Wind

Cold damp air = heavy = more rate of change of momentum [Kgm/s/s] per chunk of air slowed down by your sails = more force [N = Kgm/s/s]

Hot dry air = light = less force per wind speed.

What your wind gauge reads is the actual velocity of the air particles because (if well designed) it doesn't resist the wind and needs almost no force to keep it spinning.  Ie it's a velocity meter not a force meter.

So yes force 4 on a cold wet winter's day is windier than force 4 in a desert.  Ie the wind speed's the same but you get more push per knot!

My boats certainly go faster per wind knot in winter.  Doesn't everyone's?

There's a lot of truth in that, Rupert.

I'd just like to throw in the thought that water vapour weighs less per unit volume than oxygen or nitrogen. Question is, does it displace oxygen and ntirogen and reduce the density of air, or does it co-exist and add to it? I used to know... Something about Avogadro's hypothesis isn't it?