I've seen the formula for estimating cloud base consistently presented using Fahrenheit temperature and dewpoint (for example, see PHAK 12-14; Jeppesen also teaches it this way):

$$ \mathrm{cloud\:base} \approx \frac{(\mathrm{temperature\:°F})-(\mathrm{dewpoint\:°F})}{4.4\mathrm{°F}} \times 1000' $$

However, a METAR always gives temperature and dewpoint in Celsius, and pretty much everything else in aviation uses Celsius — even lapse rate, which is the basis for this formula, is taught as "2°C per 1000 feet". It seems to me that using °F in this formula invites error, since temperature and dewpoint must first be converted to Fahrenheit before using this formula. If the constant on the bottom of the fraction were $2.4 \mathrm{°C}$ then the formula could be used directly with numbers from the METAR.

Is there a specific reason why this formula is given in °F?

  • 1
    $\begingroup$ Where are you doing your studying? I ask because in the US, I can't instantly remember any temperatures in anything other than Fahrenheit... $\endgroup$
    – jamesqf
    Commented Dec 13, 2020 at 18:34
  • $\begingroup$ I’m studying in the U.S. (Washington DC area). However, I think all AWOS systems I’ve interacted with give temps/dewpoints in Celsius. $\endgroup$ Commented Mar 2, 2021 at 22:08

1 Answer 1


Why is cloud base estimation taught in Fahrenheit?

It's not, the source you are looking at is taught in Fahrenheit.

Jeppesen being an American company, I would assume that their teaching material uses units of measurement common in America.

There are many places which have the same formula in Celsius such as the wikipedia entry on Cloud Base among others.

I suspect that pretty much everybody looking to calculate this would understand the difference between Fahrenheit and Celsius and adjust accordingly. That's not to say that errors are impossible, but errors are always possible when calculating anything.


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