Is there a lot of humidity when air mass rises up a mountain side?

When an air mass rises up a mountain side the air cools as it rises. Cool air can not hold as much moisture as warm air so the humidity will increase until the dew point is reached whereupon the water will come out of the air (as mist/clouds and rain/snow).

On the other side of the mountain the air comes down again and as it has lost its water it is now very dry this side of the mountain is called a rain shadow.