Cloud Base Estimator

This estimator applies Espy's equation, a long-standing rule of thumb pilots use to convert the surface temperature-dewpoint spread into an approximate cumulus cloud base: divide the spread in Celsius by 2.5 and multiply by 1,000 to get feet above ground level, then add field elevation to convert to MSL for comparison against a METAR ceiling report. The 2.5°C-per-1,000-ft figure approximates how quickly a rising, unsaturated air parcel cools (close to the dry adiabatic lapse rate) versus how slowly the dewpoint drops with altitude, so the two converge at that rate and clouds form where the parcel's temperature meets its dewpoint. The calculator also derives an approximate relative humidity from the vapor-pressure ratio between the dewpoint and the temperature, and flags fog or low-visibility risk when the spread narrows to 4°C or less, with a stronger warning under 2°C — a small spread means the air is already close to saturation at the surface.

A separate freezing-level estimate uses the standard 2°C-per-1,000-ft environmental lapse rate (not the cloud-base lapse rate) to project where OAT would reach 0°C, useful for anticipating icing risk in clouds above that altitude. All of this is a quick mental-math-style estimate meant for VFR planning and situational awareness — it assumes a simple, well-mixed atmosphere and can be badly wrong under temperature inversions, fronts, or other non-standard conditions, so always verify against actual METAR/TAF ceiling and icing data before you fly.