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Cloud base calculator

Enter the surface temperature and dew point: the height of the cumulus base shows, in feet and meters.

Quick challenge

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Test yourself: every answer is computed by the tool, and the wrong choices are the most common traps.

How it works

Formula: base (ft) = (temperature − dew point) ÷ 4.4 °F × 1,000

In a rising current of warm air, the temperature drops faster than the dew point: according to the FAA, they converge at about 4.4 °F (2.5 °C) per 1,000 feet. Where they meet, vapor condenses: that's the cloud base. So divide the temperature–dew point spread by 4.4 °F to get thousands of feet above the ground. The FAA notes the estimate is reliable only with instability clouds (cumulus) during the warmer part of the day.

Worked examples

  • 80 °F with a dew point of 58 °F

    Base around 5,000 ft above the ground.

  • 90 °F with a dew point of 50 °F

    Base around 9,091 ft: high, dry cumulus.

  • Temperature equal to the dew point

    No spread: the air is saturated at the surface, fog is possible.

Common mistakes

  • Using the rule in the morning or in stable air.

    It's only for cumulus, when warm air rises in the afternoon.

  • Reading the result as height above sea level.

    It's height above the ground where you measured.

  • Mixing °F and °C.

    A spread in °F is divided by 4.4, in °C by 2.5: the tool picks the right one for your unit.

What the calculation leaves out

  • This is an FAA estimate for cumulus in the warmer part of the day; it doesn't forecast other clouds.
  • The 4.4 °F and 2.5 °C values are the FAA's rounded figures: the two units can differ by about 2%.

Cumulus base by temperature–dew point spread

Cumulus base by temperature–dew point spread
SpreadBase
2 °F455 ft (139 m)
5 °F1,136 ft (346 m)
10 °F2,273 ft (693 m)
15 °F3,409 ft (1,039 m)
20 °F4,545 ft (1,385 m)
25 °F5,682 ft (1,732 m)
30 °F6,818 ft (2,078 m)
40 °F9,091 ft (2,771 m)

Frequently asked questions

How do you calculate cloud base?

Divide the temperature–dew point spread by 4.4 °F (or 2.5 °C): the result is in thousands of feet.

Why only for cumulus?

The rule assumes air rising and cooling, which builds cumulus in warm, unstable weather.

What does a zero spread mean?

The air is saturated at ground level: fog or very low clouds.

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