Weather: Dew Point vs. Relative Humidity: What's The Difference?
PHILADELPHIA (CBS) -- You may have noticed that in our broadcasts here on CBS3, we constantly refer to a thing called the "dewpoint" during the summer months. Many have asked "what about the relative humidity?" and "what does 'dewpoint' mean and why did you just say it 20 times?" Here, I'll try to explain the difference and why dewpoint is what you should be looking at when you're assessing whether or not to go out on a jog on a July day.
I hear a lot of people say things like "the humidity is 90%" when referring to how the weather feels outside. The truth of this is that, generally speaking, that number is relatively meaningless. The reason why is right in the name - relative humidity. That means the humidity level RELATIVE to the temperature - or, in basic terms, how close the temperature is to the dewpoint (the actual formula for RH encompasses other variables, but for simplicity's sake we'll say that's what it means).
The dewpoint is the temperature to which you'd have to cool the air in order to reach complete saturation, or in order for dew to form. It's a measure of the moisture content in the atmosphere and it's by far the best way to describe what it FEELS like outside. In general terms, dewpoints under 60 degrees are comfortable. Between 60 and 70 is somewhat humid, but dewpoints over 70 is when it starts to feel muggy and oppressive outside. When the dewpoints reach the upper 70's, you're dealing with a steam bath.
So put a bit more plainly - the dewpoint is the temperature at which the air reaches full saturation, while relative humidity measures how close the air is to full saturation.
Here's why dewpoint is a more useful tool than relative humidity. Friday afternoon at 4pm, the relative humidity was 51%. That doesn't sound too bad, right? Well, at that time the dewpoint was 75 (!) and the air temperature was 96. This results in a heat index value of around 108 degrees. If you were just relying on the relative humidity, you might think it's not all that humid outside. But as we could tell stepping out the door, it was, in fact, incredibly humid.
Another example would be in the winter. If the air temperature were 45 degrees and the dewpoint were 40 degrees, you're dealing with 85% relative humidity. That sounds really steamy! Except nobody would walk outside on a day like that and pronounce it humid, because the dewpoint is low and the air is chilly.
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The ideal conditions would be dewpoints below 60 degrees and relative humidity values of less than 70%. But if you're wondering how it will feel outside, look at the dewpoint first and use our handy comfort scale to help you decide the sweat factor!
55 or less: AWESOME
55-60: PLEASANT
60-65: NOT BAD
65-70: HUMID
70-75: OPPRESSIVE
75+: INSUFFERABLE
Dewpoints this weekend may reach the upper 70's here, and while that's more common for a tropical rainforest than the city of Philadelphia, I guess it could always be worse. As far as I can find, the highest dewpoint on record in Philadelphia is 82 degrees, back in 1995. The highest dewpoint ever recorded in the world was 95 degrees in Saudi Arabia in 2003 - contributing to a heat index at the time of around 175 degrees.
