Several online weather forecasts include a ‘‘dew point’’. This information is used for what? By whom? I once had a brief course in weather while in the Air Force. I could have probably once calculated the dew point but I don’t remember what that information was used for.
I find the dew point to be the most useful piece of information in the forecast, especially in the summer. The dew point acts as a “soft floor” for the temperature, and tells you how humid it will be. So if tomorrow’s dew point is predicted to be around 70, for example, then it will be very warm and muggy at night, and appallingly hot and sweaty during the day. But if the dew point is going to be 45, then it’s going to be a dry day, chilly at night, but possibly warm in the daytime if the sun comes out.
Absolutely agreed. Relative humidity is a fairly meaningless piece of information, unless it’s at or near 100% (when fog can become an issue), because relative humidity varies by the air temperature.
On the other hand, understanding dewpoint can be very helpful for knowing how humid it is, especially if you’re exercising outdoors (I’m a runner). A dewpoint of around 50 or 55 degrees is noticeable humidity (particularly once I start exercising), 60 or 65 is quite humid, and 70 is tropical (and horrible for running in).
Also agreed that the dew point is the most useful part of the weather forecast, in summer in the north, and all the time in the south. It tells me if the day will be miserable or not. Relative humidity is useless info without the dew point.
Pilots pay attention to the dew point. If the difference between the temperature and the dew point is about 4º or less, conditions are favourable for fog.
When I was training, one clear Winter day the temperature and dew point were very close. But I lived in the Mojave Desert, so there wasn’t enough moisture in the air to worry about it.
Ditto the above.
Yup. I think the borderline between comfortable and uncomfortable is 60F for dewpoint. Summer in Chicago can be long and gross if the dewpoint is consistently above 60 and damned oppressive when it’s 70 or more. It’s a good predictor of whether I’ll need to run the air conditioning.
I remember when I was at band camp (hurr!) and publishing the daily newspaper, and one of the chaperons had us put the dew point in there with the temps. And we were like “??” and he explained what dew point was and how it was important for a group of people who needed to be outside from sun-up to sun-down with proper clothes and footewear, and we were like “??” and I still don’t remember what he said.
I wonder if I will remember what you guys explained here. Prolly not 
Dew point is SO much more useful than humidity! This post (linked below) is a really great explanation why. It’s detailed but easy to understand.
Like Johnny L.A. If the dew point & the temperature are close to each other, fog may form.
Once, when I was flying with my flight instructor into a small airport in the PNW, the dew point & the temperature were within 1 degree of each other. The day was calm with no breeze at all. While I was on approach to land, the runway disappeared in a blanket of fog as if someone had pulled a blanket over it. They had! My airplane’s prop had disturbed the air enough to cause the fog to form. Just that quick, I could not see down.
We had to “go around” since I was in Instrument Meteorological Conditions, (IMC) & I am not an instrument rated pilot. We flew to a nearby crop duster’s airstrip & waited for the fog to lift. Fun times!
Farmers also use the dew point information along with humidity & ambient temperature to determine if they can harvest their crops. Too moist at or near the dew point & the machinery gets clogged up with the grain. It does not “flow” well when it is too damp.
If you have an evaporative (swamp) cooler, you watch the dew point like a hawk!
We never did. We just accepted that it wouldn’t work well in August (‘Monsoon Season’).
Dew point is actually a good way of understanding all the fake stories have about weather. “It was 100 degrees and 100% humidity!” they say, but what the weather forecast may have really said was that it was 100% humidity at 6 am, and that the high, 12 hours later, would be 100 degrees. But of course by then, the humidity has fallen to maybe 60%. If you just remembered that the dew point was at 70 F, you’d have a good sense of high humidity for the day, but you would understand that it can’t possibly be 100% at 100 F.
As a Soaring pilot, it gives me a decent sense of how high cloud base would be.
The adiabatic lapse rate is about 5-1/2F / 1000’. So if the dew point is 60F and the ground temp is 90F, I can expect cloudbase to be around 6000’ AGL. Where I fly the dewpoint was often much lower leading to 16,000’MSL and higher cloud bases.
This is very approximate, because on a good soaring day, the actual lapse rate will be quite a bit higher than the book value.
WF Tomba really nails it. Good post.