Kevin Myatt

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About Kevin

Kevin Myatt grew up in Arkansas to the tune of tornado sirens and the rhythm of hailstones, aspiring to be a meteorologist before his studies and career were turned to journalism instead. Though he often chases storms, he prefers living in the cooler, more tranquil weather of the Blue Ridge. He moved to Roanoke in 1999 to take a job on the copy desk of The Roanoke Times; writing headlines and editing copy is his principal work for the newspaper today.

Each May, Kevin assists Pulaski County High School / Virginia Tech meteorology instructor Dave Carroll in leading college and high school students to the Plains to observe severe weather firsthand. The accounts of many of his storm chases can be found here on the storm chasing page of his weather blog on roanoke.com.

Kevin was an editor for "Hurricanes and the Middle Atlantic States," a book written by D.C.-area weather enthusiast Rick Schwartz and published by Blue Diamond Books that documents hurricanes striking the mid-Atlantic states since colonial times.

The Weather Journal column began in 2003 and appears on Friday's Virginia section front in The Roanoke Times. The Weather Journal blog began in 2006 and follows weather day-by-day between the larger columns.

Dew point more telling than relative humidity

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.

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