1
$\begingroup$

According to Buys Ballot's law:

When you stand with your back to the wind in the Northern Hemisphere, low pressure is always to your left. (In the Southern Hemisphere, low-pressure systems will be on your right.)

It implies that geostrophic wind blows in parallel to low and high pressure zone, for example I'm in the Northern Hemisphere, wind blows to my back, low pressure is always to my left and high pressure is always to my right. But I thought that wind always blows from the high pressure zone to the low pressure zone. How this contradiction may be explained?

$\endgroup$

1 Answer 1

3
$\begingroup$

Coriolis force plays an important role. A geostrophic wind forms under special conditions when the coriolis force and the pressure gradient are the only forces acting on a parcel of air, and they balance each other out. If a wind blows fast enough in a region with almost straight parallel isobars, the coriolis force can be equal in magnitude and point in the opposite direction of the pressure gradient, so in that case the wind will not change direction and so it will keep the low and high on its sides (depending on the hemisphere). Usually in reality we get near-geostrophic winds because other forces are never really absent, just order of magnitude lower than coriolis force and pressure gradient (for example very little turbulent drag)

$\endgroup$
5
  • $\begingroup$ thanks for your reply! How can I distinguish geostrophic wind from the surface wind (wind generated by pressure gradient force - airflow from the hight pressure zone to the low pressure)? $\endgroup$
    – pacman
    Feb 6, 2023 at 2:17
  • $\begingroup$ All winds are generated by pressure gradient forces, as other forces require wind speed >0. The distintion between geostrophic and surface winds is that in the latter the turbulent drag, friction from the ground and obstacles on the surface slow down the wind so that the coriolis force is very small compared to the pressure gradient force. When there is nothing to slow down the wind (usually at higher altitudes) then a wind can become near-geostrophic. In any case note that winds usually do not blow straight from high to low pressure points, but instead they circle around them $\endgroup$
    – Redirectk
    Feb 6, 2023 at 5:26
  • $\begingroup$ Air advects from the higher pressure area to the lower pressure area - can it be treated as a wind that doesn't circle around the low pressure area? $\endgroup$
    – pacman
    Feb 6, 2023 at 10:49
  • 1
    $\begingroup$ Theoretically it's definitely possible to have winds blowing directly towards low pressure areas (antitriptic wind). This happens if the coriolis force is absent or very weak, i.e. steady low speed wind (intense drag) or near the poles/equator. It is just that this is not common in mid latitudes on Earth where an air parcel would pick up enough speed whilst moving towards low pressure areas, that the coriolis force becomes noticeable, making the air parcel circle (or spiral) around high/low pressure points. Buys Ballot's law is a rule of thumb for mid latitudes $\endgroup$
    – Redirectk
    Feb 7, 2023 at 8:43
  • $\begingroup$ In "small" wind system like sea breeze - wind blows from the high pressure area to the low and doesn't significantly spiral because of the Coriolis force or is this statement mistaken? $\endgroup$
    – pacman
    Feb 7, 2023 at 14:44

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.