Like many of you I grew up flying in a bifurcated world of air temperature. On the ground we used Fahrenheit but as soon as we lifted off air temperature considerations were in Centigrade. Or Celsius some called it.
Actually, I’m not so sure it was that clean cut of a situation when I learned to fly 45 years ago. As I remember the winds and temperature aloft forecasts were reported in degrees C. But I also remember a lot of conversion from C to F going on in ground school and when making flight plans.
The rest of the world had been on the Centigrade scale for hundreds of years. There was a concerted effort in the U.S. to move entirely to the decimal system when I was in school during the 50s and 60s. We were told the decimal system made more sense, was easier to use, and the U.S. couldn’t hold out against the world.
Highway signs began to appear in both statue miles and kilometers. The same for speed limits. Even domestic cars grew a second scale on the speedometer to show kilometers per hour. The transition from the English system of weights and measurements to the international decimal system was a certainty many people believed.
But the change didn’t happen. Even the English have largely converted to the international decimal system, but in the U.S. we haven’t. That says something about the U.S. psyche but I’m not sure what. In any case, that’s beyond our concern as pilots.
Aviation in the U.S. did finally make the transition to the international system for air temperature. We haven’t gone to meters for distance and altitude, or hectopascals for altimeter seeting, but we did change to C for air temperature. I think the reason is that using degrees C for air temp is just so simple and easy in flying that why would anyone continue to resist.
In aviation we care most about two air temperatures—freezing and ISA, the international standard atmosphere. We care about freezing because that’s the temperature were water in the air can become ice. We care about ISA because all airplane performance is measured against that standard.
On the C scale 0 is the temperature where water freezes. Ok, all of you with access to Google can save your emails. I know that the C scale was modified years ago to match with Kelvin and water freezes and ice melts at about .01 degrees C above zero. Since I don’t have a thermometer that good I’m sticking with zero as the temperature for ice.
ISA air temperature at sea level is 15 degrees C. An easy number to remember. Any air temperature above ISA robs engines and airfoils of performance, while temperature below ISA adds to potential power output and lift. We need to know if we are above or below standard when planning any takeoff to know how the airplane will perform.
Another great benefit of using degree C for flying is that the standard lapse rate—the rate at which air cools with altitude–is 2 degrees per thousand feet. On the F scale the lapse rate is 3 point something degrees which is really hard to work in your head compared to working with 2.
The standard lapse rate is essential to know because if the air temperature is above standard at your altitude your airplane loses performance while it gains if the temp is colder. You can always look up ISA for your altitude in the POH, but it’s so easy to multiply your altitude by 2, adjust for the sea level standard of 15, and know ISA for your altitude. For example, at 5,000 feet the standard air has cooled 10 degrees. Subtract 10 from the sea level of 15 and you know ISA is 5 degrees C for 5,000 feet. Look at the air temp gauge and you know if your conditions are above or below ISA.
That calculation works up to 36,000 feet where ISA stops cooling with altitude and remains at -56.5 degrees C. If you are ever able to fly above 75,000 feet the air—what little is left of it—starts to warm up with altitude.
U.S. airplane manufacturers were the first to make the switch to all-C temperatures in performance charts. When we listened to ATIS the airport temperature was still being reported in degrees F until about 15 years ago, but the charts in the book used C for larger airplanes first, and then most GA airplanes by the 1970s and 80s. It was just an easier and more logical system to use.
Finally, in the mid 90s the FAA converted to the METAR international standard for airport weather observation reporting. In the old sequence report observations of ceiling and visibility came first, and surface temperature was listed in degrees F. The METAR moved the wind into first place and changed surface temperature and dew point to degrees C. Gone was the need to convert F to C to use the charts to calculate takeoff performance.
It took some time but I made the total transformation from F to C when flying years ago. The only temperature conversion I now work is after I listen to the ATIS for arrival. I still haven’t made the complete switch to C for my personal comfort so when I hear the ATIS report of airport surface temp in C I glance at the dual scale on the air temp gauge to know if I will be hot, cold or just right when I open the airplane door after landing.