Ah, yes - actually I solved this soon after posting the picture. It was relatively simple:

I already have simulation for sun position, which takes observer latitude and time of the year into account. So, days get shorter in winter and longer in summer - the more so, the norther you go. Then I only needed to make it so that when sun is down, temperature goes slightly lower - and when sun is properly up, the temperature gets warmer. Which then naturally leads to the desired effect : in the winter sun doesn't rise very high, so it only has a tiny effect on temperature. In the spring time the base temperature is still low, but increased amount of sunlight makes the temperature rise during the day (and fall again in the night). Then, in the summer sun hardly sets, so once again there isn't that much difference in day / night temperatures. Simple as that =)

(well, to make it even more detailed it should be so that the warming effect of sunlight is increased if there is snow on the ground, as white snow reflects the sunlight, warming the air above it. In the south snow melts sooner in the spring, but in the norht there is more snow - thus there is more dramatic change in day / night temperature, especially in March / April. But I was too lazy to implement this - maybe later on =) )