When is the windiest time of the day?

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This time of the year there are real differences in wind speed during various times of the day and in virtually all locations it is windiest during the late afternoon. Take a look at the above trace of weather observations at my department at the UW (you can click on it to see a bigger version). Note: times are in GMT/UTC... 00Z is 5 PM, 18Z is 10 AM, etc. Winds are light during the late evening and early morning, then pick up during the morning hours. You will also notice the wind shift...typically southwesterly in the morning and northwesterly during the higher wind period.

How about Hoquiam or Ellensburg over the past two weeks? See the attached graphics. You will see a strong daily (diurnal) variation in wind speed.
So why do winds vary during the day so much? A few reasons. The first is that winds generally increase with height in the atmosphere (since friction or drag is greatest near the rough surface). During the night when the surface cools, the atmosphere is more stable in the vertical (less mixing when cooler, dense air is low down). During the day, the surface warms and the air tends to mix in the vertical, as less dense, heated air rises. More mixing brings the winds down from aloft and thus the winds tend to be stronger.

But there is another reason...local, diurnal circulations like the sea breeze. At night land and water are similar in temperature around here, but during day, and particularly warm days, the land heats and pressure tends to fall over land compared to water. The result are sea breeze circulations that increase winds. On the coast near Hoquiam there is a classic sea breeze across the coast. Over Puget Sound there is the Sound breeze between the water-filled NW interior and central Puget Sound, which is mostly land. For Ellensburg, there is a huge "sea breeze" between the strongly heated eastern side of the state and the cooler west. If you want to learn more about these diurnal circulation, my book has a whole chapter on it.

Talking of weather, today (Saturday) is slightly cooler than Friday since there was more marine air moving in last night. Even more marine air will move into the western interior on Sunday. But temps will remain in the 70s and no precipitation is forecast. We have serious dry conditions, particularly east of the Cascade crest. The threat of major fires is quite real.
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