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Estimating cloud base while on the ground

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Estimating cloud base while on the ground

9th Jan 2012, 21:27

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You can become remarkably good at estimating cloud heights at varying layers when you have to do it every 30 minutes throughout your working day. Local knowledge also provides valuable help on days when the clouds are being a bit difficult.
How do they know if they're any good without actually going up to check if their observations are correct?

If they put out a METAR saying cloud is 1500' when it's actually 2000', do any pilots actually radio this info back to help the ground observor improve their judgement?

From my visits to towers on reasonably nice days, they seem to pluck the figure literally from the air, putting SCT 030, when i would've said FEW 040. Obviously it's always best to air on the pessimistic side.
9th Jan 2012, 22:18

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The easy way is, if you have access to both surface temperature and dew point, is to calculate the cloud base using that information.
In the International Standard Atmosphere, air cools from the surface up to 11000m at a rate of 2 degrees per 1000ft.
i.e If the tempertaure is 06 deg C and the Dew Point is 04 deg C, you can expect cloud to start forming at 1000ft.
No way.

The 2 degrees C per 1000' is indeed the ISA average lapse rate. But it's just an average and assumes a uniformly built up atmosphere.

In convective situations, the atmosphere is all but uniformly built up. Where the clouds start to form is actually the intersection of the dry adiabatic lapse rate (3C per 1000') with the saturated adiabatic lapse rate (1.5C per 1000') - or 1.5C per 1000'. The calculation for the cloudbase thus is 400 feet per each degree C difference between temperature and dewpoint.

do any pilots actually radio this info back to help the ground observor improve their judgement?
I find the info in the ATIS generally correct, but if I think there's more to it than what the ATIS tells me, I'm indeed doing a pilot report. Especially if I know that there are fellow pilots who are interested in this information on frequency.
9th Jan 2012, 22:56

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My (very limited) impression is that the days when the ATIS gets the cloudbase wrong are sometimes those when there are lots of gaps in the clouds, or when the cloudbase is a bit lumpy. Perhaps the problem is that the [email protected] doesn't hit the bottom of the main layer of cloud, but passes through to the layer above.

Again just my assumption... I was interested in Genghis rule of thumb that you can guess the cloudbase by whether or not you can see texture in the clouds. Presumably it means there's a definite 'feature' size to them. I'd always imagined bigger clouds as looking much like smaller clouds, but bigger. I'll have to look more closely.
9th Jan 2012, 23:03

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If they put out a METAR saying cloud is 1500' when it's actually 2000', do any pilots actually radio this info back to help the ground observor improve their judgement?
Yep, most definitely PIREP add to the accuracy.

I find the info in the ATIS generally correct, but if I think there's more to it than what the ATIS tells me, I'm indeed doing a pilot report. Especially if I know that there are fellow pilots who are interested in this information on frequency.
ATIS content comes from the same human observer. It should be the same as the published METAR.

The easy way is, if you have access to both surface temperature and dew point, is to calculate the cloud base using that information.
In the International Standard Atmosphere, air cools from the surface up to 11000m at a rate of 2 degrees per 1000ft.
i.e If the tempertaure is 06 deg C and the Dew Point is 04 deg C, you can expect cloud to start forming at 1000ft.
The various quoted formula will give a reasonable estimate for cumulus clouds but stratus is a different animal.
16th Jan 2012, 23:22

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Sounding Plots

If you want an accurate way then here goes.

GSD Sounding Plots

Type in icao or latlong and then hit GFS

This give you accurate data from the weather baloons.

Hope it helps.
17th Jan 2012, 09:57

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(Temperature - Dew point) x 400 has already been mentioned.

There is also a simple approximation for T-DP = (100-Relative Humidity) / 5
Dew point - Wikipedia, the free encyclopedia

So with the answer in feet you can use 80 x (100 - RH)

Some weather forecasts (eg the BBC for the UK) give RH estimates several days ahead, so this can provide a rough and ready longer forecast. Eg should I cancel something four days from now because I might finally do some night flying.

Sometimes it even works!
17th Jan 2012, 21:44

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I find the best way is to ensure you locate your aerodrome reasonably (but not too) close to a couple of big hills and a BBC transmitter, a little extra value from my licence fee
21st Oct 2012, 15:54

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To find cloud base = (Surface Temp - Dewpoint) X 1000feet and divide the answer by the dry adiabatic lape rate (DARL 3 deg. celcius/1000').

Exemple: SFC temp 15c, DP 9c.

(15-9) X 1000= 6000feet divided by 3 = 2000 feet

Cloud base at 2000 feet.

To find FZ LVL; dewpoint divided by saturated adiabatic lapse rate (SARL 1.5 deg. c/1000') multiplied by 1000. Then add cloud base and you have your freezing level.

Exemple: (9/1.5) X 1000= 6000 + 2000= 8000 feet

Freezing level @ 8000 feet.

Cheers
21st Oct 2012, 18:52

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I found my 80 x (100-RH) could be improved slightly by adding 300 ft. Perhaps this reflects mixing of the atmosphere near the surface.

Cloudbase = 300 + 80 x (100-RH) ft

It remains a very approximate formula!
21st Oct 2012, 19:15

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It was easy at Sumburgh when I did my 30 day 'penance' there back in '72.
The airfield was only just above sea level; there was a cliff called 'Fitful Head' about a mile north west which was just under 1000ft amsl. You just looked at Fitful; if the cloud was below the top it was obviously below 1000ft (say 800ft) and it was in the clear, it was something like 1200ft.
Most larger airfields nowadays (and some smaller ones) have an automatic weather station which, by means of [email protected], assesses the base of multiple layers of cloud, which can then be entered on ATIS, sometimes automatically sometimes by an observer reading it out.

Last edited by chevvron; 21st Oct 2012 at 19:17.
21st Oct 2012, 19:37

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You could always book yourself onto a course...

Aeronautical Meteorological Observing - Met Office

When I did it, many years ago, it was one or two weeks at the college and then one week at LHR doing observations. The certificate sits in my cupboard. It's a skill you need to keep fresh. I sometimes make my own assessment and then look at the METAR, to try to retain some degree of ability.

Searching for 'met office observer handbook' will find the old guidance.

Mostly, these days, it's automated. Not always for the better, though a Cloud Base Recorder was most useful.
21st Oct 2012, 20:22

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to my knowledge a device known as a ROTATING BEAM CEILOMETER is used to determine cloud bases.

PIREPS made by intelligent, thoughtful pilots can be helpful...airliners will sometimes advise departure control...bases at X, tops at Y, higher above...before they go to center.

back in meteorological school....we could and did calculate the LEVEL OF FREE CONVECTION...useful in understanding CB/Tstorms etc.

if you are flying, you should certainly read: Weather flying by BUCK...buying the FIELD GUID TO CLOUDS might help too.

a useful tool while flying is any straight edge/ruler/checklist...look down the straight edge at the tops of the clouds, move your head back and look at the straight edge and see if it is pointed UP or down from level

pointed UP? you won't top the clouds at your present altitude

pointed down...you will clear at present altitude.

have fun
21st Oct 2012, 20:35

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These formulae are very approximate.

I have been flying for 11 years and have never seen a standard atmosphere!

Last edited by peterh337; 21st Oct 2012 at 20:41.
22nd Oct 2012, 00:44

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@peter337 withEASA in charge I am sure we will see a lot more standard atmosphere

Actually due to all the hot air from Brussels the standard temp will be increasing by 1C every year :0

Caber
22nd Oct 2012, 01:19

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Sevenstrokeroll

pointed UP? you won't top the clouds at your present altitude
If you ever get to fly a jet (maybe you do?) If you see approaching jet contrails and wonder whether the jet is above or below look at the contrails

If its dark grey its below your level if its pure white its above.

I would have thought the other way around looking underneith the trails grey but bathed in light from above white? But not the case!

Pace

Last edited by Pace; 22nd Oct 2012 at 01:22.
22nd Oct 2012, 12:05

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Hi Rans, if you sign into the Met Office website, you can get all the recent European METARS, or the local UK ones.

Although the Temperature - Dewpoint calculations, using the lapse rates, are theoreticaly correct, and used as exam questions. In practice they are nearly always wrong....

Trying to work out Cloudbase from T and DP will give errors. It is possible for Temp=D.P. and have CAVOK or NSC.
Also, from the Metars, you may see an airport with 'Few at 3200ft' and 18/16, which would have given a cloudbase of 800ft, if you had used the formula.

However the calculation works backwards quite well... IF there is Fog or low cloud, then the Temp will be the same as the D.P.
22nd Oct 2012, 14:19

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IF there is Fog or low cloud, then the Temp will be the same as the D.P.
Yes, but then you are not calculating anything
22nd Oct 2012, 17:05

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Just to be clear, I never suggested that formula as a way of estimating a current cloud-base. Obviously METARS or some instrument would be better.

However, unless you live next to your aeroplane and can fly at the drop of a hat, it can improve the odds of actually flying by helping to choose days to free up for a flight. Fronts and isobars on forecast MSL Pressure charts are useful too.

Of course the TAFs on the day will be different, that's why we look at the TAFs before we fly.

Last edited by 24Carrot; 22nd Oct 2012 at 17:05.
22nd Oct 2012, 17:59

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Hi 24carrat, local variations can also make quite a difference. One day I flew to Swansea, via Shobden and the Wye Valley, both of which were covered in low ground fog. 300yards to the west and conditions were Cavok.

Hope the photo copies.....
No it didn't !
22nd Oct 2012, 21:50

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a useful tool while flying is any straight edge/ruler/checklist...look down the straight edge at the tops of the clouds, move your head back and look at the straight edge and see if it is pointed UP or down from level
At low speeds, weather can change faster than we fly. I've flown west from Inverness, in the morning, towards and above an unbroken cloud layer. It broke up completely as I flew over it. Cloud tops rise, and cloud layers form. CBs form and dissipate.

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