Propellor, FFF has it right.
I'm a bit rusty but....
Although the day is now lengthening and the sun is getting 'higher' there is still not enough sunlight hitting our latitudes to achieve thermal 'equilibrium'. Over an average 24hrs more heat is being radiated into space than is received from the sun - hence its still getting colder. Equilibrium is attained about a month after the solstice, both winter and summer, and corresponds with the coldest and hottest days respectively. Of course we're talking strict averages here, but the coldest day is Jan 21st and the hottest July 21st, [ish].