Cloud Base Calculator
Find the cloud base and freezing level from the temperature, dew point and field elevation.
We read the temperature and dew point (like 25/15 or M02/M05) and fill them in below.
In a METAR they sit together after the cloud groups: 25/15 means 25 °C with a dew point of 15 °C.
How fast the air cools as you climb. 2 °C per 1,000 ft is the usual estimate. Use the forecast or a sounding if you have one.
Height above sea level, e.g. your cruising altitude.
How your cloud base is worked out
Rising air cools until it reaches its dew point. That height is where the cloud starts.
- 1
The spread
25 °C − 15 °C
= 10 °C - 2
Cloud base above ground
10 × 410 ft
= 4,100 ft - 3
Above sea level
4,100 ft + 1,000 ft
= 5,100 ft - 4
Freezing level
25 °C ÷ 2 °C
× 1,000 ft + field
= 13,500 ft
Read more: the formulas, the pilot rules of thumb and a worked example
When the sun warms the ground, bubbles of warm air rise. As they rise they cool, and the gap between the air temperature and its dew point gets smaller. Where the gap closes, the water vapour condenses and a cumulus cloud forms. That is why fair-weather cumulus have flat bottoms all at the same height.
The formula
spread = temperature − dew point
cloud base above ground = spread (°C) × 125 m (about 410 ft)
pilot rule: spread (°C) ÷ 2.5 × 1,000 ft, or spread (°F) ÷ 4.4 × 1,000 ft
cloud base above sea level = cloud base above ground + field elevation
freezing level = surface temperature (°C) ÷ lapse rate × 1,000 ft + field elevation
Rising air cools at about 9.8 °C per km (the dry rate) while its dew point drops by only about 1.8 °C per km. So the spread closes by about 8 °C per km, which is 125 m for every degree of spread. The 2.5 °C per 1,000 ft rule (400 ft per °C) is the rounded version pilots learn. The 4.4 °F per 1,000 ft rule is the same idea in Fahrenheit. The three give answers within about 3% of each other.
Worked example
The METAR says 25/15 at an airfield 1,000 ft above sea level. The spread is 25 − 15 = 10 °C (18 °F).
- 125 m rule: 10 × 125 = 1,250 m = 4,101 ft, so the cloud base is about 4,100 ft above the ground and 5,100 ft above sea level.
- 2.5 °C rule: 10 ÷ 2.5 × 1,000 = 4,000 ft.
- 4.4 °F rule: 18 ÷ 4.4 × 1,000 = 4,091 ft.
- Temperature at the cloud base: 25 − 9.8 × 1.25 km = 12.75, about 12.8 °C, which is above freezing.
- Freezing level: 25 ÷ 2 × 1,000 = 12,500 ft above the ground, plus 1,000 ft = 13,500 ft above sea level.
- The −10 °C level is 35 ÷ 2 × 1,000 + 1,000 = 18,500 ft, and the −15 °C level is 40 ÷ 2 × 1,000 + 1,000 = 21,000 ft.
- Relative humidity is about 54%, and at 9,000 ft the air should be about 25 − 2 × 8 = 9 °C.
Lapse rates
| Lapse rate | Per 1,000 ft | Per km | When |
|---|---|---|---|
| Rule of thumb | 2 °C | 6.6 °C | Quick freezing-level estimate |
| ISA (standard atmosphere) | 1.98 °C | 6.5 °C | Average atmosphere, altimetry |
| Dry air (dry adiabatic) | 3.0 °C | 9.8 °C | Rising clear air, below cloud |
| Cloudy air (moist adiabatic) | about 1.5 °C | about 5 °C | Rising air inside cloud (varies with temperature) |
Humidity and dew point
The calculator uses the Magnus formula with the constants 17.625 and 243.04 °C (Alduchov and Eskridge, 1996):
vapour pressure = 6.1094 × e^(17.625 × dew point ÷ (243.04 + dew point)) hPa
relative humidity = vapour pressure at the dew point ÷ vapour pressure at the temperature × 100
dew point = 243.04 × γ ÷ (17.625 − γ), where γ = ln(RH ÷ 100) + 17.625 × T ÷ (243.04 + T)
Icing
As general guidance, airframe icing is most likely in visible moisture (cloud, rain or drizzle) between 0 °C and about −10 to −15 °C, where water droplets stay liquid below freezing. Freezing rain and drizzle can bring icing outside that band. The −10 °C and −15 °C levels here come from the same lapse rate, so they're estimates.
When the rule doesn't work
The rule is for cumulus made by surface heating in well-mixed air, usually from late morning to the afternoon. It doesn't predict stratus, fog, sea fog, or cloud from fronts and warm air sliding over cold air. Inversions also break the freezing level estimate. When the spread is 2–3 °C or less, fog or low cloud is likely, especially as the air cools in the evening. For real flights, use the METAR, the TAF and the forecast freezing level. You can decode them with the METAR & TAF decoder, and check take-off performance with the density altitude calculator.
For planning and education. Rules of thumb only: always check official weather reports and forecasts before flight.
Common questions
How do you calculate cloud base from temperature and dew point?
Take the spread: temperature minus dew point. Rising air closes that gap by about 2.5 °C every 1,000 ft, so divide the spread by 2.5 and multiply by 1,000 to get the cloud base in feet above the ground. A spread of 10 °C gives about 4,000 ft. The slightly more exact figure is 125 m (about 410 ft) per °C, or 4,100 ft.
What is the 4.4 °F rule for cloud base?
It is the same rule in Fahrenheit. The spread between temperature and dew point closes by about 4.4 °F for every 1,000 ft the air rises. Divide the spread in °F by 4.4 and multiply by 1,000. With 77 °F and a dew point of 59 °F the spread is 18 °F, so the cloud base is about 4,090 ft above the ground.
How do you estimate the freezing level?
Air usually cools by about 2 °C for every 1,000 ft you climb. Divide the surface temperature in °C by 2, multiply by 1,000, and add the field elevation. At 25 °C on a 1,000 ft airfield the freezing level is about 13,500 ft above sea level. Real air can differ, especially with inversions, so check the forecast freezing level too.
Does the cloud base rule work for all clouds?
No. It works for cumulus clouds made by warm air rising from the ground, usually on sunny afternoons when the lower air is well mixed. It does not predict layered stratus, fog or cloud from a front or warm air sliding over cold air. For those, use the reported cloud in the METAR and the TAF.
At what temperatures is airframe icing most likely?
As general guidance, icing is most likely in visible moisture, such as cloud or rain, at temperatures between 0 °C and about −10 to −15 °C, where supercooled water droplets are common. It can happen colder than that too. This calculator shows the heights of the 0 °C, −10 °C and −15 °C levels from your lapse rate.
How do I work out relative humidity from temperature and dew point?
Use the Magnus formula: relative humidity = 100 × e^(17.625 × dew point ÷ (243.04 + dew point)) ÷ e^(17.625 × temperature ÷ (243.04 + temperature)). At 25 °C with a dew point of 15 °C the humidity is about 54%. In Advanced mode you can also go the other way and get the dew point from the humidity.
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