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date 7.Mar.2021

■ Sailor's dilemma: are winds stronger in winter or in summer?

Winter is a dead season for most amateur sailors (yachtsmen, windsurfers, kitesurfers etc), so it offers plenty of time to waste on idle curiosity questions, like "is a 10 knots wind the same in winter as in summer?". This may not be on par with age-old questions like "do sea waters boil near the end of the horizon?" but it has grabbed my attention for a while now. As all "obvious" questions that are never asked because they seem so everyday and commonplace, it is quite tricky to fathom.

windy cyprus

To begin with, what exactly is wind speed anyway? Unlike a car's speed, wind is a compressible fluid comprising uncountably many little particles in motion. When air is cold in winter, it is more dense and one unit of its volume contains more mass compared to the same air volume in summer. If wind speed is the average speed of the individual particles, how exactly can it be measured independently from the number of said air molecules?

Supposing for a minute that wind speed is indeed the average speed of an air particle, then a 10kt wind in winter must be stronger compared to the same 10kt wind in summer, because more particles are being blown in lower temperatures, carrying more kinetic energy. That's immediately contrary to local sailor club wisdom that claims "winter wind isn't as good as summer wind". But if it is more powerful surely that's good news? Whereas a kite barely flies at 14kts in summer, it could soar in the same conditions in winter?

But what if so-called "wind speed" we measure is really wind energy? Take a plain cup anemometer, that turns faster in stronger winds. How can it be measuring anything else but the kinetic energy (including air density)? How could such a mechanical device be possibly adjusting for air temperature? It doesn't. So when we say 10kts wind measured with an anemometer, it's not speed that we mean but the impact of the wind on the blades that makes them turn at a particular rate, no?

I am no meteorological expert, just an armchair philosopher — and no stranger to controversial claims. Wherever you read about wind speed, they imply wind speed, not energy, so my above opinion is heretical. For example it may be that wind is hitting both sides of the anemometer, and this cancels out the mass, leaving just the drag coefficient in play (the difference between the convex and concave cups of the anemometer being struck at the same time). Any fluid dynamic experts out there to illuminate us blind sailors? <g> cup anemometer forces

This idle conjecture doesn't have much of a practical impact nevertheless. The difference in air temperature from winter to summer is 30C at most, and according to the ideal gas law:
ideal gas law
This approximate calculation means that the air density ρ is inversely proportional to its temperature in degrees Kelvin, so the difference of 30C is diluted to a difference of (300—270)K, or just 10% in density from the coldest winter to the hottest summer day. Therefore whichever physical law is in effect powering your sails, you won't feel much of a difference, be it summer or winter!

And on this anti-climactic note we let it rest for the time being, and pray for the winds to return in ample knots, so we don't have to split hairs like this!

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