Air temp at altitude

[Rockhound]

Sorry, I'm not an airline pilot but could someone who is settle an argument authoritatively, to wit:
Is the outside air temperature at, say, FL 350 over the North Pole significantly different from that at the same FL over Singapore?
Thanks in advance.
Rockhound

[Mad (Flt) Scientist]

Yes, they differ. To quote ESDU which lists some of the various 'design atmospheres' ...

The atmospheres are the International Standard Atmosphere, tropical maximum, temperate and Arctic maximum, tropical and temperate minimum, and Arctic minimum.

and

The standard atmosphere approximates to the climatic conditions at about 40°latitude North. To account for other conditions the international standard committee recognised other standard climates (e.g. tropical maximum, arctic minimum, temperate max and min.)

link (http://fire.nist.gov/bfrlpubs/fire04/PDF/f04064.pdf)to a pdf file with a chart comparing various design atmospheres.

If you compare, say, arctic minimum and tropical maximum at FL350, you'll see that the former is about -70C and the latter about -20C. A huge difference. For completeness, at FL350:

Tropical Max: -20C
Tropical min: -70C
ISA: -55C
Arctic max: -40C
Arctic min: -70C

All approximate values. It certainly CAN be cooler in the tropics than at the poles, there's plenty of overlap there. But it will, on average, be cooler at the poles.

[BOAC]

Rocky - you may find this link (http://www-das.uwyo.edu/~geerts/cwx/notes/chap01/tropo.html) better for the 'layman'?

Trying to remember back to the dim and distant days of met THEORY, above the tropopause the temperature is assumed 'constant' with altitude, whereas below it cools with it. I recall that where the tropopause is LOW, ie in the Arctic, the trop temperature is therefore relatively higher and therefore the 'constant' temp above will be relatively higher. 35000' will almost certainly be ABOVE the trop in the ARCTIC. Where the trop is high, ie the tropics, 35000' will probably still be in the troposphere, so still in the 'cooling' part, thus it will be colder at 35000'.

Para 3 of the link "This explains the paradox that tropopause temperatures are lowest where the surface temperatures are highest. " suggests I might not be too far adrift?

Over to the Met teacher for marks out of ten!:D

Edit:: never mind the met marks out of ten - I couldn't spell ARCTIC!

[None]

The Tropopause and Outside Air Temp (Static Air Temp) are used for many things, some of which are predicting turbulence, performance (fuel flow rates, airspeed) and planning for fuel temperature. The aircraft I fly has a minimum allowable fuel temperature of -40 degrees C. We are directed to avoid that by 3 degrees, meaning the lowest fuel temp I should see is -37 C. I have seen this once last year upon coasting in over Ireland. We climbed to get out of turbulence and found ourselves in -70 degree C air. Eventually we had to descend due to fuel temp.

The text below did not copy very well, but it shows info from a flight plan for today. The 13th note (dispatcher remarks) states the expected lowest fuel temperature for the route. Below that note are the winds aloft and temperatures for the planned and alternte flight levels. At 64N and 20W, the Tropopause is expected to be at 32,000 ft, and at 35,000 ft the Static Air Temperature is expected to be -50 degrees C. At 64N and 10W, the Trop is down to 28,000 ft, the 35,000 ft temp has warmed up to -42.

13 MINIMUM PREDICTED FUEL TEMPERATURE -21.3C AT ETE 0508
(64N020W - 64N010W) AS FLIGHT PLANNED

COMPUTED ENROUTE WINDS
FIX TROP CRZ FL TWO FLT LVLS LOWER TWO FLT LVLS HIGHER
SAT TAT SAT TAT SAT TAT SAT TAT SAT TAT

64N020W 32 35 32037 31 32052 33 32044 37 31031 39 29026
-50 -22 -51 -23 -51 -23 -47 -19 -45 -17

64N010W 28 35 25028 31 25023 33 25025 37 24031 39 24034
-42 -13 -43 -14 -43 -14 -41 -12 -41 -12

[Rockhound]

Many thanks to Mad (Flt) Sciemtist, BOAC and None for their responses to my question. I realize now it doesn't have a simple, straightforward answer.

M Scientist
I accept there can be a significant difference in the temperatures. However, am I right in believing that a T of -20 C at FL350 over the Equator would be exceptionally high and very rarely attained?
What would be a typical difference in T between Tropics and Arctic at FL350 on any given day?

None
I hope this isn't a stupid question but how well insulated from the outside air is the fuel in the tanks and lines? I mean, if the OAT is very low, does the fuel chill rapidly? From what you write, I guess it does.

Thanks in advance.
Rockhound

[Scallywag]

Hi Rockhound. Jet A-1 is probably the most commonly used jet aviation fuel worldwide and has a freezing temperature of -47c. Jet A (used in USA) freezes at -40c. There are other types as well as additives such as FSII (fuel system ice inhibitor if memory serves me right).

The point is that at airliner cruise levels the fuel will have to be warmed, particularly on long sectors at high latitudes. One common method is to use a heat exchanger between engine driven generators and fuel system. This has a dual benefit as the hot oil in the generators is cooled by the very cold fuel and the fuel absorbs the heat from the exchanger before being pumped back into the tanks, and so it goes on. On flights of 4 hours and longer, the fuel temp seems to reduce to a minimum of around -25c and then stabilise.

[Rockhound]

Scallywag,
Sorry to be so slow in acknowledging your response but I was hoping Mad Sci would reply to my last question to him.
Although you say fuel temp appears not to drop below -25 C, None claims to have experienced a fuel temp of -37. That's quite a difference. Any comments?
Thank you.
Rockhound

[swh]

Rockhound,

Most jet aircraft have a higher fuel flows to the engine than what the engines needs.

It uses the excess fuel flow to cool the engine, the heat is absorbed by the fuel warming the fuel which cools the engine and then recirculated back to the fuel tank.

Excess fuel is continuously circulated in this manor which prevents the fuel from getting too cold, and the engine too hot.

This is a very simplistic explanation of the process.

:ok:

[Flight Detent]

Mmmmm.....SWH.....very!

Just out of idle interest, which tank does the excess fuel return to from it's respective engine cooling tasks?

Cheers, FD

[SIGMET nil]

I just a checked a couple vertical soundings from 18.09.05 00 utc ...

at FL350

Alert (Canada N82 31) -59 C
Eureka (Canada N79 59) -56 C
Barrow (Alaska N71 18) -59 C
Tiksi (Russia N71 35) -52 C

Iwas unable to find anything more northerly.

Singapore -45 C (and has been like this the past 9 days)

edit: various stuff removed

[SIGMET nil]

I have checked a couple of my meteorology books at home.
The published mean temperatures for FL350 in a tropical airmass like over Singapore range fairly close to -45 C be it summer or winter.
For an arctic airmass I found summer temperatures of about -45 to -47 C in two publications, whereas winter temperatures where stated as -58 to -60 C.

Like Mad (Flt) Scientist said, the average temperature at the north pole is lower than over Singapore.