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CAVOK or BKN cloud for airshows

Eastbourne Airshow continues this weekend, also there is the Air race at Ascot, finals on Sunday. How is cloud recorded?
CAVOK or BKN cloud for airshows
Blog by Jo Farrow
Issued: 15th August 2014 15:45
Updated: 15th August 2014 15:52

Air shows galore at this time of year, although weekend shows haven't had the best weather with Bertha last week and still rather windy this coming weekend (mid August).

What you don't want for an airshow is low cloud. No point seeing amazing planes taking off and then disappearing in Stratocumulus within 2 minutes.

Airfields and airports have their own special codes for recording the weather in an observation (METAR) and giving a forecast (TAF) Terminal Aerodrome Forecast. These cover various nations and countries but there are some differences between military and civilian codes. Pilots and aircrew use these to help plan. When the pilot on your holiday aircraft tells you that your destination is 27C with sunny spells, or home is 11C and raining, he/she will probably be reading it off a METAR (Meteorological Aerodrome Report).

There are obviously lots of rules about reporting and what is included and in which order.

Here we will look at clouds

Low cloud Cumulus, Cumulonimbus, Stratus, Stratocumulus Base can be surface to 7000ft

Medium level Altocumulus, Altostratus, Nimbostratus, base 7000-17000ft

High level Cirrus Cirrostratus Cirrocumulus base 17000-35,000 ft.

It is quite a skill to be able to tell cloud bases by eye. In the days of the Met. Observers these manual observations would be recorded each hour and input into the Met Office database creating a picture across the UK. There are still individual observers but most cloud bases are recorded automatically now by a laser. There was a cloud base recorder on the field at Met Office College Shinfield Reading. With a barrier around it saying DO NOT LOOK INTO THE LASER. It shot up from the top into the air and bounced back off any cloud above. But only directly above, a real life observer could view the entire sky. Heights would be rounded down for safety and using nearby landmarks such as tall buildings, aircraft and mountains help with estimating heights. Observing at night was extra tricky. Amounts are recorded in Oktas, eighths.

If dividing a cake into slices it is much easier to divide it into 8ths than 10ths, just half, half again and split those pieces in half. The sky is the same. 0 no cloud at all. 1 okta even just the tiniest littlest bit of cloud in the sky 8 sky totally covered OVC = overcast. In-between there are groupings of Few, Scattered and Broken FEW SCT BKN. I remember a demonstration about Cumulus in the sky. Taking an A4 piece of paper and removing ¼ of the sheet. This piece was torn up into pieces and strewn over the OHP. See image. 

  It was to show that you could easily over estimate CU amounts in the sky as the clouds appeared to fill the space,

SKC 0 FEW 1-2  SCT 3-4 BKN 5-7 OVC 8

A METAR   150850 EGJO 09016G26KT 2400 DZ BR FEW003 OVC008 16/11 Q1015=

15th of month at 0850Z,  ICAO station indicator EGJO Northern European, British airfield JO, Easterly wind 16knots mean over 10mins gusting 26KT, visibility, sig weather, clouds/heights, temps (dry bulb and dewpoint, pressure QNH

Ideally, for an airshow you would be wanting CAVOK (Cloud and Visibility OK) Pronounced Cav (as in Chav) – Okay. CAVOK can be put in place of cloud/Vis and Present weather if:

No cloud below 5000ft,  No CBs anywhere, Vis 10km or more (9999), No sig. present weather

Aside from this, TCU (towering Cu) and CBs are alwasy reported. NSC No sig. cloud if other elements of CAVOK are not met.

Up to 3 cloud groups are reported with heights given in feet above the airfield

On the SYNOP station circles you can also see the hourly reported amount so clouds from the central circle. The heights type and amount of High and medium cloud appear above and Low cloud below.

Currently, Saturday looks CAVOK for Eastbourne adnd qualifying at Ascot. Sunday morning brings BKN cloud and showers but hopefully improving in the afternoon BECMG CAVOK

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