The Moon Wind:

posted in: How To Guide, Weather, woodlore | 0

What a fascinating article; I have often noticed the ‘terminator’ wind (especially at dawn here in Victoria), but I confess regretfully I have not been outdoors over the ocean enough on warm full-moon nights to notice the moon wind:

‘A “terminator wind” is a wind that blows across the “terminator”, the moving line on the earth that divides day from night. The sun heats the air on the day side of the terminator line. The heated air rises, and cold dense air from the night side of the terminator flows in to replace the rising heated air on the day side. As a result, the terminator wind always blows from night to day. This leads to a morning/night difference. In the morning the dawn breeze blows from the dark areas further to the west of my location, and towards the sunlit areas east of my location where the sun has already risen. In other words, the dawn breeze is always and forever a west wind.

In the evening, on the other hand, the sun has set in areas to the east of my location. So an east wind blows from the nighttime there, towards the west, where the sun still warms the earth. As a result, the dusk breeze is always and forever an east wind.

The opposite direction of these two winds leads to a curious phenomenon. This is that for relatively steady overall winds, the dawn and dusk winds will alternately oppose the underlying wind, or it will increase the underlying wind. This is most visible when there is a light constant east wind. At the dawn breeze is a west wind, so it opposes the light east wind and leads to a short period of calm around dawn. At dusk, on the other hand, the terminator wind blows from the east, so the dusk breeze reinforces the underlying east wind and leads to a brief gusty period around dusk … and if there is a light underlying west wind, the opposite is true.

(NB: In Victoria with its prevailing Westerly winds the opposite is true which is why there is always a ‘hush’ at dusk here – hunters should take note! – Steve)

Now, here is where the moon came in. After I’d spent enough nights at sea, watching the comings and goings of the moon, I noticed that the moon has a terminator wind just like the sun. I started calling it the “moon wind”, I didn’t know from terminators, I was on a boat in the middle of the sea, I made up a name. I first noticed the moon wind in the doldrums, where the air is often quite calm, with no wind of any kind. In those peaceful conditions, with the boat not moving at all, the terminator wind from the moon is quite apparent … at least it is to sailors hoping for any kind of wind in the doldrums. It obeys the same rule as the dusk and dawn wind in that it always blows from areas with no moon to areas with the moon. Of course, it is much weaker, and only detectable on calm nights. On a calm night it is a sliver of a breeze, just enough to send small wavelets shimmering in the emerging moonlight.’

– With felicitations to the amazing Willis Eschenbach:

Amazing that the light of the moon yet has enough heat to warm the air!

See Also: for an allied phenomenon.

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