Cloud Base Calculator
The 400 ft rule, derived rather than memorised.
Work out Cloud Base. The 400 ft rule, derived rather than memorised. Every rate is an input, not an assertion.
Cloud base above the surface
4,000 ft
1,219 m · spread of 10 °C
Rising unsaturated air cools at about 3 °C per 1,000 ft while its dew point falls only 0.5 °C, because the parcel expands but carries the same water. The spread closes at the difference, 2.5 °C per 1,000 ft, and cloud forms where it reaches zero. That is exactly the familiar "spread × 400 ft" rule, derived rather than memorised. Seeing where the 400 comes from is what lets you adjust it when the lapse rate is not standard. This applies to convective cloud lifted from the surface. Frontal cloud, orographic cloud and advection fog form by entirely different mechanisms, and this arithmetic says nothing whatever about them.
How the Cloud Base Calculator works
Convective cloud base from the temperature and dew point spread, with the closure rate shown so the familiar 400 ft per degree rule is visible as arithmetic rather than folklore.
Also known as: how to calculate cloud base · temperature dew point spread cloud · 400 feet per degree rule · cloud ceiling from dew point
Frequently asked questions
How do you calculate cloud base?
Divide the temperature–dew point spread by the closure rate, about 2.5 °C per 1,000 ft. A 10 °C spread gives a base near 4,000 ft above the surface.
Where does the 400 ft rule come from?
It is exactly this arithmetic: 1,000 ÷ 2.5 = 400 ft per degree of spread. Seeing where it comes from is what lets you adjust it when the lapse rate is not standard.
Why does the spread close as air rises?
Rising unsaturated air cools at about 3 °C per 1,000 ft while its dew point falls only 0.5 °C — the parcel expands but carries the same water. Cloud forms where they meet.
Does this work for all cloud?
No, only convective cloud lifted from the surface. Frontal cloud, orographic cloud and advection fog form by different mechanisms entirely.
What does a very small spread mean?
Air near saturation at the surface — fog or very low stratus rather than a convective base. A spread under 1 °C usually means fog is forming or already has.
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