Why Rio blows at night and AM

[windfind]

From Mike Godsey:

The short answer is no one knows. I will tell you my reasoning. When the marine layer is deep the central Bay Area cools down very rapidly in the evening as the fog pours in. So there is only a mild pressure gradient from the ocean to the central Bay Area and the winds fade. (Pt Blunt does not count since it has a local venturi) If you have ventured by boat into the fog off the Golden Gate you have noticed that while it may be howling at Crissy once enter the fog the wind fades fast.

So by evening we have a cool dense and windless air mass from the ocean to say Richmond or even Suisan bay. Now if you ever lived in the Central Valley you know the evenings can be very warm there. This mean you have less dense air making for lower pressures. The pressure gradient from the central Bay Area to the Central Valley peaks in the evening. So if the marine layer is deep with fog well into the bay you have a steep pressure gradient from the central bay to the Central Valley with the wind line starting somewhere around Davis Pt. to Pt. Isabel. (That is why I sometimes mention morning winds at Pt. Isabel and Davis Pt) This puts Sherman Island centered in the wind flow form the Bay to the Central Valley.

If the marine layer is deep this pressure gradient causes it blows all night and much of the morning. If the marine layer is thinner then as the fog burns back over the bay and the land closer to the ocean warms up, the pressure gradient drops. and the winds fade mid-morning.

Then in the afternoon the temps in the Central Valley climb the pressure gradient climbs once again and Sherman Island may see some am winds.

If the marine layer is really deep the central Bay Area may never warm much and the pressure gradient maintains all day and Sherman Island goes into Rio Daze mode with all day winds. I think....: