Cloud base estimate

Rising air cools toward its dewpoint at roughly 2.5 °C per 1,000 ft, so the spread × 400 ft estimates where cumulus bases form.

ALT & SPD

CLOUD BASE ESTIMATE9/32SOLVED

Surface temperature

°C

Surface dewpoint

°C

RULE OF THUMB

How this is computed

Rule of thumb — (Temperature − dewpoint) × 400 ft is a rule of thumb for convective cloud bases with no FAA primary source. Not a substitute for reported ceilings — check the METAR.

Rising air cools toward its dewpoint at roughly 2.5 °C per 1,000 ft, so the spread × 400 ft estimates where cumulus bases form. An 8 °C spread puts the base near 3,200 ft AGL. This is folklore-grade guidance (no FAA primary source) — use reported ceilings for decisions.

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