Teaching · For Teachers · Earth science

How high the
clouds start, from
two thermometers.

Read the temperature and the dew point in the schoolyard. The estimator uses the rule pilots learn to tell you where the bottoms of the puffy clouds overhead should be, and shows why it works.

Estimate a cloud baseWhat you get

The cloud base estimator

The worked example is made up: three courtyard readings across a sunny September day at a school 640 feet above sea level. Nothing you type leaves this browser.

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What you get

Teaching · For TeachersFree

What you finish with
For each observation, the estimated base of cumulus clouds above the ground and above sea level, the temperature-dew point spread and the relative humidity, with notes on fog risk and on when the rule applies. It prints as a log and downloads as a spreadsheet.
Who it is for
Earth science teachers and their students, meteorology and aviation students, and science clubs.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Fahrenheit and feet or Celsius and meters. Up to 60 observations. Prints a log; downloads a spreadsheet (.csv); export and import a file.
Access
Free. No account, no sign-in and nothing to install. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Choose units and, if you know it, the station elevation.
  2. Add each observation: when or where, the temperature and the dew point.
  3. Read the estimated cloud base and compare it with the clouds you see.
  4. Take readings through a day and watch the base rise as the ground warms.
  5. Print the log or download the spreadsheet.

Estimate a cloud base

How the estimate works

Two lapse rates closing a gap.

Rising air: temperature and dew point close in until they meet0 ft2,000 ft4,000 ft6,000 ftcloud base ≈ 5,500 ftair 84°Fdew point 60°FTemperature and dew point meet0 ft2,000 ft4,000 ft6,000 ftcloud base ≈ 5,500 ftair 84°Fdew point 60°F
A parcel of air rising from the ground cools about 5.4°F per 1,000 feet while its dew point falls about 1°F; the 24°F gap in the example closes at roughly 5,500 feet, where cloud forms.

The arithmetic

A bubble of air rising from warm ground cools at the dry adiabatic rate, about 5.4°F per 1,000 feet (9.8°C per kilometer), while its dew point falls only about 1°F per 1,000 feet. The gap between them closes by about 4.4°F every 1,000 feet, so the spread at the ground divided by 4.4 and multiplied by 1,000 gives the height where the air saturates and cloud forms. In Celsius that is about 122 meters per degree of spread.

When to trust it

The rule works for cumulus clouds built by rising air on a sunny day, and for readings taken in the open, in shade and away from walls and pavement. It does not describe the layered cloud a warm front brings, cloud blown in from elsewhere, or a morning inversion. Compare your estimate with the ceiling in a nearby airport report (METAR) to see how close it comes.

Sources

  1. Federal Aviation Administration. Aviation Weather Handbook (FAA-H-8083-28): temperature-dew point spread and the estimate of cumuliform cloud base.
  2. National Weather Service, JetStream online school (weather.gov/jetstream): dew point, lapse rates and how cumulus clouds form.
  3. Alduchov, O. A., and Eskridge, R. E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology, 35(4), 601–609.
  4. NOAA Aviation Weather Center (aviationweather.gov): METAR reports with ceilings to compare against.

The worked example is made up. The estimate is for teaching and fair-weather cumulus; it is not for flight planning. Your observations stay in this browser. Support: team@caveatacademic.com.