In meteorology, the negative pressure gradient \(\mathbf{G}\) is a vector
quantity that points from regions of high pressure to regions of low pressure,
normal to the lines of constant pressure (isobars).
(a) In an \(x y\) coordinate system,
$$\mathbf{G}=-\frac{\partial P}{\partial x} \mathbf{i}-\frac{\partial
P}{\partial y} \mathbf{j}$$
Write a formula for the magnitude of the negative pressure gradient.
(b) If the horizontal pressure gradient provided the only horizontal force
acting on the air, the wind would blow directly across the isobars in the
direction of G, and for a given air mass, with acceleration proportional to
the magnitude of \(\mathbf{G}\). Explain, using Newton's second law.
(c) Because of the rotation of the earth, the wind does not blow in the
direction that part
(b) would suggest. Instead, it obeys Buys- Ballot's law, which states: "If in
the Northern Hemisphere, you stand with your back to the wind, the high
pressure is on your right and the low pressure is on your left." Draw a figure
and introduce \(x y\) coordinates so that \(G\) points in the proper direction.
(d) State and graphically illustrate Buys-Ballot's law for the Southern
Hemisphere, in which the orientation of high and low pressure is reversed.