We began this chapter, the first
of three dealing with how forecasts are made, by considering a
typical, large scale weather feature: a cold, high pressure cell
in the far north. We raised the question, "When and where
will this high pressure cell move?" Since forces initiate
motion, we briefly explored the nature of forces and Newton's
three laws of motion. We next considered a variety of forces that
act on the atmosphere, including pressure gradient and Coriolis
forces, friction, and gravity. Pressure gradient and Coriolis
forces were seen to interact to produce geostrophic balance, a
useful approximation in the free atmosphere. Gradient wind was
introduced to explain the motions of air in curved paths.
Returning to the motion of the high pressure cell itself, we developed
the concept that pressure systems move through the atmosphere
not because they are blown around but in response to patterns
of divergent and convergent wind flow aloft. Highs move toward
regions of upper-level convergence, lows toward upper-level divergence.
We concluded the chapter with an exploration of relations between
temperature, pressure, layer thickness and wind flow. Greater
atmospheric thickness and high pressure aloft were seen to occur
in warm air layers, with opposite conditions prevailing (lesser
thickness, low pressure aloft) in cold air layers. This relation
explains the tilting of mid-latitude weather systems, as well
as the backing and veering of winds with height in regions of
cold and warm temperature advection.
With this introduction to atmospheric motions and structure, we
are now prepared to deal with air masses, fronts, and cyclones.
Thorough familiarity with these is crucial for those wishing to
understand how weather forecasts are made, which is the central
question of this unit.
Geostrophic Wind Simulation (2550.0K) |