The temperature/dewpoint spread in a METAR is one of the most useful numbers a VFR pilot can read. A small spread warns of fog and low stratus. A larger spread, on a day when the sun is heating the ground, lets you estimate where fair-weather cumulus will form — useful when you are planning a cruising altitude, deciding whether you can stay VFR over terrain, or flying to a field that doesn't report clouds at all. This guide explains the rule, why it works, and where it fails.
The Rule
The result is height above the reporting station, not above sea level.
Why It Works
When a parcel of air near the surface is heated and starts to rise, it expands and cools. As long as it is unsaturated, it cools at the dry adiabatic lapse rate of about 3°C per 1,000 ft. Its dewpoint drops as well, but much more slowly — about 0.5°C per 1,000 ft. The temperature and dewpoint therefore converge at roughly 2.5°C per 1,000 ft. When they meet, the parcel is saturated and a cloud forms.
Divide 1,000 ft by 2.5°C and you get 400 ft for each degree Celsius of spread. In Fahrenheit the convergence rate is about 4.4°F per 1,000 ft, which gives the 227 ft per °F factor.
Worked Example
A coastal airport reports 22/14 on a sunny morning:
- Spread: 22 − 14 = 8°C
- Estimated base: 8 × 400 = 3,200 ft AGL
Now look at the remarks. If the METAR includes T02220139, the precise values are 22.2°C and 13.9°C — a spread of 8.3°C, or about 3,300 ft. Because METAR body temperatures are rounded to whole degrees, a single degree of rounding is worth about 400 ft. When the answer matters, use the T-group.
AGL Versus MSL
The estimate starts from the elevation of the station that measured the temperature. At a mountain airport this makes a big difference:
- Field elevation: 4,000 ft MSL
- Temperature 26°C, dewpoint 16°C — spread 10°C
- Base: 10 × 400 = 4,000 ft AGL, or about 8,000 ft MSL
Sectional charts, cruising altitudes, and cloud clearance planning are in MSL, so add field elevation before you compare.
Remember the cloud clearance rules. In most Class E airspace below 10,000 ft MSL, VFR flight requires 500 ft below, 1,000 ft above, and 2,000 ft horizontally from clouds. If the estimated cumulus base is 4,500 ft MSL, cruising at 4,500 is not an option — plan for 3,500 or below, or climb above the tops if conditions allow.
When the Estimate Doesn't Apply
The spread method describes clouds that form from surface air rising. It works best for fair-weather cumulus on days with daytime heating. It is much less reliable for:
- Stratus and marine layers, which form from cooling and mixing near the surface rather than from rising thermals. A coastal marine layer can sit at 800 ft with a spread of several degrees.
- Frontal and advected cloud that formed somewhere else and moved in. The local surface spread tells you nothing about a layer that was created hundreds of miles away.
- Early morning and night, when the surface is cooling and there is no convection to lift the air.
- Very small spreads. A spread of 2°C or less points to fog, mist, or low stratus rather than a predictable cumulus base. Expect visibility problems, especially as the temperature falls toward the dewpoint in the evening.
Through a sunny day, the temperature usually rises faster than the dewpoint, so the spread widens and the cumulus base climbs. A base estimated at 3,000 ft at 9 a.m. may be 5,000 ft by mid-afternoon. Recompute with each new METAR rather than relying on the first one you read.
Reported Ceilings Come First
Any reported ceiling in a METAR — the lowest BKN, OVC, or VV layer — is an observation and always takes priority over an estimate. Use the spread method to fill in gaps: fields without cloud reporting, forecasting how the day will develop, or sanity-checking a forecast. For more on reading the cloud groups themselves, see How to Read a METAR.
Using the CockpitCalc Cloud Base Calculator
- Choose °C or °F to match your source.
- Enter the temperature and dewpoint (tenths from the T-group if available).
- Calculate to see the spread and the estimated base in feet AGL. Add field elevation for MSL.
Estimate Cloud Base
Enter temperature and dewpoint to estimate the base of convective cloud in feet AGL.
Open the Calculator →