Wind Regimes - Wind Energy Meteorology: Atmospheric Physics for Wind Power Generation - page 12

Fig. 2.1 Vertical pressure gradients in warmer (right) and colder (left) air. Planes symbolizes
constant pressure levels. Numbers give air pressure in hPa. Capital letters indicate high (H) and
low (L) pressure at the surface (lower letters) and on constant height surfaces aloft (upper letters).
Arrows indicate a thermally direct circulation
decreasing with height as well. At a height of 5.5 km the air pressure is at about
half of the surface value, and thus, the pressure only decreases by 1 hPa every
15 m. An (unrealistic) atmosphere at constant near-surface density would only be
8 km high!
The consequence of ( 2.1 ) is that the pressure in warm air masses decreases
more slowly with height than in cold air masses. Assuming a constant surface
pressure, this would result in horizontal pressure gradients aloft. A difference in
30 in air mass temperature will cause a 1.36 hPa pressure gradient between the
warm and the cold air mass 100 m above ground. This pressure gradient produces
compensating winds which tend to remove these gradients. In reality, surface
pressure sinks in the warmer region (''heat low''). This situation is depicted in
Fig. 2.1 . In a situation with no other acting forces (especially no Coriolis forces
due to the rotating Earth) this leads to winds blowing from higher towards lower
pressure. Such purely pressure-driven winds are found in land-sea and mountain-
valley wind systems. This basic effect is depicted in term III in the momentum
budget equations that will be introduced in the following section.
2.2.2 Momentum Budget Equations for the Wind
A mathematical description of the winds is most easily done by considering the
momentum balance of the atmosphere. Momentum is mass times velocity. The
momentum budget equations are a set of differential equations describing the