Snow begins in the atmosphere as water condenses into a tiny droplet. As more and more water vapor condenses onto its surface, the droplet grows. Cold air then freezes this water into an ice crystal.
Each ice crystal has a unique shape that depends on the surrounding air's temperature and water vapor content. If it is below freezing and there is a lot of water vapor in the air, the crystal grows six evenly spaced branches. More and more water vapor collects on these branches and freezes, making the ice crystal increasingly heavy. Eventually, the ice crystal falls from the sky, leaving the cloud of precipitation that it helped to form. As it falls, the crystal continues to grow by picking up more water vapor.
As it descends, the ice crystal can come into contact with warmer air that makes it melt somewhat. This melting acts like a glue, causing crystals to bond together into larger flakes, forming what many people think of as the "classic" fluffy snowflake. If the crystals melt too much and then refreeze as they get closer to Earth's surface, the precipitation falls as sleet instead of snow.
Once on the ground, snow will remain if temperatures are cold enough to keep it from melting. Glaciers that form on mountains, for example, are made up of snow that accumulates on the ground and eventually turns to ice.
Now here is the question: why does it snow on the mountains only? Why does it not fall in the plains? In fact, there are two factors on which the possibility of snow occurring at any place depends– the height of the place above the sea level and the distance of the place from the equator. The higher the altitude of a place, the greater is the chance of snowfall there. Similarly, the larger the distance of a place from the equator, the more are the chances of snowfall there. Though a very large amount of snow is formed in the atmosphere, only a very small part of it in the form of snow falls down on the mountains. The rest comes down as rain, because it melts and turns into water while passing through the warmer layers of the atmosphere. Snow does not melt on the mountains because of the low temperature prevailing there because of the high altitude. The snow goes on piling up and becomes harder under the increasing weight of its own layers.
Some air is trapped between the particles of snow, making the snow act as a bad conductor of heat. Snow also acts as a blanket for the earth. People make houses of snow in colder areas, as it protects against the cold weather. The roofs of the houses in hilly areas are made slanting in order to prevent the snow from collecting on them thus keeping the roofs from collapsing under the weight of the snow.