[chris (OP)]

Heathrow airport is fog bound and 350 flights have been cancelled today. But why have we suddenly got problems with fog, where did it come from, and what makes it go away again?

Chris

[JimBob]

I cannot believe this hasn't been answered you! It is so easy. When the teperature of the air drops to the dewpoint dew, then fog starts forming. Probably a front or an upper-level wind has blow a lot of water vapor into the London area, scrweing up your flight schedule.

The two things will clear it up are a drop in atmospheric water vapor or a rise in temperature. The last is why a fog "burns off."

[neilep]

We all know it's really Fog Fairies !!

When caught in the act of dispersing fog over areas they are colloquially known as ' Fogging Fairies '

[JimBob]

No, it is Phineas Phogg who causes the mayhem or mayher.

[chris (OP)]

Thanks Jim, but your reply doesn't really answer the question. Instead it merely restates the issue in terms of the "dew point". What is going on at the "dew point", and what defines it?

Chris

[JimBob]

The dew point is defined as the temperature at which the water vapor (water in a gaseous state) in the air condenses into a solid, the pressure being constant.

All of this has to do with the atmosphere and nothing messes with mother nature.

[paul.fr]

Just moving some old posts, and came accross this.
Fog, do you know that there is more than one type of fog?

Advection frog
This occurs when warm moist air is carried over a surface which is cooler than the dewpoint of the air. Cooling and some turbulence in the lower layer lowers temperature to dewpoint and fog forms.

Sea fog
this occurs in antactic air

Radiation fog
These occur when you have a high pressure system producing cloudless nights and moist air  The moist air closest to the colder ground quickly cools to dewpoint with condensation occurring.

there are more, evaperation fogs, steaming fogs, freezing fog, ice fog, frontal fog, rain enduced fog and ofcourse you also have smog.

[paul.fr]

A quick search on The Times website, gives this news article:
Hundreds of flights cancelled as fog blankets airports and roads

Although it say nothing about the type of fog. My way around airports is pretty limited, but is heathrow in a fog hollow? is heathrow near gatwick? because that is in a fog hollow/pocket.

Looking at that article, lets assume that because the fog lasted for at least 4 days that it was bog standard ground fog. All fogs occur when the air temperature and dew point are near to eachother, or identical. The reduced visabiliity caused by fog, depends on how concentratated the condensation nuclei are, and the droplet size/s. Rather like a normal cloud.

The standard definition od dewpoint (as noted by Jim) is:
"The temperature to which a given air parcel (roughly a cubic foot of air) must be cooled at constant pressure and constant water vapor content in order for saturation to occur."