Small Met Q
 

I think this has been posted before, done the search but to no avail, would appreciate suggested answers. Thank you in advance for your assistance. (I do not have any answers).

Q: An aircraft is en-route at FL180 in the northern hemisphere; wind is from the left, what happens to the oat when heading 360deg?

Increase
 

You are flying at FL 180 from high to low pressure (wind from left) so your true altitude will decrease. So the temperature should increase, I think!

OAT will decrease.

The wind that high is determined more by the temperature gradient than the surface pressure gradient - this is the thermal wind component. You are moving, as fullrich says, from high to low pressure, but aloft low pressure is largely caused by a column of air with low temperature. Constant pressure altitude so fullrich is wrong there; altitude is not affecting temperature, as this is related to pressure not true altitude.

That, at least, is my reading of it. Any Met instructors about?

Send Clowns
Nav & Flt Plng,
BCFT

Bear in mind also that in June's exam there was a very similar question but with a much higher FL at high latitudes. Thus you were in or above the tropopause and the temperature remained constant.

I'm with fullrich.

Buys Ballots law - if you stand with your back to the wind in the Northern Hemisphere, the low pressure area will be on your left. so if flying 360 with wind from left, you are heading towards the low and the rule High to Low, look out below applies. As you are at a flight level, so you assume ISA conditions so temp increases as you decend. (I think)

UPPER WIND RULE
With you back to the wind the cold air is to your left in the northerrn hemisphere.

(This law stems from Sent Clowns comment and he explains why we talk about temperature when it comes to upper winds)

So, draw an plan view aircraft heading up the page i.e. 360°, stick a wind from the left going across the aircraft to the right.

Now, turn the page so that the wind you have drawn is passing through your back and out your chest and point outwards infront of you. On the your left hand side is where the cold air is, which by the way, if you look at your aircraft, is exaclty where it is headed.
Therefore the cold air is infront.

JAA likes these questions and they are very simple when the penny drops. If you can't get to grips with this, dont write the paper, see your instruvtor and sort it until you can do these blind folded. This concept is tested in many ways, especially altimetry.

The same applies to jet streams, since they are just quick uppper winds

Good luck

Steve Francis

If all else fails, buy the Met CD I made, and if Pprune is worried I'm advertising, I'm unemployed right now and I don't get a penny from it's sales, students praise it enough anyway.

Fullrich, you have some very muddled ideas here, if you need it sorting gimme a holler

So what's the answer to the question Pugzi??

What happens to OAT?

Grobhead - Buys Ballot's Law only applies at the surface. As Steve says, at altitude it would apply to temperature rather than pressure system. His example of the jetstream is very instructive. Imagine the wind is a low-level jet. If coming from the left, the aircraft is flying towards the cold sector, so the temperature is reducing. From what my students have said, his CD is good.

Also, as explained at constant flight level there is no particular reason for a change in temperature due to change in altitude.

Steve - glad I got it right! Over 4 years since I sat Met.

Am genuinely interested in all the answers above and just trying to apply my ATPL knowledge to the question. Please keep your corrections to my flawed interuption coming.

It is my understanding that Buys Ballot law is a general principle and is independant of height - it applies as a general principle to both upper and lower level winds. Buys Ballot is credited with the law as he was the first to derived it empirically, the law having already been deduced theoretically by the U.S. meteorologist William Ferrel.

The way the question is asked in itself gives certain things away. The tell you what direction the wing is coming from which means that they expect you to know Buys Ballot and indeed if you apply that law the low is indeed ahead of you. They give you a flight level and not a height of FL180. Flight level by definition is a pressure altitude and is the height above the 1013.2 level assuming ISA conditions. So why is the answer not that OAT is increasing as you are actually decending through the athmosphere?.

Grobb,

The OAT decreases.

Not sure where, but there is something serious wrong about something with your knowledge. I'm not being rude, please don't think that.

Grobb, to solve the problem of your thinking I'd have to draw a picture for you, it's almost impossible with words. There is no part of any of the question that implies or states you are descending, this is what I'm puzzled about with your thinking, where are you getting this from may I ask. If i know that then I might be able to help

Steve

Hi Pugzy,
thanks for the reply - and I dont think you are being rude. As I said earlier, I am genuinely interested in the question and not trying to wind anyone up.

We all agree that with the wind from the left in the NH, the low pressure area is in front of us (whichever law you use!) and the high must be behind.

The reason I am suggesting that the OAT could be increasing is as follows. We are at FL180 so we are banging along with 1013 in the kolemans window of the altimeter. If heading from a high pressure towards a low, and not correcting the altimeter setting to local QNH, then the altimeter is lying to us and the nearer we get to the low the more it is lying, ie we are actually getting progresively lower in the athmosphere => decending and if decending the OAT is increasing!!

1. Buys Ballots rule applies at any height, but refers to the pressure distribution at that height. Thus at low level it tells you about the MSL pressure and the resulting low level winds.

2. As you go up the pressure is increasingly affected by the temperature in the air masses, so Buys Ballot now tells you about the the upper winds, a combined effect of MSL pressure and horizontal temp gradient.

3. You get a better picture of the upper air if you plot and use a contour chart. These show the height AMSL of a specific pressure level, like the 500mb or 300mb surfaces. Because in calculating the contour height you have factored in the MSL pressure the resulting contours show the upper wind - the combined MSL pressure and thermal effect - and you read them just like a constant height pressure chart.

4. Thus, using Buys Ballot, wind from the left in the N hemi says you are heading to lower height contours. Thus, if you are holding a constant altimeter reading, which is flying on a constant pressure level, wind from the left etc means you are going down.

5. This doesn't exactly say whether you are going down because the MSL pressure ahead is low, or because the air mass ahead is low temperature, for the upper wind comes from a combination of these two causes. HOWEVER, at any significant height, say FL180 and above, you can bet with some confidence that the overriding factor is the air mass temperature. Reasonable assumption therefore, as has already been given, is that the air ahead is cold.

Dick W

Grobb,

Ok, I got your issue now.

Agreed that the true altitude is decreasing and you are going lower down in the atmosphere. BUT, that low pressure up there that your heading too has been created by a shed load of cold air around. Dick is correct in saying that whilst you may be descending and the air is warming, the overiding factor is the colder mass air your about to fly into.
That is what the JAA is testing, and it's our experoience as instructors that enable us to know what concept they are trying to test, sometimes the JAA do it very badly and even we don't know what they are getting at but most of the time the questions in MET are well written.


Pugs

Yes, apologies to grobhead, I did not emphasise one important part of my last post. Buys Ballot's law is generally applied at the surface, and is then related to the pressure system - i.e. with the wind at your back the depression/trough is on the left, the anti-cyclone/ridge is on the right. These are of course determined by QFF - the pressure corrected to sea level.

You are absolutely right that the aircraft is descending, but it is keeping a constant pressure altitude. You are also right that, in general, while descending the temperature increases. However this is related to the increase of pressure - but the pressure is not increasing because the aircraft remains at FL180, a pressure altitude!

So we then have to think, if the pressure is not changing, why are we descending? Either the surface pressure is reducing (which will not consistently affect the temperature, so won't answer the question) or the column of air below is becoming denser, so increasing the rate of pressure reduction with altitude, so the pressure for FL180 is reached at a lower altitude. This denser air must be colder. As already stated the temperature effect is likely to be dominant by FL180, have more effect than any likely surface-pressure variation.

This site is great - 2 well-respected Met instructors plus another ATPL instructor helping out. Keeps my mind working too, on the obscure questions outside my subjects, ready for any interviews!

Sent Clowns, wow, nice post except that

The temperature increases as you decend not because of a rise in pressure (your thinking adiabatically which is when air is moved) but because terrestrial radiation (which heats the atmosphere) is absorbed mainly by water vapour and CO2 (amoungst others). Where this absorbtion takes place the air is warmed. Now most of these gases are found downstairs rather than upstairs, thus the lower atmosphere is warmer than the upper atmosphere, this is makes the ELR approximately 0.65/100m.

I posted for you Sent Clowns so that together we may learn and conquor the world together.

Pugs:
:ok:

Pugzi,

You have a p.m.

VC10 Rib22

you have PM too, he he

Pugzi

I would have thought that turning onto 360 you would be flying parallel to the isobars/contours and therefore the temperature would not be changing.

On a purely academic point, way beyond the question (i.e. students - don't worry about thew ponderings of instructors, unless you find these things as interesting as we do!) Pugzi - would it not depend on the amount of mixing , and therefore the stability? In a well-mixed section of the atmosphere the adiabatic warming and cooling would have significant effect on the ELR. Or is such mixing only significant in the lower areas of the atmosphere?

Tarbaby,

It doesn't matter what heading you have, if the wind is from the left you are crossing the isobars/contours, and (Buys Ballot) in the N hemi, heading for low isobars/contours. So, as I said before, you are going down. The problem is, why?

At low level it is because you are flying into a low MSL pressure area. At high level it is more likely that you are flying into a low air mass temp area.

The strong westerly winds at height are associated with flight from the warm sub-tropical air to the South toward the cold Polar air to the North - wind from the left, flying toward the cold air.

Dick W