|WHY DOES SURFACE CONVERGENCE
OCCUR WITH LOW PRESSURE?
METEOROLOGIST JEFF HABY
There are five forces that act on air. They are the
PGF (Pressure Gradient Force),
gravity. In dealing
with a horizontal wind we can ignore gravity (hydrostatic balance). We will also assume that the curvature in the
wind flow is small enough so we can ignore the centrifugal force (quasi-zonal wind). The PGF is directed from high
to low pressure. The Coriolis acts to the right of the path of motion in the Northern Hemisphere (Coriolis deflection).
In the upper levels of the troposphere friction is small enough that it can be ignored. Above the
PBL, the PGF and
Coriolis forces are the primarily forces which act on the air (quasi-geostrophic balance in the mid-latitudes).
In the PBL, the force of friction becomes important. Friction acts to slow the wind speed. This slowing of the wind
speed decreases the Coriolis force but does NOT decrease the PGF. Therefore, the PGF becomes higher in magnitude
than the Coriolis force. The result is that the wind crosses from higher pressure toward lower
pressure because the PGF is
larger than the Coriolis (creates a surface (ageostrophic) wind). The air crosses
from high toward low pressure at about a 30-degree angle (the greater the friction the greater the angle) in the PBL. It is the force
of friction that causes
surface convergence into low pressure and surface divergence from high pressure.