Barometic pressure graphs

Something thats been bothering me since I was a child is the pressure graphs weathermen use. The ones where they have lines linking points of equal pressure, like the last one on this list.

Why does the wind follow the lines, when everything I can find on wind says that wind flows from areas of high pressure to areas of low pressure?

I don’t think the wind follows the lines. The lines are called isobars.

I’ll try and drag up an example, but the wind arrow indicators on weather man’s maps do seem to point along the isobars, rather than perpendicular to it, as I would expect. Unless my memory is very faulty of course.

Have a look at this!

I think that site might answer my question. Thanks for the name of the lines, that seems to have led me in the right direction.

The winds do follow those lines, pretty much.

The wind wants to flow directly from high to low pressure at right angles to the isobars on the weather map. However, because the earth is spinning, wind gets deflected to the right in the northern hemisphere and left in the southern hemisphere. The apparant force responsible for this is called the coriolis force and it is the strongest near the poles and the weakest at the equator and tries to make the wind flow parallel to the isobars.

I’ll use a northern hemisphere example assuming that is where you live.

As the wind blows in towards the low pressure, the coriolis force deflects it right and so it swirls anti-clockwise around the low pressure area while still moving in towards the low pressure area. The stronger the coriolis force is (i.e., the closer you are to the poles), the closer the windflow is to paralleling the isobars.

You’ll find that the wind doesn’t parallel the isobars perfectly but blows across them slightly (from high to low pressure.)

Wind with lower velocity is less affected by the coriolis force and tends to blow more across the isobars. Because friction with the Earth’s surface at low altitudes slows the wind down, wind at low levels is less affected by coriolis and will blow more across the isobars than wind at higher altitudes (above around 3000 feet.)

So, at high latitudes (closer to the poles) and at high altitudes the wind will blow along the isobars. And at low latitudes (near the equator) and also low altitudes the wind will blow more across the isobars.

You can use all this to tell where the lower pressure is. If you have your back to the wind, the low pressure area will be to your left in the northern hemisphere.

PS. The coriolis force is not strong enough on a small scale to have an affect on water draining down you kitchen sink.

1920sS"DR" has given a pretty good explanation.

Compare the pressure map here: http://www.bbc.co.uk/weather/ukweather/pressure.shtml

with the wind map here: http://www.bbc.co.uk/weather/ukweather/wind.shtml

At the time of posting, the isobars run roughly SW - NE, with low pressure to the northwest.

The wind arrows show the winds are almost from the due south across much of the country, in other words the wind is crossing the isobars slightly, towards the low pressure area.