Dew Point vs. Relative Humidity....(6-20-12)
I was over at my Mom’s house thumbing through her current issue of Reader’s Digest when I came across the article “13 Things your TV Weatherman Won’t Tell You” I had to chuckle a bit when I came across #8 The dew point—not the relative humidity—is the best measure of how humid it feels outside.As you already know from reading our blogs and watching our weathercasts we emphasis the dew point over the relative humidity. For some reason, though, people are still a bit scared or confused when they hear us talk about dew point. Their eyes start to glaze over as if we are talking some scientific mumbo-jumbo. When you compare the two, however, you start to see that it is relative humidity that is the more confusing of the two.The relative humidity is just that, a ‘relative’ way to measure the moisture. It depends on temperature and will change rather dramatically during the course of the day. For example, this morning we started the day with a relative humidity of 87% and by mid afternoon we were down to 52%. The amount of moisture didn’t change that much during the day but as the temperature rose the ratio between temperature and dew point went down more than 30 percentage points.Here is an hour by hour look at the readings for Wednesday June 20th
That really shows how widely the relative humidity changes during the course of a day. That 52% during the afternoon may be the real eye opener. Typically, on hot and humid days like today you’ll hear people making comments like this: “It was very humid today. The humidity must be 100%!” Not true that 52% might give you a false sense of how humid it really is today until you walk outside. What if there was a way to measure the amount of moisture in the air like we measure how warm or cold the air temperature is? We do have a measurement and that scale is the dew point temperature. It actually works much more simply than the relative humidity and is a more absolute way to measure moisture in the air (it doesn’t depend on air temperature). As the amount of moisture increases in the air the dew point goes up and you only really have to focus on a small range of dew point temperatures to gauge whether it is humid or not. Like temperature, people react to changes in moisture a bit differently but here is a rough scale to show you the comfort level as the dew point changes.
When we talk about humid weather on the air we make it even easier by using the Humidometer which is scaled to the ranges you see above. We are peaking in the Oppressive range today with dew points in the low 70s but by Saturday will be more in the Comfortable range.So we have been telling you about the dew point for years and will keep telling you about it during those humid summer days.
Dew Point Reading
60 F or less
61 F to 65 F
66 F to 70 F
70 F or more
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