## Standard Temperature and Pressure

Most pilots in the US are more familiar with temperature in degrees Fahrenheit than in degrees Celsius as reported in METARs, with the winds aloft forecasts, and in most aviation reports. And most pilots that I know don’t think in Celsius but need to make the conversion to Fahrenheit. If you know a couple of benchmarks, it is easy to make conversions in your head.

All pilots know that standard temperature and pressure are 59°F or 15°C and 29.92 inches of mercury. Likewise, water freezes at 32° F or 0° C and boils at 212° F or 100° C. So to start we have three numbers.

0 32 15 59 100 212

The formula for converting from Celsius to Fahrenheit is

F = 9/5 C + 32.

So if the temperature is 0° C, then 9/5 * 0 + 32 = 32° F.

If the temperature is 40° C, then 9/5 * 40 + 32 = 104° F.

This seems like too much work for me to do in the cockpit. The basic thing we need to take away from the formula is that for each 1° change in Celsius there is a 2° change in degrees Fahrenheit. If you know a few relationships you can estimate the rest.

0 32 Freezing 10 50 Crisp 15 59 Standard temperature 20 68 Chilly 25 77 Just right 30 86 Start thinking about density altitude 40 104 Darn hot 50 122 Death Valley hot

If you want to estimate the temperature between these numbers just use 2° per degree celsius to calculate the temperature in degrees Fahrenheit. Here’s the same table with estimated temperatures using 0°, 15°, or 40° as the base.

°C °F Estimate 0 32 32 10 50 52 15 59 59 20 68 69 25 77 79 30 86 89 40 104 100 50 122 120

You might want to memorize the whole table or, if you fly where there aren’t temperature variations, just learn the numbers that you need. Where I fly I just need 10° in the morning and 25° in the afternoon as my baseline and I’m good to go.

Right now it is 21° here, so knowing that 25° is 77°F I just subtract 4*2=8 to get 69°F.

This morning it was 10°C so subtract 5*2=10 from 59 to get 49°. The actual number is 50° so you are close enough.

The Boston METAR says that it is 16°, so add 2° to 59 to get 61°. The temperature to three digits is actually 15.6 which converts to 60°.

Dallas/Fort Worth is 27° which is 77 + 4 = 81°. The actual temperature is 27.2°C which converts to 81°F.

A good link explaining International Standard Atmospheres is here.