Cloud Base Calculator

Estimate cloud base altitude from air temperature, dew point, and station elevation.

Cloud base height calculation
Enter surface temperature and dew point measured at the same location.

About cloud base estimates

A cloud base often forms near the lifting condensation level, the altitude where rising air cools enough to become saturated. Surface temperature and dew point provide a practical estimate of that level. Temperature describes the air's thermal state, while dew point indicates its moisture content. A small spread means the air is already close to saturation and clouds may form at low altitude. A large spread generally places the cloud base higher. This calculator uses the common meteorological rule of thumb that cloud base is about 125 meters above the observation level for every degree Celsius between temperature and dew point. The estimated height above ground is therefore 125 × (temperature − dew point). Station altitude is added to report an approximate altitude above mean sea level. For Fahrenheit inputs, the temperature spread is converted to Celsius before applying the formula. Results can be displayed in meters or converted to feet. Consider air at 25°C with a 10°C dew point. The 15-degree spread gives an estimated base 1,875 meters above the station. If the station is at sea level, that is also approximately 1,875 meters above mean sea level. At a 500-meter-high station with temperature 18°C and dew point 16°C, the estimated base is 250 meters above the station, or 750 meters above mean sea level. When both readings are equal, saturation is present at the observation level and fog or very low cloud is possible. This quick method is useful for preliminary aviation weather awareness, outdoor planning, and interpreting observations, but it is not a substitute for an official ceiling report. Actual cloud layers depend on atmospheric stability, advection, terrain, fronts, inversions, and moisture above the surface. Temperature and dew point should be current, co-located, and measured accurately. Pilots must use approved weather products such as METARs, terminal forecasts, and direct observations. The estimate describes a likely convective cloud base, not necessarily the lowest reported ceiling or every cloud layer in a complex atmosphere.

Example Calculations

Weather observationEstimated altitudeConditions
25°C, dew point 10°C, altitude 0 m1,875 metersClear, relatively dry surface air
18°C, dew point 16°C, altitude 500 m750 metersCloudy, humid conditions
12°C, dew point 12°C, altitude 100 m100 mSaturated air and possible fog

How to use the calculator

  1. Enter the current air temperature and dew point.
  2. Enter the observation station altitude, or use zero for sea level.
  3. Choose the temperature and output units.
  4. Select Calculate cloud base and interpret the estimate with current weather reports.

Frequently asked questions

What does cloud base mean?

Cloud base is the altitude of the lowest visible part of a cloud layer. This calculator estimates where rising surface air may first reach saturation.

Why does a smaller temperature spread lower the cloud base?

A small spread means relative humidity is high and little cooling is needed to reach saturation. Rising air therefore reaches its condensation level sooner.

Is the result above ground level?

The temperature-spread calculation first gives height above the observation station. Adding station altitude makes the displayed result an approximate altitude above mean sea level.

Can this replace an aviation ceiling report?

No, it is only a surface-based approximation. Pilots should use official observations, forecasts, and applicable aviation weather services.

What happens when dew point exceeds temperature?

That combination is normally inconsistent because dew point cannot exceed the current air temperature at equilibrium. Recheck the measurements and their units before calculating.