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Old 01-07-2017, 10:57   #1
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Trade Wind Trend Day vs Night

I'm looking for perspectives and experience (personal or meteorological) about trade wind patterns between night and day or general rules regarding wind speeds away from land and without a significant weather event... Basically are the trade winds stronger or calmer at night or day?

My (limited) experience in the Pacific has been pretty interesting. And suggests that if the winds are calm in the day (<5knts) they will pick up a little bit at night and if strong in the day (>15knts) they will calm down at night, as if there is a tendency towards an equilibrium when the sun isn't involved.

My guess is that it has to do with the larger air mass that is currently around and how the energy/heat of the sun affects it... so in one situation the suns heat is slowing the wind by day and in the other it's contributing to its speed. In the slow by day scenario it's as if the heat is rising so the air movement is more up (horse latitudes) so at night, or in my experience, as soon as the sun sets, there is more sideways movement and a slightly stinger breeze.
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Old 01-07-2017, 14:10   #2
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Re: Trade Wind Trend Day vs Night

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Originally Posted by theway View Post
I'm looking for perspectives and experience (personal or meteorological) about trade wind patterns between night and day or general rules regarding wind speeds away from land and without a significant weather event... Basically are the trade winds stronger or calmer at night or day?

My (limited) experience in the Pacific has been pretty interesting. And suggests that if the winds are calm in the day (<5knts) they will pick up a little bit at night and if strong in the day (>15knts) they will calm down at night, as if there is a tendency towards an equilibrium when the sun isn't involved.

My guess is that it has to do with the larger air mass that is currently around and how the energy/heat of the sun affects it... so in one situation the suns heat is slowing the wind by day and in the other it's contributing to its speed. In the slow by day scenario it's as if the heat is rising so the air movement is more up (horse latitudes) so at night, or in my experience, as soon as the sun sets, there is more sideways movement and a slightly stinger breeze.
Winds that do as you describe are not what I would call offshore tradewinds. The trades blow hard and relatively consistently. It might be 15-20kts for hours and then 18-25kts for hours and then back. Strong, or enhanced trades, often feel stronger and gustier at night, but I think that is just perception.
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Old 01-07-2017, 14:22   #3
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Re: Trade Wind Trend Day vs Night

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Winds that do as you describe are not what I would call offshore tradewinds. The trades blow hard and relatively consistently. It might be 15-20kts for hours and then 18-25kts for hours and then back. Strong, or enhanced trades, often feel stronger and gustier at night, but I think that is just perception.


Ahhhh, good point. I guess I'm speaking about general wind patterns between day and night for winds at sea... where the trade winds are usually located. Not specifically the "trade winds" General day vs night observations.
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Old 01-07-2017, 14:37   #4
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Re: Trade Wind Trend Day vs Night

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Ahhhh, good point. I guess I'm speaking about general wind patterns between day and night for winds at sea... where the trade winds are usually located. Not specifically the "trade winds" General day vs night observations.
I'm not clear what you are asking. The tradewinds are only in a general band just above or just below the equator. The trades strength are also, generally seasonal. When you get away from the area of the trades then you get into the pressure systems, lows and highs, marching themselves across the global that define what the wind is doing in your location.
The issue of the sun being up or not has most to do with the different heating of land vs sea. Something that does not occur far offshore.
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Old 01-07-2017, 14:56   #5
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Re: Trade Wind Trend Day vs Night

There definitely is a diurnal pattern.

One plausible reason:

At first light, the sea surface to the East (and the air above it) is being warmed by the sun while the sea surface to the West is at its coolest. That results in a tendency for West to East air movement. Conversely, at last light, the surface to the West is still being warmed while the surface to the East is cooling down, enhancing airflow from East to West.

As a consequence the Easterly trades tend to be stronger in the late afternoon/ early evening and lighter in the morning.
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Old 01-07-2017, 14:57   #6
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Re: Trade Wind Trend Day vs Night

Offshore, we never noticed any difference in the wind between night and day.

Inshore, different story.
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Old 01-07-2017, 15:09   #7
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Re: Trade Wind Trend Day vs Night

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Originally Posted by StuM View Post
There definitely is a diurnal pattern.

One plausible reason:

At first light, the sea surface to the East (and the air above it) is being warmed by the sun while the sea surface to the West is at its coolest. That results in a tendency for West to East air movement. Conversely, at last light, the surface to the West is still being warmed while the surface to the East is cooling down, enhancing airflow from East to West.

As a consequence the Easterly trades tend to be stronger in the late afternoon/ early evening and lighter in the morning.
The logic makes sense, though clearly not to the extent that land and sea would show differences. I can't say that on my tradewind crossing I've noticed this affect.

I guess you could take your logic and extend it to clouds, where areas around you are shared vs sunny.
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Old 01-07-2017, 21:50   #8
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Re: Trade Wind Trend Day vs Night

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Originally Posted by StuM View Post
There definitely is a diurnal pattern.

One plausible reason:

At first light, the sea surface to the East (and the air above it) is being warmed by the sun while the sea surface to the West is at its coolest. That results in a tendency for West to East air movement. Conversely, at last light, the surface to the West is still being warmed while the surface to the East is cooling down, enhancing airflow from East to West.

As a consequence the Easterly trades tend to be stronger in the late afternoon/ early evening and lighter in the morning.


This seems to be about it. I noticed it quite a few days and night in the South Pacific from Hawaii to Tahiti.
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Old 01-07-2017, 22:21   #9
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Re: Trade Wind Trend Day vs Night

I haven't noticed any long-term day vs night average wind speed variations during our trips to and from Hawaii (daytime may be slower, but not by much), but there is a profound difference in squall activity after the sun sets. Mid-day through sunset, the squalls tend to be much weaker. Once it gets dark the air cools more quickly than the water, and this enhanced difference feeds the tightly-focused squalls. These get stronger and stronger as the night wears on, and usually reach the peak just around sunrise.

These squalls seem to create perhaps a doubling or more of windspeed on the leading edge, and a zone of dead air behind them. These extreme zones are actually rotated from the direction of squall movement, and you can get clockwise and counterclockwise squalls. Most nights all the squalls have the same "sense" (clockwise or counterclockwise).

The seas can really pick up underneath the stronger squalls. Even though the squall covers a fairly small area, I think the waves move along with the squalls and in that way they experience what might be considered a long fetch.

The dead zone behind the squall can be more trouble than the strong wind in front of it. If you reef before the squall hits it's not so bad, but if you find yourself in the middle of the still air behind it can take a long time before the wind picks up again. If you can, sail away from that dead zone while you still have wind.
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Old 02-07-2017, 10:13   #10
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Re: Trade Wind Trend Day vs Night

In the North Atlantic Trades, it seems the wind will pick up a bit about an hour after sunrise for just a bit, then settle back down. I would notice it because I carry a chute most of the time and if the trades were blowing 20 to 24 and the morning lift would get above 26 it was nearly too much for the chute and had to hand steer for about 20 minutes or so until it settled back down.

I have not noticed this phenomenon in the Pacific, and the trades are a bit gentler there, around 16 to 20 kts on the average.

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Old 02-07-2017, 10:32   #11
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Re: Trade Wind Trend Day vs Night

I was told by an airline pilot that the clouds drop altitude at night thereby creating increased squalls and down draughts. His advice was not to run chutes at night. I didn't listen but I did blow out a chute . Oops
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Old 02-07-2017, 10:56   #12
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Re: Trade Wind Trend Day vs Night

If wishing to go eastward along a coast to get to another bay in the Windward/Leeward islands, I find the best time generally if at all is prior to 0800.
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Old 02-07-2017, 12:50   #13
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Re: Trade Wind Trend Day vs Night

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Originally Posted by StuM View Post
There definitely is a diurnal pattern.

One plausible reason:

At first light, the sea surface to the East (and the air above it) is being warmed by the sun while the sea surface to the West is at its coolest. That results in a tendency for West to East air movement. Conversely, at last light, the surface to the West is still being warmed while the surface to the East is cooling down, enhancing airflow from East to West.

As a consequence the Easterly trades tend to be stronger in the late afternoon/ early evening and lighter in the morning.

Interesting,,but hard to believe without hard data or some meteorologist weighing in here on the physics involved. The land/sea breeze effect is well known but i doubt an extreme slant of a rising sun would warm the waters enough to create any thermal mixing in the atmosphere. For one thing, ,the ocean surface would allow any heat generated to be distributed into some depth and even if the suns rays only heated the surface ,wave action( even ripples) would distribute this minimal heat a few inches below the surface and therefore hardly create and significant temperature differential that would generate any breeze at all, let alone enough to momentarily reverse the trades on even the lightest day. Nope, don't believe it ,but am willing to learn. Interesting!
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Old 02-07-2017, 13:47   #14
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Re: Trade Wind Trend Day vs Night

I have not noticed any difference in the trade winds between night and day, and I love sailing in the trades.

I probably would not notice a difference in the wind much at night because I automatically place a single reef in the main at sunset. So if the wind picks up, I would be less likely to notice it with less sail area up.

I hate being over canvassed at night, as I don't like sail drills in the dark. Nobody goes forward at any time without another person awake and on deck.

When we crossed the Pacific, I never noticed enhanced winds at night unless it was from increased squall activity.
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Old 02-07-2017, 13:59   #15
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Re: Trade Wind Trend Day vs Night

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I was told by an airline pilot that the clouds drop altitude at night thereby creating increased squalls and down draughts. His advice was not to run chutes at night. I didn't listen but I did blow out a chute . Oops
Squalls are more common at night in many places, but Ive never noticed a day/night variation in trade winds speed well offshore. I do percieve them as blowing stronger at night but the anemometer disagrees. 😆

Near coastal you get land and sea breeze effect which can have a significant affect on both velocity and direction.

Also near coastal the combination of Corilis effect and friction of land can have a significant affect too.

For one dramatic example of all the above look at GRIBs for the Honduran coast from the coast out to N of the Bay Islands during day and night. NE Trades can be honking at 20 knots N of Roatan and then a light NW wind near Puerto Cortez at the same time.
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