March 06, 2022

Why are winds stronger during the day?

Have you noticed?   Wind speeds are often (usually) stronger during the day.  And this has been very much true the last few days.

To illustrate,  here are the winds for Thursday and Friday at the University of Washington (top panel) with solar radiation and time (UTC)  on the bottom panel.  Black lines are sustained winds and red dots are gusts.  Winds are considerably stronger during the afternoon when the sun is in the sky!


And not to be western Washington centric, the same thing is observed over eastern Washington for the past two days, as illustrated by the winds at Moses Lake, within the Columbia River Basin.
Sustained winds at Moses Lake

So why are winds stronger during the day?   There are two reasons:

1.  Solar heating causes the lower atmosphere to mix in the vertical, bringing stronger winds down from aloft.

2.  Solar heating can produce local diurnal circulations, such as sea breezes and upslope flow on terrain that can enhance winds.

(1) is the most important for the present period.  Let me explain.

Winds generally increase with height, because the surface is relatively rough, with trees, hills, buildings, and more acting to slow down the winds.  All the surface obstacles tend to slow down the winds near the surface.  Atmospheric sandpaper.


Now the surface roughness doesn't change during the day, but something does change:  the amount of mixing in the vertical.

During the day, the surface warms compared to the air aloft.  The difference in temperatures in the vertical increases (also called the lapse rate, by the way).  And when the vertical temperature difference gets large enough, the atmosphere starts to convect-- to mix in the vertical.   Not unlike what happens in your saucepan when you make oatmeal.

Such convection-induced mixing brings air from aloft down to the surface, air that has not been slowed down by the trees, buildings, and hills at the ground.

At night, the surface cools (by emitting infrared radiation to space), producing cold, heavy, dense air near the surface, which tends to cut off vertical mixing.  Without a source of fresh, untempered air from aloft, winds tend to slow down at night.

To illustrate the changes, let me show you model soundings in the vertical for SeaTac Airport for 1 AM this morning and 1 PM this afternoon (red is temperature, blue is dewpoint, and winds are shown by the wind barbs on the right).  Temperature is on the x-axis and height is the y-axis.

At 1 AM, the surface is cool and there is an inversion (temperature increasing with height).  Very stable with little mixing.  Winds are very, very light.

1 AM Sunday

But by 1 PM, the surface has warmed, the temperature declines rapidly with height, and the atmosphere is starting to mix.  As a result, winds increase near the surface.

1 PM Sunday

Perhaps not a ten on the meteorological Richter scale, but it is interesting to know why it is a bit windier during the afternoon than when we go out first thing in the morning.





2 comments:

  1. Interesting. I've often wondered why wind storms seem to arrive more often at night than in the daytime.

    ReplyDelete
  2. Speaking of strong winds, in Ellensburg right now there are sustained winds at 43mph and gusts to 62mph. Think of the damage this would do in Western Washington. There will be little or no damage from this in the Kittitas Valley.

    ReplyDelete

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