humidity question?

Question

Why is it there can be 100% humidity and won't rain for days? I was watching man vs. wild, and he said humidity is often 100%. So, does it involve pressure to get right until it will rain or what?

Answer 1

Fog is type of cloud on ground.When fog exists,the humidity will be almost equal to 100 percent.Still,fog hangs on the air upto a few metres from the ground ,without disappearing(ie without producing any precipitation near the ground) for hours together.Fog disappears only when the sun's radiation evaporates it or strong wind blows it away.This is because the fog contains droplets which are so tiny that they seem to be wieghtless and hence hang in the air.
This is the case of the clouds at higher levels.They also contain such tiny droplets and they float in the air in the form of cloud.Rain occurs only when such millions of droplets combine together to form bigger rain drops which fall as precipitation due to increase in the weight.Some triggering action is required to produce such rain drops and make the cloud to produce precipitation in case it does not happen in the natural way, even though the humidity may be 100 percent inside the cloud.
In fact, this is what is done(introducing some triggering action) during artificial rain-making.
Sometimes the humidity at the ground and inside the cloud may be 100 percent.But, the air in between the cloud and the ground may be dry.In that case, the rain drops may evaporate before reaching the ground and it may look as if there is no rain at that time.
So,even though the humidity is 100 percent inside the cloud,at the ground level and in the atmosphere in between, it is necessary that the tiny droplets shoud combine to become bigger rain drops to produce precipitation.In the absence of any such natural triggering action,some external triggering action shoud be introduced to make the cloud to produce the precipitation.

Answer 2

When the humidity in a particular "chunk" of air reaches 100% the water vapor in the air begins condensing. If the air is right at the ground, dew forms. The condensing water forms tiny fog drops a few inches or a few feet above the ground. Higher in the air the condensing water forms the tiny drops we see as a cloud.

Humidity is a measure of the amount of water vapor - an invisible gas. Liquid water, such as rain, is not humidity.

Eventually, many clouds will begin to produce rain, which will fall into the drier air below the cloud. That is, the air below the cloud has a humidity less than 100 percent.

Some of the water in the falling rain evaporates into the drier air below the cloud, which increases the humidity of that air. But, unless the air is already very humid, it won't increase it to 100 percent.

If the air is already very humid, the evaporation from the falling rain can make it humid enough for lower clouds or fog to begin forming.

In other words, most of the time a place that the rain is falling on does not have 100% humidity, but the clouds that the rain is coming from are at 100% relative humidity.