Cloud Base Calculator

Cloud base from the temperature-dewpoint spread

For the conditions right now, type an airport identifier and choose Use current METAR. The calculator takes the temperature and dewpoint from the latest report, using the tenths in the remarks when the report has them, and the station elevation from AviationWeather.gov. It works out the estimate and shows the sky the report actually observed beside it, so you can see how the two compare.

Clouds form where air cools to its dewpoint. On a warm day, air heated at the surface rises, cools as it expands and eventually reaches saturation; the bottoms of the puffy cumulus clouds mark that level. Because the temperature and the dewpoint both fall at known rates in rising air, the gap between them at the surface tells you roughly how high the air must rise before the gap closes.

The FAA method

The Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25C, chapter 12) gives the numbers. When lifted, unsaturated air cools at 5.4 °F per 1,000 feet, and the dewpoint decreases at 1 °F per 1,000 feet, so the two converge at 4.4 °F per 1,000 feet. To estimate the base:

1. Spread: temperature minus dewpoint, in °F. With 85 °F and a 71 °F dewpoint, the spread is 14 °F.
2. Divide by the convergence rate: 14 ÷ 4.4 = 3.18.
3. Multiply by 1,000: the base is about 3,180 feet above the ground.

That is the handbook's own worked example. To see it in the calculator, choose °F and enter 85 and 71: it shows 3,182 feet, because the handbook rounds 3.18 before multiplying. The starting values, 29.4 °C and 21.7 °C, are the same readings rounded to tenths of a degree Celsius, which is why they give about 3,150 feet. Enter a field elevation to add the base above sea level as well.

In Celsius

METARs report temperatures in whole degrees Celsius, such as 18/17 for 18 °C and a 17 °C dewpoint, with M in front of a temperature below zero. The same rule in Celsius is about 2.44 °C per 1,000 feet, or roughly 400 feet per degree of spread: an 8 °C spread puts the base near 3,300 feet. Switch the unit at the top of the calculator to enter either scale; the result is the same.

An estimate, not an observation

The spread rule describes one process: surface air rising until it saturates. It works best on days when that is how the clouds form, typically in the afternoon with cumulus building over warm ground. Air also reaches saturation in other ways that the handbook lists separately: warm air moving over a cold surface, warm and cold air mixing, and air cooling against the ground at night. Low stratus, fog and the layered clouds ahead of a front form that way, and the spread at the surface says little about their bases.

So treat the number as a planning cross-check. A small spread means saturated air close to the ground, which is worth noticing whatever the clouds are doing; a large one suggests high bases if cumulus develop. The reported sky in the METAR, the forecast in the TAF and pilot reports describe what is actually there. The METAR decoder spells out every cloud layer in a report, and How to read a METAR explains the sky condition codes, from FEW to OVC and vertical visibility.

Base above sea level

Cloud heights in METARs and TAFs are above the ground at the station, the same reference as this estimate. Charts, airspace floors and altimeter readings use mean sea level, so the calculator adds the field elevation when you enter one: a 3,180-foot base over a 1,500-foot field sits at about 4,680 feet MSL.