Dew Point vs. Wet-Bulb Temperature

Dew point and wet-bulb temperature are both "below dry-bulb" numbers that shrink the gap as air gets more humid — which is exactly why they're easy to mix up. They answer two different physical questions, and this calculator computes both from the same inputs so you can see how they diverge.

What each one actually means

Dew point is the temperature air would have to be cooled to, at constant pressure and constant moisture content, before water starts condensing out of it. It's a pure property of how much water vapor is already in the air — no evaporation or airflow involved in its definition.

Wet-bulb temperature is what a thermometer reads when a wetted wick around its bulb is exposed to moving air and allowed to cool by evaporation until it reaches equilibrium. It depends on the same humidity, but also on the fact that evaporation is actively happening at the thermometer's surface, which is a different physical process than dew point's simple "cool until condensation starts."

Worked comparison at four humidity levels

Holding dry-bulb at 25°C and sea-level pressure (101.325 kPa) fixed, here's what this calculator's own formulas produce as relative humidity drops:

RH	Dew point	Wet-bulb	Dry-bulb
100%	25.0°C	25.0°C	25.0°C
75%	20.3°C	21.7°C	25.0°C
50%	13.9°C	18.0°C	25.0°C
25%	3.6°C	13.6°C	25.0°C

Computed directly from this site's Tetens formula and psychrometer equation at Tdb=25°C, sea level.

The pattern: dew point always falls faster

Notice dew point pulls away from dry-bulb much faster than wet-bulb does as the air dries out — at 25% RH, dew point has dropped more than 21°C below dry-bulb while wet-bulb has only dropped about 11°C. That gap exists because evaporative cooling (what the wet-bulb thermometer experiences) is a self-limiting process — as the wet wick cools, it also raises the local humidity right at its own surface, which slows further evaporation. Dew point has no such limiting feedback; it's a direct, static readout of how much moisture is actually present, so it can swing much further from dry-bulb in dry air.

Why this ordering is always true

For any unsaturated air, this ordering always holds: dew point ≤ wet-bulb ≤ dry-bulb. All three collapse to the same value only at 100% RH, where air is already saturated and there's no room for either cooling-to-condensation or evaporative cooling to produce a lower number.

Which one should you use?

Use dew point when the question is about condensation risk — will moisture form on a cold surface, a window, or a duct (see the condensation risk page). Use wet-bulb when the question involves evaporative processes — cooling tower approach temperature, evaporative cooler performance (see the evaporative cooling page), or when it's the number you're directly measuring in the field with a sling psychrometer.

FAQ

Can wet-bulb ever be higher than dew point?

Yes — in fact it's always the case for unsaturated air (dew point ≤ wet-bulb). They're only equal to each other (and to dry-bulb) at 100% RH.

Which one determines how "sticky" humid air feels?

Dew point correlates much more directly with perceived mugginess — it's a direct measure of moisture content independent of temperature, which is why meteorologists report dew point, not RH, as the better comfort indicator.

Why does the gap between dew point and wet-bulb widen so much in dry climates?

Because evaporative cooling is self-limiting (it raises local humidity as it cools), while dew point has no such limit and tracks moisture content directly — in very dry air this produces a large divergence, which is exactly the effect evaporative coolers exploit (see the evaporative cooling page).

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