D.C.
All easy 737 are able to undertake cat3a landings( 200m vis and 50' decision height) so landing in fog is ok as long as these minimum requirements are met.
On several occasions recently, the vis has dropped below this( have heard it down to under 75m) and as already stated, an approach cannot be legally made! Aircraft can be launched with the aim of landing at ltn if the forecast shows that the wx will improve, however this does not always happen.
Hence a diversion may occur if the extra fuel carried for low vis ops is used up. An event that is unpleasent and tedious for both pax and crews.