ATPL Meteorology: Buys Ballots Law and wind?

[PilotJames]

Heya guys I was wondering if you could help me out with something,
Buys law states that “if you stand with your back to the wind in the northern hemisphere, then the area of low pressure is on your left and the area of high pressure is on your right”
However the horizontal component of the earth’s wind (virtually all of it) is due to air moving from areas of high pressure to area of low pressure. Well that type of wind anyway ;).
So surely if you stand with your back to the wind then the low pressure area is in front of you and the high pressure area is behind you (as wind moves from high to low) :confused:??

Thanks for any responses in advance

[punk666]

You are correct in saying pressure goes from High to Low but remember the earth is spinning. So Coriolis force is acting upon the wind causing it to shift to the right.

So in the NH you will get a starboard drift and in the south you will get a port drift.

So if you put your back to the wind the low pressure will be to to the left.

Hope I have made this clear enough for you to understand.

Punk

[pkkapkka]

The resultant wind direction, in theory, is due to two forces (far from the ground) 1) the difference in pressure between two areas (the gradient pressure force) 2) the Coriolis force due to the rotation of the Earth. The Coriolis force on the Norther emisphere turns the wind direction to the right, until it is balanced by the gradient pressure force (always directed from high to low areas). So in the end, winds do not blow from high to low but in a normal direction to the gradient pressure force. If you put your back to the wind, the low pressure area is to your left, and the high pressure area to your right. You can find more information (and a better explanation) on any ATPL meteorology books.

[Da-20 monkey]

You forget about coriolis. As wind starts to move it will bend to the right (in the Northern hemisphere).
Air starts to move from a high pressure area to a Low, but as it starts to move it will bend to the right. The bending is proportional to the PGF, the more wind, the more bending.
As an example, you can explain with this why most jetstreams blow from the west. There are bands of low and high pressure around the earth, and if there was no coriolis the wind would blow north or south. However, as the wind picks up speed, coriolis will bend it, because jetstreams are fast, it will bend nearly 90 degrees so that is blows from the west.

Another example: in the NH the wind will veer with an increase in altitude, as seen from the ground. This is because the wind close to the surface (up to +- 300oft agl) is slowed due to surface friction. Above this height wind speed increases, so coriolis also increases (it's horizontal component). So the wind will bend to the right with 20 degrees or so.

[Da-20 monkey]

To add a bit, what you say about a L in front of you and a H behind you if you stand with your back into the wind is true if you are close to the equator, where coriolis is weak. There is a L band close to the equator (the ITCZ) and wind from noth and south blows directly toward it. Wind directly from H to L is called Antitriptic wind, these are the monsoon winds.

There is another force present on higher latitudes, centrifugal force. When isobars are curved, so around a H or L on higher latitudes, there is resultant centrifugal force. Directly in between H and L, isobars are straight. Here there is no resultant centrifugal force, this is the Geostrophic wind.

There is a formula for this: (not required for the atpl exams) Geostrophic wind = PGF/ ( (2)(earth rotation)(rho)(sin latitude) )