Calculate cloud base height from temperature and dew point or humidity, in °C or °F, as feet or meters above ground, with optional altitude above sea level.

Elevation and observed comparison
Blank omits altitude MSL. Enter 0 for sea level.

Optional. Requires station elevation; both use the same vertical datum.

Blank omits the observed comparison.

Cloud Height Formula

The following rule-of-thumb formula is used to estimate the cloud base height above ground level (AGL) from the temperature–dew point spread.

Default rule: HAGL (m) ≈ 125 × (T − Td), with temperatures in °C. The FAA alternative uses HAGL (ft) ≈ 1,000 × (T − Td) / 4.4, with temperatures in °F. These are distinct approximations. The Bolton option estimates lifting condensation temperature and converts its temperature drop using a constant dry lapse rate of 9.8 K/km.

Variables:

  • HAGL is the estimated cloud base above the station ground level, in the height unit specified by the formula.
  • T is the surface air temperature, using the temperature unit specified by the selected formula.
  • Td is the surface dew point, in the same temperature unit as T; it must not exceed T.

Subtract dew point from air temperature to obtain the spread, then apply the selected approximation. The default 125 m/°C rule and FAA 1,000 ft per 4.4°F rule give slightly different estimates. Add an entered station elevation to estimate altitude MSL. Temperature and relative humidity can instead derive a water-based dew point using the Magnus fit; both temperature and derived dew point must be within −40 to 50°C. Terrain and observed-base comparisons are optional; a sensitivity test is not an uncertainty interval. These estimates are not measured cloud ceilings or operational flight clearances.

What is a Cloud Height?

Cloud height (often called cloud base height) refers to the vertical distance from the ground at a given location to the base or bottom of a cloud (AGL). It is usually reported in feet or meters. In aviation, cloud bases and ceilings are important for flight planning and safety. If you need an altitude above mean sea level (MSL), you can add the local ground/station elevation to the AGL cloud base.

How to Calculate Cloud Height?

The following steps outline how to calculate the Cloud Height using the given formula:

  1. First, determine the air temperature (T) in °F.
  2. Next, determine the dew point temperature (Td) in °F.
  3. Subtract the dew point temperature (Td) from the temperature (T) to get the difference.
  4. For the FAA method, divide the Fahrenheit spread by 4.4 and multiply by 1,000.
  5. Finally, calculate the estimated cloud base height (H) in feet AGL.

Example Problem:

Use the following variables as an example problem to test your knowledge:

Temperature (T) = 75°F

Dew Point Temperature (Td) = 60°F. The FAA estimate is (75 − 60) / 4.4 × 1,000 ≈ 3,409.09 ft AGL (1,039.09 m). The default 125 m/°C method gives 1,041.67 m; neither result is a measured ceiling.