Help with Met question

[luda171]

Hi guys, im currently studying for my ppl met exam, failed it last week by 1 mark :(. So ive got my hands on the ppl confuser questions and one i think its got the wrong answer but was hoping someone could put my mind at rest.

Given a surface temp of +21 C and a Dewpoint of +7 C, at aprox what height will the base of the Cumulus cloud be found?
A - 4000ft
B - 7000ft
C - 8500ft
D - 5500ft

The answer in the book says D but i can't work that out to be right. Ive used the DALR as -3 C /1000ft which makes the base between 4-5000ft. Any help would be appreciated thanks.

[Prazum]

Calculate the temp difference, in this case 21 -7 = 14 (this is known as the spread)
Divide the spread (14) by 2.5 = 5.6 then times this by 1000.

5600 is the answer, which is close to 5500.

Or as its in Celsius, you can multiply the spread by 400 (14*400) =

[Heston]

But the dewpoint will change with height also...

Rule of thumb for cumulus clouds is that the base will be at 400x(difference between surface temp and surface dewpoint) in feet.

For 21C and 7C the difference is 14C. 400x14=5600. So you estimate the height of the cloud base will be 5600 feet. Answer D.

Edit: crossed with Prazum - and note we are saying the same thing.

[2high2fastagain]

DALR is 3C/1000ft, but dewpoint lapses at 0.5C/1000ft as Heston says. Subtracting one from the other gives you the 2.5C/1000ft lapse rate

As Prazum says, (temp-dewpoint) * 400 gives a good answer. You can replace temp and dewpoint with max day and min night temp from the BBC forecasts to get a view of what's going to happen in the next day or so.

I also found a little formula that says...
Cloud Temperature = -0.003 * cloud height (feet) + air temp (C)
(note the minus sign at the beginning)
I can't say whether it works or not (anyone know?), but for clouds at 5600 feet and a temp of 21C on the ground, this calculation predicts the cloud temp will be 3.8C, which means IMC rated pilots could go up into it without too much fear of icing.

[India Four Two]

Cloud Temperature = -0.003 * cloud height (feet) + air temp (C)

This is just a complicated way of saying 3 degrees per 1000'

[2high2fastagain]

LOL. Indeed it is! Someone shows me a formula and I automatically think it is magic. One of the benefits of being first through the comprehensive system all those years ago I suppose. Bring back slide rules! (Oh, they never took them away in aviation, at least the circular one that lives in the darkness of my flight bag...)

[2high2fastagain]

Nope, according to our Prime Minister it's something about love I think.

[Flyingmac]

Two high two fast.
Is LOL a formula?

[keith williams]

Post 7 gave the answer to the question in post 8 before it was asked.

Was that a time warp or what?

[Saab Dastard]

The time stamp fairy strikes again! :)

Don't worry, the database is just having a hissy fit again.

SD