Did you notice? Wind speeds are usually (usually) stronger during the day. And this has been very true for the past few days.
To illustrate, here are the winds for Thursday and Friday at the University of Washington (top panel) with solar irradiance and time (UTC) on the bottom panel. The black lines are sustained winds and the red dots are gusts. Winds are significantly stronger in the afternoon when the sun is in the sky!
And not central western Washington, the same has been observed in eastern Washington over the past two days, as exemplified by winds at Lake Moses, in the Columbia River basin.
Stormy winds at Lake Moses
So why is the wind stronger during the day? There are two reasons:
1. Solar heating causes the lower atmosphere to mix vertically, bringing stronger winds down from above.
2. Solar heating can create local circulations during the day, such as sea breezes and reverse currents over terrain that can enhance winds.
(1) is the most important for the current period. Let me explain.
Winds often increase with altitude, as the surface is relatively rough, with trees, hills, buildings and many effects slowing the wind. All surface obstacles tend to slow down winds near the surface. Atmospheric sandpaper.
Now the surface roughness does not change during the day, but something will change: vertical mixing.
During the day, the surface is warmer than it is in the air. The vertical temperature difference increases (which, by the way, is also known as the lapse rate). And when the vertical temperature difference is large enough, the atmosphere starts convection – vertical mixing. Not like what happens in your pot when you cook oatmeal.
Such convection-induced mixing brings air from above to the surface, which is not slowed down by trees, buildings, and hills at ground level.
At night, the surface cools (by emitting infrared radiation into space), creating cold, heavy, dense air near the surface, which tends to cut off vertical mixing. Without a source of fresh air, with no wind from above, the wind tends to slow down at night.
To illustrate the changes, let me show you the longitudinal acoustic model for SeaTac Airport at 1am this morning and 1pm this afternoon (red is temperature, blue is dew point and wind is measured). shown by the wind bars on the right). Temperature is on the x-axis and height is on the y-axis.
At 1 a.m. the surface cools and there is an inversion (temperature increases with altitude). Very stable with little mixing. The wind is very, very light.
1 am on Sunday
But by 1 p.m., the surface warmed, the temperature dropped rapidly with altitude, and the atmosphere began to mix. As a result, winds increase near the surface.
1pm on Sunday
Probably not a meteorological 10, but it’s interesting to see why in the afternoon the wind was a little lighter than when we went out in the morning.
