Skip to main content
Calcimator

Cloud Base Estimator

Estimate cloud base altitude from temperature and dewpoint spread using Espy's equation. Quick weather assessment for VFR pilots.

Inputs

°F
°F
ft MSL

Results

Estimated Cloud Base (AGL)

4,000 ft

≈ 13 football fields

Cloud Base (MSL)

4,500 ft

≈ 15 football fields

Fog / Visibility Risk

Low

Temp/Dewpoint Spread122°F
Relative Humidity54%
Estimated Freezing Level13,000 ft MSL
How to Use This Calculator
  1. Get the current temperature and dewpoint from METAR or ATIS.
  2. Enter the field elevation for MSL conversion.
  3. The cloud base estimate tells you the approximate ceiling height.
  4. NOTE: This is an estimate — always check actual METAR ceilings.

How the result changes across these scenarios

ScenarioEstimated Cloud Base (AGL)Cloud Base (MSL)Fog / Visibility Risk
Low Overcast800 ft1,000 ftHIGH — fog or low clouds likely
Clear Dry Day10,000 ft11,000 ftLow
Fog Likely400 ft500 ftHIGH — fog or low clouds likely

What each input means

Temperature (OAT)
Surface temperature in Celsius from METAR or field observation.
Dewpoint
Surface dewpoint temperature from METAR. Closer to OAT = higher humidity = lower clouds.
Field Elevation
Airport elevation to convert AGL to MSL.

What each result means

Estimated Cloud Base (AGL)
Approximate altitude above ground where clouds begin to form.
Temp/Dewpoint Spread
Smaller spread = more moisture = lower clouds.
Estimated Freezing Level
Rough estimate of where icing conditions may begin.

How this is calculated

Worked example, using the default values

  1. Temperature-Dewpoint Spread
    Spread = OAT − Dewpoint
    25°C − 15°C = 10.0°C
  2. Estimated Cloud Base (AGL)
    Cloud Base = (Spread ÷ 2.5) × 1000
    (10.0 ÷ 2.5) × 1000 = 4,000 ft AGL
  3. Cloud Base (MSL)
    MSL = AGL + Field Elevation
    4,000 + 500 = 4,500 ft MSL
  4. Relative Humidity
    RH = (Vapor Pressure ÷ Sat. Vapor Pressure) × 100
    (17.06 ÷ 31.69) × 100 = 54%

Engine last updated . Checked against 1 independently-derived test — how we verify calculators.

Related Calculators

The questions that sit next to this one — chosen by subject, including calculators filed under a different category.

Aviation & Pilot Tools

Density Altitude Calculator

Calculate density altitude for flight performance planning.

Aviation & Pilot Tools

Standard Atmosphere (ISA) Calculator

ISA temperature, pressure, and density at any altitude.

Meteorology

Visibility Calculator

Estimate atmospheric visibility from humidity, precipitation rate, and particulate concentration. Provides aviation flight category and driving safety guidance.

Aviation & Pilot Tools

Pressure Altitude Calculator

Field elevation corrected for non-standard pressure.

More in Aviation, Marine & Transport.