Sun's Effect

[sigurdur]

Ever notice how one can have a strong wind all day, yet when the sun sets the wind dies down?

Or there will be a strong wind all night, and at sun rise the wind will be almost calm?

What does the sun do that causes these very noticeable flucuations?

[aj1983]

The sun changes the temperature variation with height. The temperature variation, pressure and humidity, control the stability of the lower atmosphere. Stability and mixing/turbulence are related. Close to the surface the winds are damped by friction with the ground and objects. With stronger stability ("cold air below warmer air", usually during nighttime) there is less mixing of high momentum air (strong winds created by horizontal pressure gradients above the surface than if the atmosphere is unstable ("warm air below colder air", usually during daytime especially if the sun shines). Essentially, this determines the wind speed difference between the surface and higher (i.e. a few km) in the atmosphere. So it is actually solar heating changing atmospheric dynamics.

Note: it is somewhat more complicated than stated here, because of the factors which contribute to stability.

[jurinko]

Wind = move of the air between areas of different pressure. Sun heats the ground unevenly (clouds, ground albedo etc.) and air gets warmer or less warm, expands and pressure differences occur.

[sigurdur]

True, but after the sun sets, a lot of times the wind will pick up again. Same at sunrise.

I am questioning how a pressure gradient can change just in the area of sunset and sunrise, then after the sun has broken the plane fully, return to what it was.

[radiant]

Sept 30, 2009 13:15:37 GMT sigurdur said:

True, but after the sun sets, a lot of times the wind will pick up again. Same at sunrise.

I am questioning how a pressure gradient can change just in the area of sunset and sunrise, then after the sun has broken the plane fully, return to what it was.

Well there are some observations like:

By day you generally get a sea breeze front just inland from large bodies of water. This takes air from the surface of the water across the land for a few miles or so and then convects it upwards. Air on the land side of the front is also moving towards the sea breeze front. More or less once established they last all day long. At night the flow from the sea reverses but is very much weaker.

Because air has huge mass over town size areas of air, the convection at the surface will disrupt the airflow at the surface and make it very hard for the air mass outside the warmed area to accelerate it and move it sideways once it travels upwards. So the wind will tend to flow around and above thermal areas more or less directing the wind to flow unimpeded higher up. At night this disruption will end and maybe the wind will come closer to the surface.

Air of the same temperature flows like water does. It will always follow the land if possible and be resistant to rising higher and will prefer to flow around obstructions. So once the obstructions are gone the prevailing wind comes back to the surface.

[icefisher]

Sept 30, 2009 2:31:13 GMT sigurdur said:

Ever notice how one can have a strong wind all day, yet when the sun sets the wind dies down?

Or there will be a strong wind all night, and at sun rise the wind will be almost calm?

What does the sun do that causes these very noticeable flucuations?

Here in southern California we have prevailing onshore winds. During the day the land heats up and starts convecting the air upwards accelerating the onshore wind. As night falls the land cools faster than the water and you get a slowing in the prevailing wind sometimes a slight reversal if the prevailing wind is weak. This is generally controlled throughout the summer by a semi permanent high pressure area off the southern California coast.

In the other seasons you have moving pressure zones and we get what we call a Santa Ana wind condition that blows harder at night as a high pressure area settles in over Utah pushes winds from the Northeast and the wind accelerates at night (with the cooling differential between land and water) and when the sun comes up often the wind slows down, dies or reverses as a warming differential takes hold.