Kevin Myatt is The Roanoke Times' weather columnist.

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I get lots of questions about the dew point -- what it is, why it matters, its relationship to humidity, etc. I've never reprinted a column before, and I don't plan to get in the habit of doing it a lot, but I figured the best way to answer some of those questions was to rerun most of a column I wrote about humidity and dew point, published on July 3, 2005.

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Relative humidity is, most simply put, how full the air is with moisture. If the humidity is 50 percent, for instance, it means that the air is holding about half the moisture it could hold. The ability of air to hold moisture changes with its temperature. The warmer it is, the more widely spaced air molecules become, which means there is more room to squeeze water molecules between them.

So the same amount of moisture can produce radically different humidity readings depending on temperature.

The same amount of moisture that would cause 100 percent humidity at 60 degrees would produce 71 percent humidity at 70 degrees, 51 percent at 80 degrees, and 37 percent at 90 degrees.

That is why meteorologists tend to prefer the dew point as a better measure of actual moisture in the air.

The dew point is the temperature at which the air would be completely saturated with moisture, or have 100 percent humidity.

In the above example, the dew point would be 60. At 60 degrees, the air is saturated -- most likely, there is dense fog in that circumstance.

But if the amount of moisture doesn't change as the sun comes out and warms the day up to a high of 90, the relative humidity falls all the way to 37 percent by afternoon. It would be easy to presume that the atmosphere got a lot drier during the course of the day, with the humidity falling from 100 percent to 37 percent.

But actually, with the dew point staying at 60, the amount of moisture in the air did not change. Instead, the atmosphere's ability to hold moisture increased with heating, so the same amount of moisture fills a much smaller percentage of space available at 90 than it did at 60.

Think of it this way:

You have enough boxes to fill a closet. If you put those same boxes in the living room, the boxes hold as much stuff as they did in the closet, but they fill a smaller percentage of space in the living room than they did in the closet. The boxes fill 100 percent of the space in the closet, but perhaps only 37 percent of the space in the living room.

In the summer, we often talk of humid afternoons. You might be surprised that what would be considered an extremely sultry afternoon in Roanoke (temperature of 95, dew point of 72) would have a relative humidity of less than 50 percent (47 percent, in this case). It's the dew point number, not the relative humidity percentage, that reflects the humidity.

Weather Journal appears on Monday, Wednesday and Friday.