Humidity is the amount of water vapor held in a volume of air at a given time. The hotter the air is, the more humidity it can contain. That is why on many hot summer days, it is extremely humid outside. When the air is one hundred percent humid, it is totally saturated with water vapor(think of a sponge that can't hold another drop of water). If the temperature should suddenly drop, precipitation will occur because a cooler air mass would not be able to hold the same amount of humidity as the warmer air did. That is why many times after a very hot day, it rains. At night, the temperature drops and this can cause the moisture to condense and produces rain.