A World of Weather, Edition 3: Chapter 12 Introduction

CHAPTER 12

MID-LATITUDE II: THE CYCLONE MODEL By way of review, we start with key observations from the last chapter. The sea-level pressure at the center of a powerful mid-latitude low pressure system might read 980 mb, while a strong high-ressure area might have a central pressure of 1030 mb. That there's only about a 5% barometric difference between stormy lows and placid highs over the mid-latitudes illustrates the atmosphere's dedication to a system of checks and balances that keeps local air columns from getting too heavy or too light.

Controlling the weight of a column of air is a matter of exercise and diet. First, let's review the exercise program for a slimfast low. In the upper troposphere, aerobic jet stream winds perform calisthenics of divergence, a high-altitude tretching that removes mass from the column of air above the low. In a spirit of check and balance, surface winds converge toward the center of the low, thereby tending to add weight to the air column and offsetting the loss of mass aloft. But as long as upper-level divergence slightly exceeds low-level convergence, there is a net loss of mass in the air column and surface pressure should fall.

We say "should" here because upper-level divergence must also compensate for a low's "fatty diet." Recall that the atmosphere avoids excessive congestions of mass from convergence and depletions of mass from divergence by generating patterns of upward or downward motions. For the case of a developing low, air converging around the low's center cannot just continue to accumulate near the ground. Instead, as air starts to pile up, it is compelled to rise, cooling during its ascent. This cooling tends to raise the mean density of the air column, which serves to slightly increase column weight.

Thus, the clueless explanation offered by a few television weathercasters that "rising air causes low pressure" is completely opposite from the truth. Rising air and its associated cooling tend to increase surface air pressures, not lower them. So cooling during ascent is just another check-and-balance that the atmosphere employs to keep surface pressures from falling too rapidly. In the final analysis, upper-level divergence over a low must also offset the increase in column density associated with cooling from ascent.

That's a tall order for upper-level divergence. Logically, as the low strengthens and convergence increases near the ground and cooling from ascent quickens, upper-level divergence has to increase in order to handle the increasing demand for maintaining a relatively light column weight.

How does upper-level divergence meet these demands? When does the demand become too great for upper-level divergence to keep pace? And what happens just ahead of the low's path? That is, how does a low carve out a low-barometric-pressure niche so that it feels right at home once it arrives, or does it just move along like a helpless stick in a stream of steering winds? The answers to these questions are crucial to predicting the evolution and life cycle of mid-latitude low-pressure systems. As a forecasting tool, meteorologists have encapsulated the common characteristics and behaviors of mid-latitude lows into a template called the Cyclone Model (cyclone is a generic name sometimes given to low-pressure systems).

So attention all mid-latitude cyclones: This is your life!