Onshore Flow Aloft
precursor to the land breeze at the surface
Since warm air is less dense than cool air, the air over land
contracts in response to radiational
cooling from the ground below. This contraction leads to a decrease
in the distance, or "thickness", between
constant pressure surfaces within
the cooled air. Over water, where the air cools much more slowly, such
contraction does not occur and the distance between pressure surfaces
remains about the same.
In response to additional cooling, an area of
low pressure (red "L") develops at higher
levels over land while an area of
high pressure (blue "H") develops over water.
pressure gradient force
causes air at higher levels to flow onshore (from