SolriseWhat is dew point? Dew point vs humidity
Dew point is the temperature the air would need to cool to before it is fully saturated with moisture. Unlike relative humidity, it is an absolute measure of how much moisture is in the air, so it is the better guide to how sticky it will actually feel outside.
What dew point is
Every parcel of air holds some water vapor. If you cooled that air down, at some temperature it could not hold all of that moisture any more, and the extra would start to condense into droplets. That temperature is the dew point.
The key idea is that dew point does not change just because the temperature does. It only moves when the actual amount of moisture in the air changes. That makes it a steady, honest number for comparing one day with another.
Dew point vs humidity
Relative humidity is how much moisture is in the air right now, relative to the maximum it could hold at the current temperature. Because it is relative, it swings with temperature even when the actual moisture stays the same. That is why humidity often spikes overnight as things cool down, then drops in the afternoon, without the air getting any drier or wetter.
It also means the same humidity can feel completely different on different days. 90% humidity at 50°F feels very different from 90% humidity at 90°F. Dew point is the more consistent comfort signal if you are planning time outdoors.
The dew point comfort scale
| Dew point | Level | What it feels like |
|---|---|---|
| Under 55°F (under 13°C) | Dry | Dry and comfortable. |
| 55 to 64°F (13 to 18°C) | Sticky | Becoming sticky, with muggy evenings. |
| 65°F and up (18°C and up) | Oppressive | Lots of moisture: oppressive, and sweat struggles to dry. |
Source: National Weather Service dew point comfort guide.
Why dew point changes
Dew point rises when there is more actual moisture in the air. That typically happens after rain, near large bodies of water, or as warm, moist air pulls in from the south. It falls when drier air moves in, for example behind a cold front.
The gap between the temperature and the dew point, called the spread, is useful too. When the spread is small, the air is close to saturated. A spread of 4°F (2°C) or less means fog or low clouds are likely to form.
What dew point means for you
Everyday
Above about 55°F it starts to feel sticky outside, and above 65°F it feels oppressive for most people, regardless of the actual temperature. It is also the real reason hot runs or walks feel brutal: sweat cannot evaporate to cool you down when the air is already close to saturated.
Outdoors
With a dew point above 65°F, slow your pace, drink more, and plan hard efforts for early morning. When the dew point is within a few degrees of the overnight low, expect wet grass, dew-soaked gear, and possibly fog at dawn.
Flying
Pilots can estimate the base of puffy cumulus clouds: divide the temperature and dew point spread (in °F) by 4.4, then multiply by 1,000 feet. High humidity also raises the risk of carburetor icing, even on warm days.
Common questions
What is a comfortable dew point?
By the National Weather Service comfort guide, a dew point under 55°F (13°C) feels dry and comfortable. From 55 to 64°F it becomes sticky, and 65°F and up feels oppressive.
What is the difference between dew point and humidity?
Relative humidity is moisture relative to what the air could hold at the current temperature, so it swings as the temperature changes. Dew point is an absolute measure of moisture, so it is the more consistent comfort signal.
What dew point causes fog?
Fog or low clouds are likely when the temperature and dew point are within about 4°F (2°C) of each other, because the air is close to saturated.
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