Met exam is only 20 questions ?!?

[PompeyPaul]

Any idea why ?

With Air Law (40 questions) if 1 or 2 things come up that you don't know then you are generally OK. With Met if 1 or 2 things come up then you are on a one way ticket to failsville.

Any idea why they made it such a small number of questions ? As it stands my practice exams are ranging anywhere from 50% -> 90%

1 hour for 20 questions ? That's like 3 minutes a question!

[PompeyPaul]

16) The surface temperature is 21 degrees Celsius, and the dew point is 15 degrees Celsius. At what approximate height would you expect to find the cloud base?
A 1000 ft
B 2000 ft
C 2900 ft
D 3500 ft
You chose C, but the correct answer was B.

I thought that the formula was (temp - dew * 400) = cloud base.

In this case that's ((21-15) * 400) = 6 * 400 = 2400ft. Which is as near as dam it right in the center (ok, by 50 ft closer to B). Is there a more accurate way of calculating this ?

[Blinkz]

You gotta love the CAA for being awkward buggers :ok:

Thats the formula that I would use, and that gives 2400 which is closer to 2000ft!

Just seen you live in Guildford Paul? Where abouts do you fly from?

[bigelz1215]

i was always told 2 degrees centigrade drop in temp per 1000 feet - so a drop in temp of 6 degrees = 3000 ft , so I would have got it wrong . However the met office give the ELR (environmental lapse rate ) of on average 6.5 degrees per 1000 metres (3281 feet) which means using their reasoning gives an answer of 3028 ft - nearer answer C - so I am confused

[Blinkz]

2deg/1000ft (well actually 1.98) is the ISA standard ELR, however is generally not this lol.

This get very confusing with the DALR,SALR and the ELR.

Basically (if I can remember my met correctly :{) As the air rises it will initially cool at the DALR (approx 3deg/1000ft and is generally pretty uniform) once the air reaches its dew point and becomes saturated is where the clouds form and the air will then cool at the SALR, which is more variable depending on how saturated the air is. These two rates compared with the ELR will determine the stability of the atmosphere and as such how the clouds will develop etc.

As to the actual cloud base I was taught the formula above, (temp-dew)x400

[BackPacker]

I passed my met exam (so I can be considered an expert, not? :}) and here are the two values that I remember in this respect:

ELR - 2 degrees C (5 degrees F) per 1000 feet is the ISA standard but in real life it will be different, particularly if there's an inversion present. In fact you can detect an inversion by looking at the actual numbers on metform 415 (I think it was).

DALR and SALR - two numbers that are meaningless (and not worth remembering) individually, but combined lead to the formula cloud base (assuming ISA and homogeneous atmosphere) = 400 * ( temp - dewpoint). The 400 are coming from the intersection of the DALR and SALR if you put them in a graph.

I'd have to find my met book and look up where the 400 actually came from but since you're training for met right now you will probably have the books at hand.

[OpenCirrus619]

I think that the missing bit of the puzzle here is that the Dew Point also reduces by around 0.5C/1000'. This is where the 2.5C/1000' reduction in difference between temp and dewpoint comes from (3C drop in temp, 0.5C drop in dew point results in 2.5C reduction in difference between temp and dew point).

My reaction is that this is one of the CAA/JAA questions where you have to pick the "least wrong" one - 2400' is closer to 2000' than any other answer.

OC619

[S-Works]

Or it could be just one of the many wrong answers in the confuser.....

[rodthesod]

If you work on 3 degC / 1000ft to dewpoint (cloudbase) then 1.5 degC (saturated) your sums will work out ok (good for Fohn Effect etc).
ELR 2/1000
DALR 3/1000
SALR 1.5/1000
These are also useful for working out the various types of stability.
I hope I'm right - it's 30 years since I did my ATPL exams and I retired 3 years ago - but some things just won't go away.

rts

[Whopity]

From Pilots Weather by Brian Cosgrove, the Microlighters Met Book

Surface temp = 21
Dew point = 15

Difference = 6
DALR = 3 degrees per 1000 ft
therefore estimated Cloud base = 6/3 x 1000 = 2000 ft

[BEagle]

Agreed, Whopity!

K I S S !!!

[Whirlybird]

Whopity and BEagle got there before I could. Don't complicate matters. This is not a difficult question. Air cools at the DALR until it's saturated; then it forms cloud.

I don't think I've ever heard of this (temp-dew point) x 400. But I might have done because I've forgotten so much stuff you could write half a dozen books with it all if you could find it! But I can't see the use of it really, so think I might forget it again.

[kevmusic]

I learned this when I was gliding and the Thom book confirms it - wot rodthesod, BEagle, Whopity and Whirls just said.

DALR at 3deg/1000', (SALR at 1.5deg/1000'.)

21 - 15 = 6deg, therefore answer B, 2000'.

That's obviously the way the CAA want you to do it!

[Whirlygig]

Whirls isn't that clever! Mind you, in common with Whirly, I hadn't heard of (temp-dewpoint) x 400 either!

Cheers

Whirls

[denhamflyer]

The classic simplistic approach - the CAA are testing your syllabus not your knowledge. That question takes no account of the fact that the dew point changes with height! But it is what is taught at the basic level.
Do further research and you will see that the dew point change (approx 0.5 per thousand) gives a net 2.5 change per thousand feet (1000/2.5=400ft per degree) giving:
base=(temp-dew)*400.
Normally this is given as (temp-dew)*125 = base in m (metric).
Remember you are being tested on what is being taught :ugh: not necessarily the correct answer!

[gcolyer]

temp - dew x 400 is taught in the AFE books.

[kevmusic]

You've got me in a Whirl now! :O

(Back to topic........)

[Astral_Flyer]

I would have come up with 2000' :)

Since the year dot. Which goes back a few decades. I was always told that the dry lapse rate was 3°C/1000' and in saturated air it was 1.5°C/1000'

I hadn't heard of temp - dew x 400 until now. Perhaps the reason why is that I didn't read the right books.

* Astral throws her 'wrong' books away and heads out to get some 'right' ones.

[Whirlybird]

The classic simplistic approach - the CAA are testing your syllabus not your knowledge. That question takes no account of the fact that the dew point changes with height!

The question does say APPROXIMATE height.

And for those of you who are confused by the Whirleys....
WhirlyBIRD = Whirly
WhirlyGIG = Whirls

Someday - at the Gatbash, even - you will see us together, and panic because you think you're seeing double. But we're TWO different people, honest!

[BackPacker]

But I can't see the use of it really, so think I might forget it again.

Neither did I.

Until I flew back to home base from a fly-in. I checked the weather and the reports were the same everywhere. Something like temperature 15, dewpoint 13, scattered 1200. So off I went, only to find 800 broken upon departure. But, hey, the weather further on should be 1200 scattered, not? So I pushed on, eventually finishing the flight at 700-800 feet. Class G and SVFR through the D, fortunately, but not very comfortable. Now, if I see a two degree t/d spread and the cloudbase reported at 1200, I get suspicious 'cause I know it is more likely going to be 800.

Sure, if you're a whirly instead of fixed wing you have slightly different weather minima so cloudbase might be slightly less important. But cloudbase observation by the official met observer in the tower, AFAIK, is only done with the Mk 1 eyeball and is notoriously hard to get right. The t/d spread gives me a way to second-guess his opinion and if I don't trust it, I now have promised myself to ask for pilot reports before I go.

[Whirlybird]

Backpacker,

The t/d spread gives me a way to second-guess his opinion and if I don't trust it, I now have promised myself to ask for pilot reports before I go.

Absolutely! I couldn't agree more. What I meant was that I'd never heard of that way of doing it. I would have said that with a DALR of 3 degrees per 1000 ft, a two degree difference in temp and dewpoint means the cloud starts at approximately 700 ft. OK, 800 ft with your formula, but does it matter exactly; it's low enough for anyone (helicopters included) to wonder if going flying is a good idea.

So DEFINITELY work out for yourself where the cloudbase is likely to be. But I still don't think my poor little brain needs to remember two formulae, and even if my way (and the way everyone I've ever talked to does it) isn't precisely accurate, it'll do fine. :ok:

[DX Wombat]

Met exam is only 20 questions ?!?
Shhhh! Someone from you-know-where might hear you and decide it needs to be a 100 questions exam with a pass mark of 95%. :ooh: Be grateful for small mercies. ;)

[Gavin53]

The easiest way is just 21-15=6.
6 divided by 2.5= 2.4

Thats it, overall altitude is 2400ft, but closest answer on question paper is 2000ft, therefore B!

[ThreadBaron]

http://i10.photobucket.com/albums/a113/threadbaron/S1248.gif

[J.A.F.O.]

I'd never heard of the 400*the number you first thought of thingy.

I'm with Whirly, Whopity, Beagle, et al.

Course, the real answer is when the Met Man tells you the Temp & Dew Point you ask him how high the cloud is.

[Whirlybird]

change my Whirls to a Whirly so as not to offend.

Ah, they're learning. :D;)

But don't worry, JAFO, neither Whirls nor I would have been offended. It's just that everyone else would have got totally confused - sent a pm to the wrong one, chatted up someone who hardly knew who he was, thought we were the same person and accused us of being inconsistent in our posts. All those have happened! And it can all be very simply solved, as you've realised, by remembering that there are two Whirlies, and WhirlyBIRD = Whirly, WhirlyGIG = Whirls.

We just decided that to help you all out with a terribly difficult problem, that's all. :)

[J.A.F.O.]

As though I could ever confuse the Whirlybird with anyone else. ;)