Highs, Lows, and Fronts
In a low pressure system, the air spirals in toward the
center driven by the pressure gradient force. This inward motion causes the
air in the center to "pile up." With nowhere else to go, the air
at the center will be pushed upwards, carrying moisture with it to form
clouds and cause rain or snow. The opposite occurs in a high pressure
system. The pressure gradient force causes the air to spiral outward,
away from the center. The air at the center of the system will be dragged
downwards, reducing cloud formation and giving clear skies.
This is why we view the arrival of low pressure as a possibility
of precipitation and the arrival of high pressure as indicating sunny days.
But, it is at the boundary between high and low pressure systems
- a weather front - that the weather can be the most turbulent!
In the physical theory of heat, it is usually possible to dissolve
more of a substance into a fluid when the fluid is warmer. This is also true
for "dissolving" water vapor into air; warmer air can hold more
moisture than cooler air. Further, it is usually true that substances expand
as they get warmer, becoming less dense and more buoyant.
At the boundary between a cold air mass and a warm air mass,
the cold, dryer air tends to sink lower while the warm, moister air rises
over the cold air. As it rises, the warm air will start to cool, making it
less possible to keep all of its water vapor dissolved. Areas of water droplets
will form as the water vapor comes out of solution in the air, forming clouds.
The clouds can be fed rapidly by the turbulent air motion at the weather front,
rising very high. The unstable clouds that are formed can then lead to precipitation
in some form: rain, sleet, hail, or snow. The turbulent air motion can lead
to straight high winds, microbursts, and tornados!