Met theory question

[GrahamK123]

I'm sure there is a simple answer but I am struggling with this one..

You are flying east to west in the Northern Hemisphere at the 500 hPa pressure surface. Which of the following statement is correct?

Then the answers are something like

1) Wind is from the south you are gaining altitude
2) You have a headwind you are gaining altitude
3) You have a tailwind you are gaining altitude
4) If the wind is from the North you are gaining altitude

The answer is if wind is from the North you are gaining altitude... but I can't see you have enough information to get to this or what the question is testing...:ugh: any help greatly appreciated

[FREDSIMTH]

I think it is to do with the pressure. If the wind is from the north the you are heading towards higher pressure see Buys ballot law

[OpenCirrus619]

IIRC:

The assumption is everything is simple - just highs and lows with the wind blowing around them.

Draw a little diagram with one high and one low, wind blowing clockwise around the high and anti-clockwise around the low.

If you are flying E->W with wind from the N (right) you must either be approaching a high or leaving a low.

Another assumption is you are holding a steady indication on your Altimeter. So using:
High to Low: Watch out below
Low to High: Safe up in the sky
you will be climbing.

OC619

[Wazzoo]

As Fred and OpenCirrus say, when flying in the Northern Hemisphere and the wind is coming from the port side (i.e. starboard drift) then you are flying from a relatively high pressure to a relatively low pressure area. This is based on Buys Ballot's law and is the opposite in the Southern Hemisphere. Therefore, flying E-W with a wind from the North gives a port drift..as such you are flying from a low pressure to a high pressure.

If you are flying from a low pressure to a high pressure at a constant pressure level of 500m, in the high pressure area the 500mb level will be at a higher altitude than in the low pressure area. Thus flying at the 500mb level will give an increase in true altitude (but a constant indicated altitude..since your altimeter is merely a glorified barometer)

Best way I've remember these things is using Hi-Lo-Hi and the convese, Lo-Hi-Lo. If you are flying from a Hi-press to Lo-press then your altimeter will overread - Hi - decreasing true altitude. If you are flying from a Lo-pres to a Hi-pres, then your altimeter will underread - Lo - increasing true altitude. The same works for flying from high temperatures to low temperatures but in the opposite sense to the pressure..i.e. High temp/Low Press, Low temp/High Press.

Hope that helps!:ok:

[GrahamK123]

Thanks very much for the replies guys!
:)

[degothia]

I also have met question that I need help with. The answer is D/ It is warmer than ISA, but I have some problems finding out why. I would realy like to know where in Oxford/Jeppesen I can read about this.

*An aircraft is flying over the sea at FL 100, with a true altitude of 10000 feet; local QNH is 1003 hPa. What assumption, if any, can be made about the air mass in which the aircraft is flying ?
A/There is insufficient information to come to any conclusion
B/Its average temperature is about ISA
C/It is colder than ISA
D/It is warmer than ISA

D.

[GrahamK123]

I would say answer D is correct because if the QNH is less than 1013.25 the air is warmer than ISA.

[pugzi]

This one is relatively straight forward once the penny drops.

To know the temperature of the air you need to know two things, the indicated altitude on QNH and the true altitude. You know this because if it's warmer than ISA then TRUE > INDICATED and if it's colder than ISA then TRUE < INDICATED.

In the question then, whilst you have the true altitude they have not given you the indicated on QNH. They have given you a flight level which you can use to work out the indicated on QNH. So, get off the flight level 1013 and onto 1003. You now see that the INDICATED on QNH is 9730 FT. Notice that this is less than the TRUE altitude of 10,000ft. Therefore the TRUE altitude is more than the INDICATED altitude.

Looking back to the second paragraph this can only happen if the air is warmer!

Hey presto.

[degothia]

Well, we dont have pennies in Sweden, maybe thats why it didnt drop...
Thank you, it is al very clear now:)
D.

[pugzi]

Graham,

Be very careful, that rule you state is not correct, it may have worked for the example above but it is not a correct statement and it will get you in trouble with your understanding. The question below demonstrates this, using your method you would have got this wrong.

An aircraft is flying over the sea at FL 100, with a true altitude of 9000 feet; local QNH is 1003 hPa. What assumption, if any, can be made about the air mass in which the aircraft is flying?

When you carry out the working I described, True Altitude is less than Indicated Altitude on QNH, therefore the air colder than ISA.

[first_solo]

This is also a nice comparable question found on another forum:
You must make an emergency landing at sea. The QNH of a field on a nearby island with an elevation of 4000ft is 1025 hPa and the temperature is -20°C. What is your pressure altimeter reading when landing if 1025 hPa is set in the subscale?

- 4000ft
- 0ft
- more than 0ft, but less than 4000 ft
- less than 0 ft

[pugzi]

Actually the above question is based upon you knowing the relationship between QFF and QNH. In this question the QNH is less than the QFF. So if the QNH is 1025 then the QFF must be more than this, lets say it's 1026.

Draw a picture to help in the next bit.

Since QFF is a more "accurate" sea level pressure it means that the pressure at sea level is 1026. Draw sea level and label the water line as 1026. Now, the altimeter, set to QNH thinks that 1025 is where sea level is, i.e. where it will read 0 ft. But 1025 will be found about 27 ft above the surface of the sea since the sea surface is 1026. So draw another horizontal line just above sea level and label 1025 hpa. Therefore the actual sea level is below the place the altimeter thinks it is, hence when you ditch the altimeter will read less than zero, in my example it will read -27ft.