Relative Humidity
Listen in RealAudio
Email your weather question

Okay, you probably already know that relative humidity (or RH) is a measure of water in the air, but how does it get there? And how can it be raining when the humidity isn't 100 percent.

Hi, I'm Bryan Yeaton for The Weather Notebook. Today, as a public service, we will try to clear up any confusion.

The first thing to remember about humidity is that air holds invisible moisture-tiny water molecules. When a weather system crosses a large body of water, it evaporates some of this water and carries it along. So, the moisture is already in the air, even though we don't see it.

The next important item is that warmer air can hold more water than cooler air. So a relative humidity of 80 percent in the winter, can actually be drier than 60 or 70 percent in summer. Think of it this way-that air is a glass that can only hold so much water. The glass can be empty, half full-50 percent-full-100 percent-or anything in between. When you have a warmer mass of air, it's just like using a bigger glass.

When air is forced upward, as when it rides up a mountain slope, it cools until it reaches its dew point-the temperature at which it condenses: when the water molecules bind together enough to become visible. We call this a cloud, or fog: they are the same thing.

Now, the water droplets which make up a cloud are so tiny, they need to crash into each other around a million times to make a raindrop, a process called "coalescence." So, although raindrops are falling from a cloud which is fully saturated-100 percent relative humidity-they may fall back through air which is much less saturated.

The Weather Notebook is produced at the Mount Washington Observatory, and supported by Subaru.