Estimated convective cloud base
When the sun heats the ground, air rises and cools until its moisture condenses. That condensation level is the base of the cumulus you see on a summer afternoon. This estimate puts a rough height on it from two surface readings.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The result is a height above the surface, not above sea level, unless you enter a surface elevation as well. It marks the level where a parcel of surface air, lifted by heating, would reach saturation.
This is an estimate and nothing more. It is the oldest rule of thumb in practical meteorology, useful for sizing up an afternoon before you fly, and no substitute for a forecast or an actual report.
When this is useful
- Judging whether a planned VFR cross country will stay comfortably below cloud on a convective day.
- Glider and light aircraft flying, where the cumulus base marks the usable top of a thermal.
- Sanity checking a forecast cloud base against the temperature and dew point given in a surface report.
How it is calculated
A rising parcel of unsaturated air cools at about 3 C per 1,000 ft, while its dew point falls at only about 0.5 C per 1,000 ft. The two converge at roughly 2.5 C per 1,000 ft of the original spread, which gives about 400 ft of lift for every degree Celsius between the temperature and the dew point.
The rule is known as Espy's equation and it has been in use since the nineteenth century. The calculator applies it directly and adds your surface elevation if you supply one.
cloud base above the surface = (temperature - dew point) x 400 ft, with both in Celsius
Worked example
- Surface temperature
- 22 C
- Surface dew point
- 10 C
- Surface elevation
- 0 ft
The spread is 12 C, so cumulus would be expected to form near 4,800 ft above the surface.
Check: The same rule written in Fahrenheit is a spread divided by 4.4, multiplied by 1,000. A 12 C spread is 21.6 F, which gives 4,900 ft, agreeing with the Celsius form to within the precision the rule deserves.
Limitations
- It applies only to cumulus formed by surface heating on a well mixed afternoon. It says nothing about layer cloud, frontal cloud, orographic cloud or fog, and it will give a confident looking number for all of them.
- It assumes the temperature and dew point were measured at the surface, in representative conditions, at the time the convection is happening. Morning readings will not describe an afternoon sky.
- It ignores what happens above the condensation level entirely, so it cannot tell you whether the cumulus stays small, builds into a shower, or becomes a thunderstorm.
- Very small spreads point towards fog or low stratus rather than convective cloud, and very large spreads often mean no cumulus forms at all.
Common mistakes
- Reading the answer as a height above sea level when no elevation was entered. Without an elevation it is a height above the ground.
- Entering a dew point higher than the temperature. That is physically impossible and the calculator will reject it.
- Treating the figure as a cleared VFR cloud base. Use an actual aerodrome report or forecast for that.
Sources
- Aviation Weather Handbook (FAA-H-8083-28) , US Federal Aviation Administration
- Lifted condensation level , American Meteorological Society Glossary