Cloudbase/dewpoint calculation

[PilotOnline]

I'm a bit stuck on this, can anyone help me out?

How do you calculate the cloudbase from the temperature and dewpoint?

Many thanks!

[matspart3]

Approx 400' per degree IIRC

[FlyingForFun]

Actually, I don't think it's possible.

Assuming the air has the same moisture content all the way up to the cloudbase, which I would think is quite unlikely, it might be possible if you knew the lapse rate. ISA gives us a lapse rate of 2 degree per thousand feet, but we know that ISA usually doesn't match the real world. The adiabatic lapse rates we learn for our exams don't help, since we're not talking about adiabatic lapses here. The type of clouds may give you a bit of a clue: cumulous clouds indicate unstable air, in other words a high lapse rate, so you'd expect a lower cloudbase than you would for stratus cloud and the associated stable air, if the temperature and dew-point were exactly the same, but I don't think that's going to be very helpful.

If there's any kind of frontal activity, then all bets are off, because there will be a different air mass at altitude to that at ground level, with different associated temperatures, lapse rates and moisture contents.

The best way of finding the cloud base is either to look at the TAF/METAR, or to fly up to the base of the clouds (remembering, of course, that you need to have 1000' vertical seperation from clouds above 3000' or fly IFR).

FFF
--------------

[engineless john]

It does work for cumulus, as they are formed directly by rising air. Theres bucket loads of stuff on lapse rates and such, and as a glider pilot I would love to tell you all about it. Unfortunately it fell out the back of my head last October as the soaring season finished.....

Try looking at

http://www.itadvice.co.uk/weatherjack/wx-FAQs.html

and

http://www.itadvice.co.uk/weatherjack/STARS.htm

which has a tutorial on soundings on a link at the bottom of the page.

Cheers
John

[Tinstaafl]

Cloud forms because the airmass is forced to rise eg a wind arriving at ridge. As it rises it will cool at the dry adiabatic lapse rate. As it cools it will become less able to hold moisture becoming more & more humid.

If it continues to rise then eventually it's humidity will become 100%. Any further cooling will force water to condense out of that parcel of air. In other words cloud/fog will form.

While the rising parcel of air has less than 100% humidity it will cool at the 'dry adiabitic lapse rate' (DALR) ==> 3 deg C / 1000'.

If/when it gets to the point of condensation then any further cooling will be at the 'wet adiabitic lapse rate' (WALR) ==> 1.5 deg C / 1000'. The WALR is less than the DALR because the condensing moisture releases its 'latent heat of evaporation'.


Knowing the starting temperature & the dewpoint you can derive the cloudbase.

As the air rises it initially cools at the DALR until the dewpoint temp. is reached, then it cools at the WALR.

eg. Starting conditions: Sea level, 25 deg C, Dew point 13 deg C.

At first the airmass changes temp at DALR (3 deg C / 1000') so at 1000' its temp will be 22 deg C,

2000' == 19 deg,
3000' == 16 deg,
4000' == 13 deg Dew point: moisture condenses/cloud or fog forms. From here it cools at WALR.

5000' == 11.5 deg
6000' == 10 deg

Until the mechanism forcing it to rise stops.

[High Wing Drifter]

Beware! The Thom book is wrong on this.

The lapse rate for cloud formation is 2.5deg/1000ft.

The DALR is 3.0deg/1000ft and the Dewpoint Lapse Rate is 0.5deg/1000ft. DALR-DPLR = Lapse Rate for cloud base calc (3.0-0.5=2.5). This is what the exam will expect you to use.

[Tinstaafl]

Oz uses as I've given unless the syllabus has changed in the last few years.

Accurate enough for pilot purposes but as HWDsays, it doesn't allow for dew point lapse rate.

[bluskis]

Take temperature 7 dewpoint 6, strong possibility fog will form if temperature drops 1 or 2 degrees. However above the fog level it may well be clear for 1000s of ft. even if the lapse rate is the relatively common 2C per 1000 ft.

Cumulus in the UK summer is usually 2500 to 4000 ft, in winter 1200 to 2000 ft, and stratus can be down to the ground summer and winter.

[Gerry Actrick]

Pilotonline,
I think some have misunderstood the question.
You want to know how high the cloudbase will be when you only know the temp and dewpoint?
I don't know the answer, but I know a man who may do.
The 'Wendywindblows' automatic weather stations (as used by hang gliders) works out cloudbase height this way. Find the man, and there is your answer. I'll dig about - if that's the question.
Gerry

not quite what I was thinking about, but try http://www.wendywindblows.com/techy.html
Gerry

[High Wing Drifter]

think some have misunderstood the question.
You want to know how high the cloudbase will be when you only know the temp and dewpoint?
I don't know the answer, but I know a man who may do.
The 'Wendywindblows' automatic weather stations (as used by hang gliders) works out cloudbase height this way. Find the man, and there is your answer. I'll dig about - if that's the question.
Errrm, well that is what I (me me me me) has answered??

2.5/1000'. So if yer temp is 8/4 then the cloud base will be at 1600' ((8-4)*1000/2.5 you see).

[Doghouse]

Obviously cloud is caused to form by a number of different factors (some mentioned above) and certain of these are difficult to calculate.

Tinstaafl and the next post are correct in the basic manner of calculating a cloud base. Their equations are commonly reduced to:

Temp - dew point x 400 = approx. cloud height

Good for CAA exams but not terribly reliable in real life.