# Dew Point Calculator

Calculate dew point temperature from air temperature and relative humidity using the Magnus formula. See comfort levels and temperature-dew point spread.

## What this calculates

The dew point is the temperature at which air becomes saturated and water vapor begins to condense. Unlike relative humidity, the dew point gives you an absolute measure of moisture in the air. A dew point of 65°F (18°C) feels sticky regardless of the actual temperature, while anything below 50°F (10°C) feels dry and pleasant.

## Inputs

- **Air Temperature** (°C) — min -45, max 60 — Current air temperature.
- **Relative Humidity** (%) — min 1, max 100 — Relative humidity as a percentage (1-100).

## Outputs

- **Dew Point** (°C) — Dew point temperature in Celsius.
- **Dew Point** (°F) — Dew point temperature in Fahrenheit.
- **Comfort Level** — formatted as text — How the dew point feels in terms of human comfort.
- **Temperature-Dew Point Spread** (°C) — Difference between air temperature and dew point.

## Details

This calculator uses the Magnus formula (Alduchov and Eskridge, 1996):

Td = (b * alpha) / (a - alpha)

Where alpha = (a * T) / (b + T) + ln(RH/100), with constants a = 17.27 and b = 237.7°C.

**Dew point comfort scale:**
- **Below 50°F (10°C):** Dry and pleasant
- **50-55°F (10-13°C):** Comfortable
- **55-60°F (13-16°C):** Slightly humid
- **60-65°F (16-18°C):** Getting sticky
- **65-70°F (18-21°C):** Uncomfortable for many
- **70-75°F (21-24°C):** Oppressive
- **Above 75°F (24°C):** Dangerous

For example, on a 90°F day at 40% relative humidity, the dew point is about 63°F, which is the edge of sticky. On the same 90°F day at 70% humidity, the dew point jumps to 79°F, which is dangerously oppressive.

Pilots use the temperature-dew point spread to predict fog and cloud bases. When the spread narrows to about 2-3°C, fog formation becomes likely. The cloud base height in feet is roughly 400 times the spread in °F.

## Frequently Asked Questions

**Q: What is the dew point?**

A: The dew point is the temperature at which air becomes fully saturated (100% relative humidity) and water vapor starts condensing into liquid. It is an absolute measure of how much moisture is in the air, unlike relative humidity which changes with temperature.

**Q: Why is dew point better than relative humidity for comfort?**

A: Relative humidity is relative to the current temperature, so 50% humidity at 90°F contains much more moisture than 50% humidity at 60°F. The dew point gives you a direct, absolute number. A 70°F dew point always feels oppressive regardless of the air temperature.

**Q: How do pilots use the dew point?**

A: Pilots monitor the temperature-dew point spread to predict visibility. When the spread drops below 3°C (5°F), fog is likely. The spread also estimates cloud base height: multiply the spread in °F by about 400 to get the approximate ceiling in feet above ground level.

**Q: Can the dew point be higher than the air temperature?**

A: No. The dew point can equal the air temperature (at 100% relative humidity) but never exceed it. If conditions change so that the dew point would theoretically exceed the air temperature, condensation (fog, dew, or rain) occurs to remove excess moisture.

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Source: https://vastcalc.com/calculators/physics/dew-point
Category: Physics
Last updated: 2026-04-08