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Is there a cost to flying a constant pressure surface?

Looking at this

if the surface pressure difference of 55mb was reflected at say FL300, the aircraft (flying west) would have to climb about 1600ft just to maintain FL300.

To what extent are surface pressure variations reflected higher up?

Admittedly the crosswinds will bring a greater penalty in the above example because if you have a 200kt crosswind you will end up flying an awful lot further

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

To what extent are surface pressure variations reflected higher up?

My intuitive answer would be that the scale of the variation is linearly connected to the pressure level, i.e. your 55hPa pressure variation at the 1000hPa pressure surface (sea level) would be reduced to a 27hPa variation at the 500hPa level (FL180). Rather less then more, as the generally stronger winds in the upper atmosphere tend to even out those pressure differences. But this should of course be researched more in detail for a precise answer.

Regarding your mentioned “cost” of of having to climb 1600ft in order to maintain a constant pressure level (versus maintaining a geometric altitude): This should be more or less be compensated out – at least for turbine and turbocharged engines – by remaining inside an air mass of constant low (maybe optimal) pressure instead of flying into denser air whilst maintaining a geometrical altitude.

EDDS - Stuttgart

Depends very much on the situation. For example:

  • in a cold-core low, there is a cold airmass surrounded by warmer air; since the cold air is denser, the pressure surfaces are closer together, and so the low deepens with altitude
  • in a thermal low, the opposite is the case (the low pressure at the surface is caused by rising warm air, but the pressure surfaces are farther apart in the warm air, so in the upper troposphere you get a high)

In general, the pressure configurations might be tilted “forward” (i.e., in the direction of propagation) or backward with altitude, or they might be unrelated to each other.

Looking at the GFS for today, SLP and 200 hPa, the height difference going ridge-to-trough through a Rossby wave at 200 hPa is about 1 km (which is pretty typical). The SLP pressure difference between the Azores high and Icelandic low is about 50 hPa, so the equivalent height difference on an equal-pressure surface would be around 400 m (which is stronger than typical).

The Greenland SLP is a special case, since you have a very dense blob of air that then gets extrapolated down to sea level.

EDAZ

On a horizontal plane, rather than vertical one, Arthur Clouston in The Dangerous Skies, on record-breaking trips UK to South Africa or Australia, flew direct-to headings without correcting for wind. On 2500NM legs, he would cross one or more pressure systems and his ground track would be S-shaped, but he would arrive in the right place.

Whilst the bureaucracy of landing permits etc by post/telegram should be easier now than in the 1930s, and gps and better meteorological services make navigation easier, they wouldn’t have been restricted to a flight level or avoiding Prohibited zones.

I’m not sure how much science went into this

Last Edited by Capitaine at 11 Oct 16:19
EGHP - LFQF - KCLW, United Kingdom

The cost could be considerable if you had a midair because you weren’t sticking to the flight level allocated.

Darley Moor, Gamston (UK)

My original post assumed you are sticking to the FL. Then your actual altitude varies according to the pressure, temperature, the vertical profile of the air below… and the Q was whether this costs more than flying at a constant baro altitude.

he would cross one or more pressure systems and his ground track would be S-shaped, but he would arrive in the right place.

That makes sense; airliners do that all the time, flying a different latitude W → E versus E → W on the N Atlantic routes, for example. I’ve done the same here

Administrator
Shoreham EGKA, United Kingdom

Peter wrote:

if the surface pressure difference of 55mb was reflected at say FL300, the aircraft (flying west) would have to climb about 1600ft just to maintain FL300.

To what extent are surface pressure variations reflected higher up?

You need the synoptic chart at FL300 (not at SLP chart) to make an exact statement
Also, you may have to climb at lot to get a tiny 1mb change (1 mb is not longer 30ft)

In general, pressure variations are very large in low levels then things gets fairly standard between FL300 and FL600 (no interesting weather between 10km to 20km), then very interesting again above that…

EGSX, United Kingdom

Ibra wrote:

1 mb is not longer 30ft

It is at altitude. One hPa corresponds to about 70 ft in the standard atmosphere at a pressure altitude of 30 000 ft.

London
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