Wind Weight

In summer, thermal mixing causes the breeze to pick up during the day (I don't mean sea-breeze, I mean inland). That's why on a fine summer day it is often calm first thing and in the evening. In some circumstances you can get a strong temparature inversion within a few metres of the ground. Often you will see mist trapped close to the ground by this effect.

However if there is a strong breeze driven by a pressure system, turbulence causes the mixing to take place anyway, so you don't get much diurnal variation.

I'm not really certain that this will cause wind weight differences between summer and winter though.

Personally, I think it is more psychological and physiological. It is just harder to sail in the same weight of breeze in winter. You are encumbered with extra clothing and your muscles don't work nearly as well when cold. I was very aware of the latter when I used to windsurf in the winter.