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.

Redback asks what difference moisture makes the the density of air. The answer is actually the oppopsite to what one would think - moist air has a lower density than dry air. Odd, but true - and here is the boring mathematical bit to prove it (remember Gas Equation from school):

Density = Pressure/RxTemperature

R= Gas constant, which for water vapour is 1.6 times dry air. Hence density of moist air less than that of dry air.

Aircraft cruise high (low dencity) and cold ... gives higher speed at design Mach No. for wing and reduced drag. Not sure it reads across to a 0.03 Mk No (20 kts) at sea level.

Dencer air = more molecules in contact with sail = more drag, another factor to consider.