Snow is made of ice, but unlike hail it forms as hexagonal shaped crystals within clouds. The crystals are often complex in their structure and can combine to form flakes of snow in a variety of sizes.
Ice crystals within the upper levels of clouds are not a rare phenomenon, but in order for them to be called snow they must have the hexagonal crystal shape, and they must also be large enough to fall to the ground.
In order for snow to form, three conditions are necessary: there must be moisture in the atmosphere; some means must exist to convert this moisture into precipitation; and the ambient temperature must be at or near 0 degrees C.
Moisture or water vapour enters the atmosphere in three ways: through evaporation from wet surfaces (or through sublimation from ice or snow), by transpiration from plants and trees and by perspiration from animals and humans. The atmosphere’s ability to contain moisture is determined almost entirely by its temperature. Warm air is able to hold far more moisture than cool air, and therefore when air is forced to cool by some means, it will lose its ability to hold moisture and condensation will occur, converting the moisture into water particles - clouds.
In the basic water cycle moisture enters the atmosphere through evaporation, transpiration or perspiration, then, if the air is cooled, the moisture condenses into water droplets (clouds), eventually falling back to earth to continue the cycle. So the remaining important factor to consider is what would cause the atmosphere to cool.
Warm air is relatively light and therefore tends to rise. Air which is heated by the sun will rise in convection currents and, as it rises, it will expand and cool. Air which is forced to rise as the wind blows it towards a range of hills or mountains will expand and cool in exactly the same way. It is also possible for air to be forced to rise above a front in a depression, or when two masses of air come together at ground level, in both cases warmer air will rise over cooler air.
The speed at which the air rises will determine the speed of condensation, the height of the clouds being formed and the intensity of the precipitation. Generally speaking the fastest rising air is that caused by convection or by air rising above a cold front.
So, to summarise, in order for snow to form, air containing water vapour must be caused to rise and the resulting clouds must exist at a temperature low enough for the water particles in the cloud to freeze and form the hexagonal crystals which are snow. Furthermore, in order for the snow to fall to the ground as snow, the temperature at ground level must be at or about zero degrees Celsius.