Estimating cloud base while on the ground

[rans6andrew]

Is there a reliable way to estimate the cloud base from the ground, using a minimum of equipment?

How do the folk that provide the data for Metars do it?

You just can't tell, by eye.

This afternoon, it looked good, a bit white/grey overcast but brighter and clear to the east (where we were heading), a bit misty to the horizon in all viewable directions but not enough to cause concern. Tree tops all clear, so we went for it. Took off (towards the west) and climbed away only to find almost zero horizontal viz before reaching a couple of hundred feet. Not ideal in a 3 axis microlight. Managed the situation and got away with it.

I called the airfield and was able to stop a chap in a C42 making the same mistake, I hope he got back to Lower Upham safely.

Rans6.......

[abgd]

The met people apparently have lasers that they fire into the air, then they use trigonometry to find the altitude of the bright point where the laser hits the cloud.

You could use a fireworks rocket and count the seconds until it disappears... But not at the airfield!

Perhaps the more experienced people have some rules of thumb. I can imagine that if you know the windspeed and there are some cloud features, you could use that to guide your guesstimates.

Sounds like a scary ride - glad you got away with it.

[abgd]

Interesting... What's so bad about Light Amplification by Stimulated Emission of Radiation that the acronym has to be mangled?

Lasers (First letter capitalised)
lasers (all lower case)
LASERS (all upper case)
LASER (singular)

No, I'm not trying to insult anyone.

[BackPacker]

T/D spread times 400' works just fine for me.

But obviously you have to know both the temperature and the dewpoint for that. The latter typically comes from a METAR or ATIS broadcast, and these typically include cloud information as well.

Alternatively, you can get your own Psychrometer...:p
Hygrometer - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Psychrometer)

abgd, it's a (misguided, if you ask me) PPRuNe policy to prevent kids from getting the idea to shine those devices towards aircraft cockpits for fun. This way the thread doesn't show up when you do a Google search.

[rans6andrew]

ah, so what I need is a lzr beam on a calibrated pivot on each wingtip, pointing towards the sky and a knob to adjust the angles till the two spots coincide. Then you just read off the angles and convert to a range. Sorted.

Rans6....

[Saab Dastard]

What's so bad about Light Amplification by Stimulated Emission of Radiation that the acronym has to be mangled

To prevent google ads inserting (in)appropriate ads in threads about the items being aimed at aircraft.

SD

[GeeWhizz]

it's a (misguided, if you ask me) PPRuNe policy to prevent kids from getting the idea to shine those devices towards aircraft cockpits for fun. This way the thread doesn't show up when you do a Google search.

Its illegal and potentially dangerous to not just flight, but the eyes too; something I know some police constabularies are very hot on.

As for estimating the cloud base... no idea. For me a game of 'met-man says'. If its within limits go see for real, if not sit on the ground until it is. At least if I crash it the AAIB can rule out that I weren't qualified to fly in the weather conditions at the time.

[Gertrude the Wombat]

To prevent google ads inserting (in)appropriate ads in threads about the items being aimed at aircraft.
I'm sure that was the way to do it Once Upon A Time, but do Google not offer more sophisticated filtering mechanisms these days?

(And anyway, what's to stop someone buying the word "l @ s e r" ... ?)

[RTN11]

Back to the original thread...

It's a bit of a black art, but you can get better with time. It can be very deceptive, and what looks like 1000' can easily be 500' or less particularly if it's stratoform.

[rans6andrew]

abgd, I was too busy coping with the situation to be scared.

And because:

the aircraft is pretty stable and trimmed at the airspeed

there were no high obstacles in the direction of flight

if you keep the compass steady, the ball in the middle, the VSI just above zero and the airspeed steady you won't stall or spin

my GPS has terrain and obstacle warnings which I kept one eye on

thus I was able to manage without gyro instruments.

The difficult decision was whether to climb up higher for increased terrain/obstacle avoidance but risk being in cloud for longer or to descend to get a constant view of the ground and risk running into something. A gentle climb seemed to make the most sense, which I did.


Rans6.........

[maxred]

Well this is good - www.orbifly. com, UK TAFS & Metars. Mark one eyeball does the rest. I tend to judge by horizontal viz, against hill backdrops if there are any.

[Foxy Loxy]

The difference between temperature and dewpoint x 400 works fairly reliably for cumulus.

Either a cloud base recorder or Mk 1 eyeball with experience does accurately for the rest.

:ok:

[ExSp33db1rd]

Know the height of many visible local peaks, i.e. if you know one is at 1,000' above sea level, and you can see the top of it, then you have at least 1,000' cloudbase.

Course, wouldn't work in most of Norfolk !

[Genghis the Engineer]

"M'lud, of course my client had good reason to believe that the light emitting device he was using was safe around aircraft, he saw the advert for it on a well known pilots website"....

That is basically what the policy is aiming to avoid.


My crude rule - if I can see the texture of the cloud, then the base is below about 1500ft and I should consider staying on the ground unless I actually want to go IMC, if I can't, it's higher than that and it's *probably* okay to fly.

Crude, but surprisingly reliable.

G

[Vino Collapso]

A few decades back I attended a Met Observers Course for ATC personnel. Only certificated staff could issue Metars that were accepted by the Met Office as being 'official'.

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. The worst is an all over grey type with no visible texture.

At night we used to use a cloud base searchlight that shone a spot of light on the clouds. The angle from horizontal was measured by an Alidade then trigonometry used to work out the cloud height.

These days it is more common at regional airports to find cloud base ceilometers zapping l@ser (now common spelling it seems)light onto the clouds and measuring the time for the reflection to return. Only drawback is that they tend only to look at one point in the sky and miss the important stratus just out of view. So they are more of an aid to the observer rather than a replacement. the use of multiple ceilometers can over come these shortcomings.

[trex450]

I am surprised that no one has already suggested looking at the metars for nearby airports. If they are talking about (for example) 1000ft base and vis below 10K and you can see a poorly defined horizon/thick haze with grey overhead then maybe it would be wise to stay put. It is difficult to judge cloud base, especially if there are no know features nearby to use but if you are flying anything with only basic instrumentation then obviously you are VFR and need the relevant weather to safely fly.

[cats_five]

The difference between temperature and dewpoint x 400 works fairly reliably for cumulus.
...

I love that the difference is in degrees C but multiplying by 400 gives the cloudbase in feet.

[Sir George Cayley]

If you can't see the hill, the cloud is low.

If you can see the hill, the cloud is about to be low:ok:

SGC

[Danscowpie]

Don't mess about with lasers, that's a big boys toy in a properly constructed cloud base recorder only to use by trained chaps.

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.

Not entirely a precise science, but close enough for Government Work (and big boys).

[J.A.F.O.]

Exsp33db1rd

Actually something similar works in south Norfolk - you can see the Mendlesham mast from there and it's 1234' high.

[RTN11]

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.

[BackPacker]

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.

[abgd]

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 laser 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.

[Vino Collapso]

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.

[debiassi]

If you want an accurate way then here goes.

GSD Sounding Plots (http://rucsoundings.noaa.gov/plot_soundings.cgi?data_source=GFS;latest=latest;start_year= 2011;start_month_name=Sep;start_mday=21;start_hour=6;start_m in=0;n_hrs=1.0;fcst_len=shortest;airport=eham;plot=Java-based%20plots;hydrometeors=false&start=latest)

Click on load soundings.

Type in icao or latlong and then hit GFS

This give you accurate data from the weather baloons.

Hope it helps.

[24Carrot]

(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 (http://en.wikipedia.org/wiki/Dew_point)

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!:)

[mrmum]

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 :ok:

[shurikn]

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.:ugh:

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.

(DP/SARL) X 1000 and add cloud base to answer.

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

Freezing level @ 8000 feet. :ugh:

Cheers

[24Carrot]

I had forgotten about this thread.

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!

[chevvron]

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 lasers, assesses the base of multiple layers of cloud, which can then be entered on ATIS, sometimes automatically sometimes by an observer reading it out.

[TheiC]

You could always book yourself onto a course...

Aeronautical Meteorological Observing - Met Office (http://www.metoffice.gov.uk/training/met-service/aeronautical/observing)

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.

[sevenstrokeroll]

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

[peterh337]

These formulae are very approximate.

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

[caber0]

@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:)

[Pace]

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

[phiggsbroadband]

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.

[peterh337]

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 :)

[24Carrot]

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.:)

[phiggsbroadband]

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 !

[Maoraigh1]

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.