I observe that the blades are slanted to an angle, but if it produce a lower pressure in one region, is it not possible that the wind from both side will go in?
[Physics] How does rotation of fan blade produce wind
fluid dynamics
Related Solutions
I believe you're exactly right: it's the complexity of hills, buildings, trees, asphalt, water, etc that make surface winds complicated. As you go higher in the atmosphere, these surface effects disappear and the winds become much more steady. You can see this in the winds aloft forecasts issued by the FAA for use in aviation:
In the upper left there is a drop-down box allowing you to select altitudes from the surface (SFC) to 48,000 feet (FL480). As you go up in altitude, the relatively chaotic surface winds blend into a much smoother (and faster!) flow.
I assume the effect is similar at sea, given the simpler boundary conditions.
(I admit this does not directly address your question of, "how variable are the winds at sea"? I too am curious to hear an authoritive answer.)
First, the reason why the finger becomes more wind-sensitive with some saliva isn't that the saliva evaporates but because the saliva, or water, is a good thermal conductor. The finger has to be warmer than the air so the heat flows from the finger to the air and a good thermal conductor such as saliva helps this flux to take place.
Second, because it's the temperature and not evaporation that matters, we must care about the temperature of the air because this is what is actually cooling the finger. Without any wind, the human body – which is warmer than the air – creates a thin layer of warmed air in the very vicinity of the skin which acts as a thermal insulator and slows down the cooling of the human body.
But the wind disrupts this insulating layer on the front side of the finger and the cooler air gets directly in contact with the finger (or the with saliva on the finger). The air on the opposite side of the finger is either stuck in vortices – that may keep on recycling some warmer air (previously heated by the finger), or if there are no vortices, the air on the opposite side of the finger is still warmer because it was heated up by the finger "recently" when the air was flowing around the finger.
Best Answer
Take a ceiling fan. If the blades were not slanted, as the instantaneous momentum of the rotation will be in the plane of the blades, only the thin sides would move air, very inefficiently.
Slanting the blades pushes air in a perpendicular direction to the plane of rotation , the air filling up the vacuum behind comes in the same direction as the one pushed, so an overall draft is established. The direction of the wind depends on the direction of the slant with respect to the direction of rotation.
A a draft is built up , directed where change of air is needed.