What is the Dew Point?
In a word: humidity.
In four words: humidity regardless of temperature.
How does a dewpoint "feel"?
Subjectively, here's how depoints feel:
| Dewpoint | How it Feels | Good for Evaporative Cooling? |
|---|---|---|
| 85°F and higher | Like being in a wet sauna, thick with steam. Dangerous for exertion. | Never. |
| 75-85°F | Extremely sweaty, oppressive, very uncomfortable. Dangerous when hot. Much worse than "moist". | Never. |
| 65-75°F | Sweaty, oppressive. Sweaty. Unpleasant. | Usually not. |
| 55-65°F | Sticky, uncomfortable. Muggy. | Often not, but sometimes yes. |
| 45-55°F | Comfortably dry. Pleasant. | Usually yes. |
| 35-45°F | Still comfortably dry, but lips and eyes may start to suffer. | Yes. |
| 25-35°F | Noticably dry. Requires drinking substantial water. Moisturizer and chapstick probably a good idea. | Certainly! |
| Below 25°F | Exceptionally dry. Pay close attention to your hydration. Dry lips mean you aren't drinking enough water. | Absolutely. |
How is dewpoint different from % Relative Humidity?
Relative Humidity means very different things at different temperatures. For example, 55% Relative Humidity at 65°F(18°C) is perfectly comfortable and safe. However, that "same" percentage, 55% RH, at 95°F is dangerously high, and you risk heat stroke and death at those conditions after a long enough exposure.
Suppose you have a dewpoint of 45°F(7°C). That will "feel dry" at any temperature.
Here's a table showing the same %RH at different temperatures, and what that equals for dewpoint:
| Temperature | Relative Humidity | Dewpoint |
|---|---|---|
| 55°F | 55% | 39°F |
| 65°F | 55% | 48.4°F |
| 75°F | 55% | 58°F |
| 85°F | 55% | 67°F |
| 95°F | 55% | 76°F |
| 105°F | 55% | 85°F |
| 115°F | 55% | 95°F |
Here's the same temperatures, except this time we keep the dewpoint constant at the same comfortable 48.4°F:
| Temperature | Relative Humidity | Dewpoint |
|---|---|---|
| 55°F | 78% | 48.4°F |
| 65°F | 55% | 48.4°F |
| 75°F | 39% | 48.4°F |
| 85°F | 28% | 48.4°F |
| 95°F | 21% | 48.4°F |
| 105°F | 15% | 48.4°F |
| 115°F | 11% | 48.4°F |
You can see above, the same dewpoint at different temperatures means wildly different % Relative Humidity.
The word "Relative" in the term "Relative Humidity" actually means: Relative to the Temperature! That's why it's "relative" at all.
The next time you hear someone say that it's not humid out because it's "only 39% humidity" be sure to think about the temperature. If it's 75°F outside, that 39% humidity will feel great (at a dewpoint of 48.4°F). But if it's 95°F outside, that same 39% means the dewpoint is 66°F and you will feel incredibly sticky.
Once you know that relative humidity is... relative... it's hard to think about humidity the same. Dewpoints give the win, when it comes to knowing comfort levels.
Technical Definition, Explained
The dewpoint is a way of looking at the amount of water vapor in the air. The units of dewpoints are degrees of temperature, like °F and °C.
When the dewpoint of the air is very close or just above the actual air temperature, then you get condensation and even fog. The % Relative Humidity in this case is nearly 100% as a result.
The "dew" part of the term "dewpoint" literally refers to the temperature at which dew will form on an object, if that object is at or cooler than the dewpoint of the air.
This is why the units for dewpoint, degrees F or C, are the same as the units for measuring temperature.
Bonus Points
Can dewpoint ever be higher than air temperature?
While the dewpoint is almost always lower than the actual air temperature, it sometimes can equal air temperature.
But it can't ever be higher than the air temperature, because that would mean dew only forms on surfaces warmer than the air itself... That's not how condensation works. A surface warmer than the air itself will warm the air, causing the air to dry out, in relative humidity terms.
So, short answer: no, dewpoints are always less than or equal to the air temperature.