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Humidity versus Dew Point: What’s the difference?

The next couple of days are going to feature air that feels a bit more muggy! It's a classic sign that we're turning the corner out of cold, wintry weather.

However, many times we confuse humidity with a "humid" atmosphere. (I know, that makes no sense-- Blame the guy who came up with the calculation for relative humidity. It's his fault.)

That's because when we calculate a humidity percentage, we're actually taking the ratio of temperature to the dew point.

In reality, you can have a temperature of 32º, with a dew point of 32º which yields a relative humidity of 100%.

In that case, it's either snowing, or cold and foggy... We don't typically consider that a very humid atmosphere.

However, think of a July afternoon. It's 94º out and we give you the weather report saying "there's a dew point of 72º-- drink plenty of water, it's hot and humid out today!" (By the way, that's a heat index of 102º... yuck)

What's important here is that that's only a humidity of 49%!

On the surface, that may not seem like it would be all that humid... but stepping outside, you'd definitely say that it felt like it!

When it feels "humid", what you're actually feeling is a higher dew point! There's some science we can get to in arguing relative humidity, specific humidity or absolute humidity... maybe we can save that for another blog.

A helpful analogy you could use is one that involves the idea of thermometers. A temperature reading on a thermometer give you a gauge of the amount of heat present in the air, while dew point reading can be thought of as a "thermometer" to measure moisture present in the air.

There's actually an instrument that measures dew point too, it's technically not a thermometer. It's called a hygrometer.

Over the next few days, our dew point values will reach into the 60s. Anything above 60 is when you can really begin to feel it outside.

Our dew point tracker does a good job at picking up moisture building back into the region both Thursday and especially Friday. This moisture will be the fuel to fire off thunderstorms as dry air is expected to punch back into the region by Saturday.

This clash of moist and dry air is the perfect recipe for thunderstorm development-- we'll certainly have plenty of that to go around into the weekend.


Insider Blog / Weather
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Luke Victor

Luke Victor gives forecasts on ABC 17 News broadcasts and reports on weather stories on air and online, giving viewers and readers a deeper look at what causes different types of weather.


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