Thanks to all for the valuable e-conversation on this topic.
I am using personnaly, in order to modelize the colony behaviour during the year, a synthetic parameter for climate : the % of the time useful for bee to forage (this parameter, applied to the day-lenght duration, gives a "number of efficient foraging hours by day". It seems rather theorical, but I verified that it gives a nice view of the climate of the year for bees.
I attached an old but interesting representation, where we see huge differences between four following years. I am in France in the Alps (Annecy), in "low mountain" conditions. In 2007 the "season" begun at the beginning of february, and in 2008 at the end of march.
To read the graphic : the x-axis is the number of the week from 1 to 52, the y-axis represent a % of the time where bees can forage (0= the bees do not fly at all, 1 = the bees can fly during all the daylenght). I am using hourly climatic data, including temperature, rain and wind conditions, to exclude the periods where bees cannot flight for foraging. I do a mean by week, that is my time step. 
Thanks
Michel BOCQUET


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