Why no fog?

[topcat450]

One for the weather buffs:

I live in central England - and have been expecting to awake to fog these past few mornings - but nope. Nothing. Is this because the dew point is below freezing and so water vapour sublimates directly to its solid state, ie frost? or is there another piece to the jigsaw I'm missing?

Also, what is different about tomorrow morning that widespread fog is (or was when I last looked) now predicted?

[Romeo Romeo]

I think for fog you need light winds to provide enough mixing to keep the water droplets in the air. With no wind, they fall out of the air and form dew (or frost if it's cold enough). If the wind's too strong then the fog lifts and you get a low overcast.

Looking at the weather chart for last night, it looks like there was no wind. Tomorrow there might be a bit.

[fireflybob]

For radiation fog to form you need a light wind of between 2 to 8 kts to stir the air sufficiently. Less than 2 kts and no fog, more than 8 kts low stratus (usually).

Commonest time for fog to form - what's called "after dawn thickening" - the sun comes up and gives the air a bit of a stir with some mild thermal activity and, hey presto, fog.

It has also been known for aircraft taking off to be the catalyst for the formation of radiation fog.

Hope this helps.

[High Wing Drifter]

As said above not enough wind. Just to add, that explains the heavy dew (i.e. frost).

[topcat450]

Cheers folks. in my head I tend to associate fog with calm weather - but the winds must be too light as you said.

As the met office is now showing a big fat 0kts for tomorrow's windspeed I may revise my own personal forecast and get up nice and early incase it is clear.

[Sir George Cayley]

Funny, I've suffered from "after dawn thickening" for years

Sir George Cayley

[B Fraser]

Do you live on high ground and the cold air has "drained away" into the valleys or frost hollows below ? Gatwick is a prime example of a frost hollow which explains why their early morning weather is often pants when all around is clear.

A fair amount of mixing is required for fog which is why the viz drops just after sunrise as the air gets stirred up and the fog deepens.

I used to get a thickening just before Dawn..... I wonder how she is these days

[wrecker]

I always remember be taught that between 2 and 8 Kts of wind needed for the correct amount of mixing