Sea level:
EGKA 140950Z 30004KT 5000 -RA FEW012 SCT015 10/08 Q1026
EGHI 140950Z 23005KT 170V260 8000 -RA FEW018 SCT025 BKN037 09/08 Q1026
EGMD 140950Z 26009KT 9999 7000SW -RA SCT012 10/07 Q1027
200ft AMSL:
EGKK 140950Z 22004KT 180V240 9999 -RA SCT048 09/07 Q1026
600ft AMSL:
EGKB 140950Z 22009KT 8000 -RA BKN035 08/06 Q1026
OK, not a good example as the cloud is obviously patchy
and really I should spend hours digging out stable conditions with OVC005 on the coast 
But I often find the cloudbase at EGKB is definitely not the cloudbase on the coast minus 600ft. In fact if it was, EGKB would be useless as a diversion for EGKA, but this has never been the case. I can do the ILS at EGKB every time while the MDA at EGKA 02 is 430ft and nothing short of a Saturn 5 rocket engine would lift the stuff there.
It’s apparent that the clouds do go up with the land. But why?
Years ago I asked a Q on forums why cloud never goes all the way to the ground. I don’t think I ever got anything other than cryptic answers but it must be something to do with the radiation from the ground causing the temp and dewpoint to separate, close to the ground (say the bottom 200ft).
Peter wrote:
It’s apparent that the clouds do go up with the land. But why?
But not always by the same amount. So I wouldn’t depend on being able to continue below a cloud cover in rising terrain.
How do you spell advection?
Shoreham has coastal effects which may lower the ceiling with respect to Biggin Hill. Also there may be times when the London built up area has retained heat keeping the dew point a couple of degrees lower.
But moist air moving over colder, rising terrain will in general turn to fog when you reach dew point.
Cloud can of course go to the ground, but it changes its name to mist/fog. The obvious example is in the mountains, but it’s even visible at much lower altitudes. Where I live (Cotswold escarpment) the elevation is 500ft, but when I walk to the nearby beacon (elevation 900ft) I often enter the cloud. So the cloud doesn’t always lift.
Another trick the hills play is when valley fog creeps up a hillside during the morning. This means we can take off early from Glenswinton (500-540 ft) in CAVU but return an hour or two later to dense fog – requiring diversion to a higher or lower field.
Sometimes hill fog rises just so far and then stops. Somewhere I have a photo of our runway with the bottom half in dense fog and the top 200 metres in brilliant sunshine. This persisted for about an hour, resulting in all kinds of unparliamentary language…
I think the topology ahead has the greatest impact on the cloudbase. For example today the cloudbase in each of the CIs is the same. I have found over the years that the base between ORTAC and St Catherine’s will usually be pretty uniform in a SWly for much the same reasons. You can usually count on the base rising north of the South Downs and North of North Downs which is why Gatwick and Biggin will often have a higher base related to the strength of the wind. This falls apart when there is very little wind. Equally how often in a northerly flow is KA relatively clear whereas bases are low over the Weald.
The thread on scud running is relevant. I think with experience you develop a better feel as to how to read the weather, especially over shortish distance. Over longish distances an understanding of the weather patterns is as important. I have found if you properly assess the conditions, are familiar with the topology and deliberately plan to fly below the base there are rarely any surprises.
