This whole thing puzzles me. On one of the few sunny days in January, the sky was blue from horizon to horizon and any airliner visible left a non-persistent contrail if it left one at all. Wind forward to about 5:30 pm yesterday (another, slightly milder, sunny day), the whole sky was criss-crossed with persistent and drifting contrails. I can see why some people get agitated.
So the question is, why nothing on one cold sunny day and everything on another? What's the science? I have real trouble imagining that the chisellers who run airlines would accept the cost of trucking any extra weight in the form of "stuff" to chuck out over London; that aspect in itself seems to bust any claims. So, what combination of factors (meteorological, fuel burn temps etc) would cause this one one day but not on another, comparable, day?