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Fog calculation/estimate?

 
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chrisale



Joined: 09 Nov 2005
Posts: 187

PostPosted: Wed Feb 08, 2006 9:47 pm    Post subject: Fog calculation/estimate? Reply with quote

This is more of a general weather/climatology question.

It's (very) often foggy in this fine town. Is there a way to calculate the probability of it currently being foggy at the level of the station. Ground fog is created when the local atmosphere reaches saturation right? So I imagine there is some sort of calculation to figure this out?

I though maybe the "cloud base" value was this... but it seems to be more a function of where actual clouds will form.

Just curious.

Thanks

Chris
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bodemory



Joined: 06 Dec 2005
Posts: 43

PostPosted: Thu Feb 09, 2006 8:29 am    Post subject: Reply with quote

First of all, we have to know what kind of fog we're talking about.
Radiative (from earth), advective, coast...

The last one is easy to forecast : when sea wind arises, as soon as coast is getting warmer, thermal will make the air condensate at moderate height, forming fog (it occurs before noon).

Advective fog is easy to forecast too, by looking at weather maps. When tropical air arrives over cold areas fog develops on the surface.

The first one is the most common and appears just after the sunrise, when the temperature is at its lowest.
It is particularly difficult to say when it'll disappear, because it needs soil to be warmed sufficiently, and for this, you need the sun !
We had this situation here for 2 weeks...

To forecast, it is relatively simple, you need those conditions :

    1. High RH (temperature reaching the dew point, saturation)
    2. Winds calm
    3. Sky clear (up to few thousand meters), at least for radiative fog


The two first points are easy to monitor.
The third one is related to the fact that fog often forms by radiation from earth, cooling the surrounding air, thus lowering its temperature without changing much the dew point.
This is more efficient when sky is clear, allowing conductive heat transfer throughout atmospheric layers.

You can put an alarm relying on those conditions. For the last one, the most
obvious criterion would be the atmospheric pressure, since HP are favouring the third criteria.

Taking cloud base into account is useless in this case since it is another way to measure (DP-T), which is RH. The best way to measure it would be to take into account the atmospheric lapse rate which is known thanks to soundings.

HTH,
Brice, JAA FCL PPL(A).
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chrisale



Joined: 09 Nov 2005
Posts: 187

PostPosted: Thu Feb 09, 2006 2:13 pm    Post subject: Reply with quote

very interesting stuff.

Yes, I'd say 99% of the fog in our "Alberni Valley" is the radiative kind. Though we do sit on a very long (30km?) Inlet I don't think the sea fog actually gets to us.

It's generally rainy in the winter... so when it's not raining, the valley is very moist... when it finally clears, the cold air gets trapped below, inversion takes hold and fog forms.

What you said is definitely enlightening, I'll see if I can work out some sort of conditional alarm that might predict when fog is likely to form.

I'm surprised there isn't a mathematical formula, but one of the main factors, clear skies, is observationally rather than imperically gathered.

Thanks!

Chris
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