Humidity is a measure of moisture in the air, specifically that of water vapor, which is water in its gaseous state. Find out how to measure absolute humidity and how to determine the ratio of dry air to water vapor with help from a meteorologist in this free video on understanding weather.
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Today, we're talking about humidity. First of all, what is humidity? Humidity is simply a measure of moisture in the air, more specifically, it's a measure of the water vapor in the air. And water vapor is simply water in its gassiest state. There are a couple of different ways to measure humidity. Number one, you can measure the absolute humidity. You can take a measurement in a specific amount of air and then you determine how much moisture -- how much water vapor -- is in it, and then determine the ratio of dry air to water vapor. That would give you the absolute humidity. How much moisture? What percentage of moisture is that air? Now, the relative humidity's determined a little bit differently, and we'll hear relative humidity most often when you're hearing the weather report on the evening news. Relative humidity is actually a ratio figured out when you figure out the actual air temperature and also the dew point. And there's a conversion -- a chart that'll show you what the relative humidity is with the dew point relative to the temperature. So there are a couple of different ways, but simply humidity is the amount... It's a measurement of the amount of moisture in the air. Sometimes it's determined in a percent. For instance, if the air's fairly dry, you may find the humidity to be only 20 or 30 percent. If the air is very moist, 70, 80, 90 percent, you're likely to see rain -- the higher the amount goes. So if we have a 100 percent humidity measurement, that means that you're seeing rain or you're seeing fog. So it's the amount of moisture contained in the air, and the two most common ways to measurement...to measure it are absolute humidity and relative humidity.