E6B Calc How do you calculate cloud base and freezing level?
Multiply the gap between temperature and dew point (in °C) by 400 ft to get the cloud base above ground, and add 1,000 ft to the field elevation for every 2 °C of surface temperature to get the freezing level. At 25 °C with a 15 °C dew point at a 1,000 ft field that is a base of 5,000 ft MSL and freezing at 13,500 ft, and E6B Calc's free Cloud Base & Freezing Level screen does both from three numbers.
Why 400 ft per degree
A rising parcel of air cools about 3 °C per 1,000 ft, while its dew point falls only about 0.5 °C. The gap closes by roughly 2.5 °C per 1,000 ft, so each °C of spread is 400 ft of climb before the cloud forms. That gives cumulus bases. It says nothing useful about layered cloud ahead of a front.
The freezing level is a plain lapse-rate sum: surface temperature divided by 2 gives thousands of feet above the field.
Doing it in the app
- On E6B, open Cloud Base & Freezing Level under Weather.
- Type Temperature and Dew point from the observation.
- Type Field elevation in feet.
- Read Cloud base AGL, Cloud base MSL, Freezing level and Spread.
The row labelled Method prints "400 ft per °C of spread", so you can see which rule it used. Results are rounded to 100 ft, and the unit follows your temperature setting: 77 °F with a 59 °F dew point gives the same 4,000 ft and 13,500 ft, with the spread shown as 18.0 °F.
| Temp / dew point | Field elevation | Spread | Cloud base AGL | Cloud base MSL | Freezing level |
|---|---|---|---|---|---|
| 25 / 15 °C | 1,000 ft | 10.0 °C | 4,000 ft | 5,000 ft | 13,500 ft |
| 31 / 12 °C | 5,000 ft | 19.0 °C | 7,600 ft | 12,600 ft | 20,500 ft |
| 14 / 12 °C | 500 ft | 2.0 °C | 800 ft | 1,300 ft | 7,500 ft |
| 8 / 3 °C | 0 ft | 5.0 °C | 2,000 ft | 2,000 ft | 4,000 ft |
On the third row a Small spread warning appears: a spread of 3 °C or less means fog or low stratus is likely, and 800 ft over the field is a low-cloud day whatever the formula says.
Where the numbers stop being honest
Both figures come from one surface reading. A temperature inversion or a front puts the freezing level somewhere else entirely, and 20,500 ft for a 31 °C afternoon is the model's answer, not a forecast. Treat it as a sanity check on a briefing and nothing more.
METARs report dew point in whole degrees, so a one degree rounding error moves the base by 400 ft. If the surface is below 0 °C the same formula returns a level under the field (-4 °C at a 1,000 ft field gives -1,000 ft); read that as freezing at the surface.
A fixed limitation: the 400 ft and the 2 °C per 1,000 ft values do not change. Cold, saturated air lapses nearer 1.5 °C, and there is no setting to change it.
The screen is free and works without a connection. Paste a METAR into the decoder and tap Use this report and it gives a convective cloud base too, but the decoder allows only 3 free visits before E6B Pro.
Questions and answers
How do you calculate cloud base from temperature and dew point?
How do you find the freezing level with the surface temperature?
Does the cloud base formula work for all clouds?
Can it tell me whether there is icing in flight?
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