In Lleida, for local and visiting climbers the months of December and January may present challenging climatic conditions. The problem is "La Boira " — fog. We're not talking about flimsy morning mists, burned off by the first rays of the sun, but thick, blanketing pea-supers, caused by temperature inversion, which in the stable anticyclonic weather systems typical of Lleida at this time of year may last all day, all week, even 2 or 3 weeks at a time!
It comes in three varieties: straightforward Boira, Boira Gebradora (freezing fog), and Boira Pixanera, literally "pissing fog" (wet fog). None of these is good news for climbers, but the latter two are particularly troublesome, either turning the landscape into a perpetually frozen winter-wonderland (Gebradora), or soaking the ground and rock sufficiently to make climbing all but impossible (Pixanera).

Vilanova de Meià, with the 'Boira' lurking just below.
The good news is, that although many of the low-lying crags in the region are affected, in 95% of cases all you need to do is keep going UP and you'll eventually break through into crystal blue skies and bright, warm sunshine.
Of the zones covered in the Lleida Climbs guidebook, the worst affected are: Camarasa, Sant Llorenç de Montgai, Alòs de Balaguer, Santa Ana and El Cogul. The least affected are: La Pauta, Àger, Vilanova de Meià, Abella de la Conca, Perles and  Coll de Nargó.
Terradets and Oliana are difficult to predict — the altitude of the bases of cliffs here is not particularly low, but the huge walls in these gorges tend to hem the fog in if it forms at all. 
Of course, the granite walls in the Pyrenees will almost certainly be fog-free too, but here at this time of year you're more likely to be climbing ice than rock.