Cloud Base Height Calculator
Cloud base from the temperature and dewpoint — and why the famous 125 metres per degree is a subtraction of two lapse rates.
This gives the lifting condensation level — where a parcel raised from the surface would first saturate. That is the base of convective cloud and it is not the base of every cloud: layers formed by fronts or by air riding over terrain start somewhere else entirely. Nothing is uploaded.
The famous 125 metres per degree is a subtraction
A lifted parcel cools at 9.76 °C per kilometre. Its dewpoint falls too, but far more slowly — around 1.8 °C per kilometre, because the parcel keeps its water while it expands. So the gap between them shuts at 7.96 °C per kilometre, and one over that is 125.7 metres per degree. That is the rule, derived rather than remembered.
| Spread | Cloud base | In feet | Temperature there |
|---|---|---|---|
| 1 °C | 126 m | 412 ft | 20.8 °C |
| 2 °C | 251 m | 825 ft | 19.5 °C |
| 5 °C | 628 m | 2061 ft | 15.9 °C |
| 8 °C | 1005 m | 3298 ft | 12.2 °C |
| 12 °C | 1508 m | 4947 ft | 7.3 °C |
| 20 °C | 2513 m | 8246 ft | -2.5 °C |
In imperial the same subtraction gives about 229 feet per degree Fahrenheit, which aviation rounds to "a thousand feet per four and a half degrees". A zero spread puts the base on the ground, which is not a special case in the arithmetic and is exactly what fog is. And at the base itself the two curves have met — that is the definition, and a useful check on any figure.
And it survives because one term dominates it
The dewpoint lapse rate is the soft number here — an approximation, not a constant. But the dry rate is more than five times larger, so it dominates the subtraction. Ignoring the dewpoint fall entirely would give 102 m per degree instead of 126, and the whole plausible range does not even double the answer.
| If the dewpoint fell at | Metres per degree | Base at an 8 °C spread |
|---|---|---|
| 0.0 °C/km | 102.5 m | 820 m |
| 1.0 °C/km | 114.2 m | 913 m |
| 1.8 °C/km | 125.7 m | 1005 m |
| 2.5 °C/km | 137.8 m | 1102 m |
| 4.0 °C/km | 173.7 m | 1389 m |
Which is a satisfying reason for a rule of thumb to exist. It is not that forecasters agreed on a convenient number — it is that the quantity being approximated is a small correction to a constant, so almost any reasonable assumption lands near 126. Rules of thumb survive when the thing they simplify barely varies.
How to use
- Enter the surface temperature and dewpoint.
- Leave the dewpoint lapse rate alone unless you know better.
- Read the cloud base in metres and feet.
- Check how little the assumption changes it.
Frequently asked questions
How do I calculate cloud base?
Take the temperature minus the dewpoint and multiply by about 126 metres per degree. An 8 C spread puts the base near 1000 m, which is around 3300 feet.
Where does the 125 metres per degree come from?
It is a subtraction. A lifted parcel cools at 9.76 C per kilometre while its dewpoint falls at only about 1.8, so the gap between them shuts at roughly 8 C per kilometre — and one over that is 126 metres per degree.
Why does the dewpoint fall at all?
Because the parcel expands as it rises, so the same water is spread through more volume and condenses at a slightly lower temperature. It falls far more slowly than the temperature does, which is what makes the gap close.
How much does that 1.8 figure matter?
Surprisingly little, and that is why the rule survives. The dry rate is more than five times larger and dominates the subtraction, so the whole plausible range of dewpoint lapse rates does not even double the answer.
What if the spread is zero?
The base is at ground level, which is not a special case in the arithmetic and is exactly what fog is. The air is already saturated where you are standing.
Does this work for every cloud?
No — it gives the lifting condensation level, which is the base of convective cloud. Layers formed by fronts or by air riding over terrain start somewhere else entirely and this says nothing about them.
What is the imperial version?
About 229 feet per degree Fahrenheit, which aviation rounds to a thousand feet per four and a half degrees. Same subtraction, different units.
Does this send anything anywhere?
No. Every figure is computed in your browser, and nothing is uploaded or stored.
🔒 This tool runs entirely in your browser. Nothing you enter is uploaded, logged, or stored.