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Estimating cloud base while on the ground

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Estimating cloud base while on the ground

Old 8th Jan 2012, 20:42
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Estimating cloud base while on the ground

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.......
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Old 8th Jan 2012, 20:55
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The met people apparently have [email protected] that they fire into the air, then they use trigonometry to find the altitude of the bright point where the [email protected] 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.
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Old 8th Jan 2012, 20:57
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Interesting... What's so bad about Light Amplification by Stimulated Emission of Radiation that the acronym has to be mangled?

[email protected] (First letter capitalised)
[email protected] (all lower case)
[email protected] (all upper case)
[email protected] (singular)

No, I'm not trying to insult anyone.
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Old 8th Jan 2012, 20:57
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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...
Hygrometer - Wikipedia, the free encyclopedia

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.
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Old 8th Jan 2012, 22:33
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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....
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Old 8th Jan 2012, 22:33
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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
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Old 8th Jan 2012, 22:35
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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.
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Old 8th Jan 2012, 22:52
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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" ... ?)
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Old 8th Jan 2012, 23:03
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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.
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Old 8th Jan 2012, 23:07
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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.........
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Old 8th Jan 2012, 23:07
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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.
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Old 8th Jan 2012, 23:40
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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.


Last edited by Foxy Loxy; 11th Jan 2012 at 07:00.
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Old 9th Jan 2012, 07:19
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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 !
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Old 9th Jan 2012, 07:57
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"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
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Old 9th Jan 2012, 10:07
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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 [email protected] (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.
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Old 9th Jan 2012, 11:53
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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.
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Old 9th Jan 2012, 12:20
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Originally Posted by Foxy Loxy View Post
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.
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Old 9th Jan 2012, 20:43
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If you can't see the hill, the cloud is low.

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

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Old 9th Jan 2012, 21:01
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Don't mess about with [email protected], 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).
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Old 9th Jan 2012, 21:23
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Exsp33db1rd

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