Why is precipitation in a mountain higher than in the plains?

Question

Mountains generally have very cold temperatures at the peak, right? Cold air has trouble holding precipitation and turns into snow. Why is there greater precipitation in the mountain than in the plains?

Answer 1

Air has mass (weight). The more air above you the more mass (weight) there is pushing down - air pressure. As you get higher there is less air above you and so less pressure. Because there's less pressure the air expands (it becomes rarified). Because the air expands the heat it contains is spread out further and it cools down - about 1 degree Celsius for every 150 metres ascended (2 degrees F per 500 feet).

So far so good?

The warmer air is the more moisture it can contain. When air reaches the point at which it can contain no more moisture it's described as being Saturated (Saturation Vapor Pressure). Excess moisture has to be offloaded - inside you'd see this as condensation on windows, mirors etc, outside you'd see it as dew or frost. Up in the air the excess vapor forms water droplets which fall as precipitation.

Mountains act as a barrier, squashing the air mass as it's forced upwards. More water vapor is being squashed into each unit of air so making it more likely to become saturated and at the same time the air is cooling so it can contain less vapor anyway.

So it's a combination of cooling and compressing that causes the precipitation on the summits and windward sides of mountains.

Once the air passes over the summits it contains less water vapor, can expand again and warm up as it descends back down to lower levels.

Answer 2

Because along the plains the temperature remains fairly constant, when the cloud is pushed up the mountain its temperature drops and this causes the precipitation to start, as rain at lower altitudes and as snow at the higher altitudes.
You will notice markedly drier climates on the Far side of a mountain range.

Answer 3

The air is thinner at higher elevations!