Dew Point at 90°F, 30% humidity
The dew point at 90°F, 30% humidity, using the Magnus formula. Adjust either value below to try your own numbers.
Dew Point
54°F
Pleasant, dry-feeling air with no noticeable mugginess.
At 90°F with 30% relative humidity, the dew point is 54°F — generally described as "comfortable."
What is a Dew Point Calculator?
A dew point calculator finds the temperature at which air becomes fully saturated with water vapor and condensation begins to form — the "dew point." Given the current air temperature and relative humidity, it computes this temperature using the Magnus formula, a well-established approximation used throughout meteorology.
Unlike relative humidity, which is relative to the current temperature and can be misleading on its own, dew point is an absolute measure of how much moisture is actually in the air — meteorologists generally consider it a more reliable indicator of how muggy or comfortable the air will actually feel.
The Magnus Formula
Where T is air temperature in °C and RH is relative humidity as a percentage. This calculator converts your Fahrenheit input to Celsius internally, applies the formula, and converts the result back to Fahrenheit. The constants 17.625 and 243.04 come from a well-tested empirical fit (the Alduchov-Eskridge approximation) to the physical relationship between temperature, humidity, and water vapor saturation pressure.
Dew Point Comfort Levels
Meteorologists and building-comfort standards generally agree on rough comfort bands for dew point: below 55°F feels dry and comfortable; 55-65°F is comfortable for most people with slight noticeable humidity; 65-70°F starts to feel sticky, especially with activity; 70-75°F is humid and uncomfortable for most; and above 75°F is oppressive, muggy air that makes it hard to cool down even in the shade. Unlike relative humidity, these dew point ranges stay meaningful regardless of the actual air temperature, which is why weather forecasters increasingly report dew point alongside or instead of relative humidity.
Why Relative Humidity Alone Can Mislead
Relative humidity is relative — it measures how close the air is to saturation at the current temperature, not how much moisture is actually present. A 40°F morning at 90% relative humidity contains far less actual water vapor than an 85°F afternoon at 50% relative humidity, even though the first number sounds "more humid." Dew point sidesteps this confusion because it's tied directly to the absolute amount of moisture in the air, making it comparable across different temperatures.
Example — Your Current Inputs
At 90°F with 30% relative humidity, the dew point is 54°F — generally described as "comfortable."
Additional Example — Morning Fog Formation
On a clear evening, the air temperature is 68°F with 80% relative humidity, giving a dew point of about 61°F. Overnight radiative cooling can drop the air temperature down toward that dew point — once the actual temperature reaches 61°F, the air becomes saturated and fog or dew begins to form. This is exactly why forecasters use the gap between air temperature and dew point to predict overnight fog: the smaller the gap, the more likely fog is by morning.
About These Parameters
- Air Temperature
- The current shaded-air temperature. Dew point can never exceed air temperature — when the two are equal, the air is fully saturated (100% relative humidity).
- Relative Humidity
- The current relative humidity percentage, as reported by most weather stations and apps. Combined with air temperature, this determines the absolute moisture content that the dew point represents.
Frequently Asked Questions
Can dew point be higher than air temperature?
No — physically, dew point can never exceed the current air temperature. When dew point equals air temperature, relative humidity is exactly 100% and the air is fully saturated. If measurements ever show a higher dew point than temperature, it indicates measurement error, not a real atmospheric state.
Why do pilots care about dew point?
Aviation uses dew point (alongside temperature) to assess carburetor icing risk and fog formation. When the "spread" between air temperature and dew point is small (roughly 4°F or less) and both are dropping, fog is likely to form — a significant factor in visibility and flight planning, especially for takeoffs and landings.
What indoor dew point is considered comfortable?
OSHA and most HVAC comfort guidelines recommend indoor conditions of roughly 68-76°F with 20-60% relative humidity, which generally keeps indoor dew point in the comfortable 45-60°F range. Indoor dew points creeping above 60°F often feel muggy even with air conditioning running, and can also promote mold growth over time.
Is dew point the same everywhere at the same relative humidity?
No — because relative humidity depends on temperature, the same 60% relative humidity reading corresponds to a very different dew point (and very different actual moisture content) on a cold day versus a hot day. This is exactly why dew point is the more useful number for comparing how muggy two different days or locations actually feel.