Free cloud base calculator
Enter the surface temperature and dew point to estimate the cumulus cloud base in feet and metres above ground level — using the temperature–dew point spread method, updated live, as you type.
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An estimate for convective cumulus only — not an aviation weather briefing. How accurate is this?
Results are estimates. Consult a professional.
How the cloud base calculator works
Cloud base is the height of the bottom of a cloud above the ground. For fair-weather cumulus — the puffy clouds that build on a sunny afternoon — that height can be estimated from just two surface readings: the air temperature and the dew point. The calculator takes both, finds the gap between them (the "spread"), and returns the estimated cumulus cloud base in feet and metres above ground level (AGL).
The method works because of how rising air cools. A parcel of sun-warmed air rises and cools at the dry adiabatic lapse rate, about 5.4 °F per 1,000 ft, while its dew point falls much more slowly, about 1.0 °F per 1,000 ft. The two readings therefore close in on each other at roughly 4.4 °F per 1,000 ft. Where they meet, the air is saturated and cloud forms — that height is the cloud base.
Why the temperature–dew point spread predicts cloud base
Cloud forms when air cools to its dew point — the temperature at which the water vapor it carries starts to condense. At the surface, the air temperature is usually above the dew point, so the air is clear. The spread between them is a measure of how much the air would have to cool before condensation begins.
Lift does that cooling. On a sunny day, the ground warms the air just above it, that air becomes buoyant and rises, and as it rises it expands and cools. Because the temperature drops about 4.4 °F per 1,000 ft faster than the dew point does, a larger surface spread means the parcel has to climb higher before the two converge — so a dry day (big spread) gives high cloud bases, and a humid day (small spread) gives low ones.
The two inputs: surface temperature and dew point
Both numbers come from the same place — a thermometer, a weather app, or an aviation METAR report — and both must be surface readings taken at the same time.
Surface temperature
This is the ordinary (dry-bulb) air temperature near the ground. It is the higher of the two numbers and sets the starting point the rising parcel cools down from.
Dew point
This is the temperature the air would need to reach to become saturated. It can never be higher than the air temperature; when it equals the air temperature the surface air is already saturated. The dew point is the better measure of how much moisture the air actually holds, which is why it — not relative humidity — drives the cloud base estimate.
A worked example using the cloud base calculator
A glider pilot checks the conditions before launch: the surface temperature is 25 °C (77 °F) and the dew point is 15 °C (59 °F). She wants to know roughly where the cumulus bases will sit so she can plan her climbs.
Step 1 — Find the spread
Subtract the dew point from the temperature: 25 °C − 15 °C = 10 °C of spread.
Step 2 — Apply the rule of thumb
Multiply the spread by the per-degree rates: 10 × 400 = 4,000 ft and 10 × 125 = 1,250 m above ground level.
Step 3 — Read the result
Cumulus cloud bases should form around 4,000 ft (1,250 m) AGL — a comfortable working height for the day. Crucially, this is height above the ground, not above sea level: add the field elevation to get an altitude.
Cloud base by temperature–dew point spread
This table converts a temperature–dew point spread straight into an estimated cumulus cloud base. Find your spread, read across for the height in feet and metres AGL.
| Spread (°C) | Spread (°F) | Cloud base (ft AGL) | Cloud base (m AGL) |
|---|---|---|---|
| 0 | 0 | 0 | 0 |
| 2.5 | 4.5 | 1,000 | 313 |
| 5 | 9 | 2,000 | 625 |
| 10 | 18 | 4,000 | 1,250 |
| 15 | 27 | 6,000 | 1,875 |
| 20 | 36 | 8,000 | 2,500 |
Estimated convective cumulus cloud base from the spread method (≈ 400 ft or 125 m per 1 °C of spread). Heights are above ground level. A spread of 0 means saturated surface air — cloud sits at the ground as mist or fog.
How to use the cloud base calculator
- Get the surface temperature. Use the ordinary (dry-bulb) air temperature from a thermometer, weather app, or METAR, in °C or °F — toggle the unit and the sliders follow.
- Get the dew point. Most weather apps and aviation reports give it directly. Enter the current surface value; it must be at or below the air temperature.
- Read the cloud base. The hero shows the estimated cumulus base in feet AGL, with the metric equivalent beside it.
- Remember it is AGL. The result is height above the ground beneath you. Add the ground elevation to convert it to an altitude above sea level.
- Compare scenarios. Lower the dew point while holding the temperature steady to watch the base climb as the air dries.
The dew point itself can be found from temperature and humidity with the dew point calculator, and pilots can pair this with the density altitude calculator to judge aircraft performance.
Cloud base and ceiling definitions
How accurate is the cloud base calculator?
The spread method is a well-established rule of thumb, not a precise forecast. For fair-weather convective cumulus on a well-mixed afternoon it is typically good to within a few hundred to a thousand feet — close enough for soaring, ballooning, and general situational awareness. The two unit forms (the 4.4 °F divisor and the 400 ft/°C multiplier) agree to within a few percent.
Its limits are specific. It estimates the base of convective cumulus formed by surface heating only — it does not predict overcast or stratus decks, frontal cloud, or cloud forced up by terrain, none of which form from surface-based parcels. It assumes the surface readings represent the rising air and that conditions are well mixed, so it is least reliable in the early morning, near fronts, or when a layer aloft is much moister or drier than the surface. The result is height above ground; add the field elevation for an altitude.
Frequently asked questions about the free cloud base calculator
About this Cloud base calculator
This cloud base calculator runs entirely in your browser. Nothing you enter is sent anywhere — the surface temperature and dew point you type stay on your device, and the estimated cumulus base updates instantly as you adjust them.
It applies the temperature–dew point spread method used in aviation weather, and pairs with the dew point calculator and density altitude calculator. Browse the full set of weather calculators or the whole calculator library.