Simulations

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)