PPL exam questions

[gerardflyagain]

Can someone explain the answer to this question provided by PPL cruiser please?

The surface temperature is 25 degrees Celsius, and the dew point is 22 degrees Celsius. Approximately what height would you expect the cloud base to be, given that the average environmental lapse rate up to 10,000ft agl is 2 degrees Celsius per 1000ft?


Correct answer: C - 1000 ft

[TheOddOne]

A 'rule of thumb' I was given many years ago is that the difference in degrees Celsius multiplied by 400 should give the cloud ceiling. I would therefore expect in your example a value of 1,200'. It's not an exact science, though (like the rest of meteorology, where old wives' tales and seaweed do as well as the massive computer in the basement at Exeter...)

TOO

[Low Level Pilot]

Below the cloud base the air is unsaturated so cools at the dry adiabatic lapse rate, which is approx 3 deg C per 1000ft. Therefore, the cloud base should be about 1000 ft.

[OpenCirrus619]

I think whoever set the question has made the same mistake as Low Level Pilot.

While the Dry Adiabatic Lapse Rate is approximately 3.0C / 1000' you also need to consider the Dew Point Lapse Rate - which is around 0.5C / 1000'.

Combine these and you get 2.5C / 1000' - hence 400' for every degree C difference between Ambient and Dew Point on the ground.

OC619

P.S. That said the aim of the OP is to pass the exam - so go with the wrong approach while sitting the exam :\

Edited to add:
From this: Dewpoint Lapse Rate (https://books.google.co.uk/books?id=TmdlBqzl9WIC&pg=PA172&lpg=PA172&dq=Dewpoint+Lapse+Rate&source=bl&ots=ON7I8vo4YF&sig=V3dtW37Y5440yW_GNHZYcYRR5_k&hl=en&sa=X&ved=0ahUKEwi537yiofXNAhXFHxoKHQlrAvsQ6AEIjgEwFQ#v=onepage&q=Dewpoint%20Lapse%20Rate&f=false)
"we see that dew point and temperature converge at a rate of about 0.8K per 100"
Since 100m = 328'
an 0.8K = 0.8C
This translates to (around) 2.5C / 1000'

[B Fraser]

Only totally dry air cools at 3 deg per 1,000 feet. The actual environmental lapse rate will be less.

[eckhard]

But the ELR is simply the rate at which the undisturbed atmosphere's temperature varies with altitude.

The formation of cloud depends on the cooling of a parcel of air which is lifted within this atmosphere. The rate of cooling of this parcel is determined by its humidity. The buoyancy of the parcel is determined by the relationship between its density (temperature) and that of the surrounding atmosphere.

So, the DALR, SALR and ELR all come into play.

I too use the 400ft rule of thumb. Seems to work most of the time in the UK during the morning.

[Low Level Pilot]

Quite right the the elr is less than the dalr, but elr is also varible depending on the actual conditions. Using the dalr will give you the "worst case" scenario, i.e. the lowest cloud base in theory. The actual cloud base will probably be higher as elr is less than dalr. However the aim is to pass the exam and then get experience in real life scenarios. The exam obviously uses dalr therefore the "correct" answer will only be found using dalr.

[India Four Two]

gerard,

Did the question really say 2 degrees per 1000' or was that a typo?

[SATCOS WHIPPING BOY]

I gather that this was a multiple choice question given that you say the correct answer was "C- 1000ft"

Perhaps this was the closest answer, and that A B and D were wildly adrift.


(just my thoughts)

[Level Attitude]

OpenCirrus619,

Was one of the possible answers 1,500'?
This would have been put there to give "confirmation" to a wrong initial quick answer/guess.

Although some people might think the wording was designed to entrap, I think it is an excellent single question to confirm someone's knowledge of cloud formation.

Remembering that this is a PPL level question:

ELR = 2 Deg C is completely irrelevant and put there to test that you know that is the case.

A parcel of air (even slightly) warmer than its surroundings will be less dense and hence start to rise.

As it rises it will expand and it is this expansion that cools it.

It will cool at the DALR which, as Low Level Pilot has said, is 3 Deg C per 1,000' - and they are expecting you to know this (it is not an open book exam).

The temperature difference given between the surface (25C) and the Dew Point (22C) is 3 Deg C - therefore the base of any cloud will start to form at a height of 1,000'

Which makes:
Correct answer: C - 1000 ft

[oggers]

OC619 is correct. The air cools at the Dry Adiabatic Lapse Rate until reaching the dew point. But the Dew point temp is a moving target.

http://www.pprune.org/members/125420-oggers-albums-misc-picture421-dalrvdewpoint.jpg

sfc T = 25º
Dew Point = 22º
∴ T-D Spread (TDS) = 3º
Dew Point Lapse Rate = 0.5º/1000'
Dry Adiabatic Lapse Rate = 3º/1000'
DALR-Dewpoint Convergence (CR) = 2.5º/1000'

TDS ÷ CR = 3 ÷ 2.5 = 1.2 or 1200'

The 400'/deg C of temp-dewpoint spread mentioned in one of the posts is correct. The answer "C - 1000ft" is inaccurate but may be best of the choices.

[gerardflyagain]

Please see the explanation below. Why should you look out below when flying from a high to a low pressure? I thought a lower pressure would show you are lower than you actually are so, if anything you should look out above.

I understand that flying from high to low temperature you should look out below. It's the high to low pressure that I don't get. :sad:
Thanks

PPL Cruiser:

Question: If you are flying in an unusually cold air mass, an altimeter is likely to indicate...

Answer: that the aircraft is higher than it really is
Explanation: 'As a pilot just remember......from high to low temperature and/or pressure LOOK OUT BELOW.'

[gerardflyagain]

Thanks everyone.

Yes India, it did say 1,000ft, I copied and pasted it.

I'm pretty sure 1,500 was an option for the answer. So I think the posters are right who say the stated ELR is a red herring to test you know that DLR is the relevant figure. A cheeky question if you ask me! But effective in proving a point... if only the explanation on the site had been as thorough as the one I've got here :)

[worrab]

Air pressure drops as you get higher so an altimeter will show a higher reading as the air pressure drops. If you're maintaining a constant height as measured by your altimeter you will find yourself descending as the air pressure reduces.

[foxmoth]

Think about winding the altimeter, if you wind the pressure down the altimeter goes down (try it in an aircraft if you are not sure)so:-
Sitting at 3,000' with 1030 set because that was QNH when you left, get further down route and the pressure is 1020 - wind it off and the altimeter goes DOWN by 300' so your actual height is 2,700', with a hill at 2,500' you definitely want to be looking out below!

[gerardflyagain]

Thanks both. When you fly into a low pressure area, isn't the temperature higher? That's what this diagram seems to suggest?

http://www.metoffice.gov.uk/media/image/f/e/Figure-10-Highs-and-lows(edit).jpg