Homework 4 Solutions
The Coriolis force does influence large
scale movement of air, so it is observed in the northern
hemisphere that winds rotate counterclockwise around areas of low pressure and clockwise around areas
of high pressure. The reverse is true in the southern hemisphere. However, on the much smaller spatial
scale of a draining sink or toilet, the Coriolis force is almost negligible. The only forces governing the
motion of the draining water are the pressure gradient force (directed towards the axis of the drain) and
the centrifugal force, so the water can actually drain in either direction. The particular way a toilet or sink
drains has much more to do with factors such as the shape of the drain or initial motion of the water
rather that which hemisphere it±s in.
level winds in the mid
latitudes, these winds are typically observed to blow parallel to
the lines of constant height, and not directly from high to low pressure, because the wind is in geostrophic
In this situation, the pressure gradient force, directed from high to low pressure, is equally
balanced by the Coriolis force, directed to the right of the mean wind (in the northern hemisphere).
the tropics, the magnitude of the Coriolis force is much weaker because the latitude is near zero.
winds in the tropics tend to be less geostrophically balanced, or blow more along the direction of the
pressure gradient, leading to a much smaller spatial scale of air motions and upper
level features (such as
ridges and troughs) as compared to the mid
Because the Coriolis force is exactly zero at the
equator is impossible to form hurricanes there, as we±ll discuss later in the course.
level winds in a constant pressure gradient field, In the situation where the air is not curving
about an axis of rotation geostrophic balance applies (see answer to previous question). However, when
the wind is curving about an axis of rotation, namely around the base of a low pressure trough or the top
of a high pressure ridge, there is an additional centripetal force that is directed towards the axis of
This situation is called gradient balance.
The additional centripetal force effectively reduces
(increases) the strength of the pressure gradient force at the base of the trough (top of the ridge), causing
the wind to slow down (speed up) at this point.
Thus, the wind must speed up between the base of the
trough and the top of the ridge, causing divergence of the air and upward motion to the east of the
trough. To the east of the upper
level trough is typically where ²interesting³ weather is found, as there is
scale lifting to create clouds, and subsequently, precipitation.
Applying the same logic, sinking air