Richard L. Collins - Flying the Weather Map

Low Pressure

If flying the weather was a game with only one specific question allowed before each flight, what would be the best thing to ask? There is no doubt that potentially the most useful question would be: Where are the low pressure centers?

Lows are weather makers. They can affect wide areas, and if we dont understand anything else about meteorology, we had better understand some basics about low pressure. When all the forecasts seem in error, and there are a lot more questions than answers, a low is probably misbehaving. It is moving faster or slower than anticipated, or perhaps it is deepening (strengthening) or filling (becoming weaker). Or maybe a low formed in an unexpected location. Only if we know the characteristics of a low can we construct a mental image of weather situationsespecially of situations that are changing rapidly and defying the efforts of forecasters.

The basic circulation around a low is counterclockwise in the northern hemisphere, which will be the area of discussion throughout the book. Air also flows into a low. If it flows into a low and stops, the low begins to fill and the area of low pressure will simply go away. For the low to live on, air must flow into the low and then move upward in the atmosphere.

We must understand that upper air patterns have a lot to do with surface systems. The most important key to upper level influence is found on the 500 millibar chart. This reflects the pressure patterns at approximately 18,000 feet, which is the center of the vertical distribution of mass. There are charts for higher levels, but there is little change in the patterns from the 500 millibar surface on up. We will discuss the relationship of the 500 millibar chart to surface weather in Chapter 3, and use it extensively in examining weather situations in the last four chapters of the book.

Moisture

Moisture supply has a lot to do with weather, and if we compare geography with the counterclockwise circulation around low pressure centers, we get a good idea of how this works.

When a low is to the west of a given point, the circulation at that location will be from the south. That means relatively warm air, and potentially moist air, depending on the area from which the air is being drawn. Basic meteorology teaches us that air assumes the properties of the surface over which it flows, and the effects of southerly circulation at different locations clearly illustrates this.

Start at Denver, where a south wind comes from over old and New Mexico. This is dry country, and Denvers average annual precipitation totals only fifteen inches a year.

Move now to Wichita, 400 miles to the east. There is more potential circulation from over the Gulf of Mexico and the average annual precipitation moves up to twenty-eight inches.

Move even farther east, to Little Rock, directly north of the Gulf, and note an average annual precipitation of forty-eight inches. This forty-eight inches, incidentally, is about an average for cities that are directly north of the Gulf of Mexico and inland a few hundred miles. Montgomery, Alabama, and Atlanta both have about the same average annual precipitation as Little Rock.

Around the Low

Put a low pressure center on a map and visualize the circulation around it. Figure 1 is an example. The low center over central Arkansas has three circulation arrows for illustration. Relate each to the surface over which it flows. The one coming from the south brings warm and moist air to the low from over the Gulf. This is the basic feed of the system. The easterly circulation brings moisture from over the Atlantic, but this is not as warm and perhaps not as wet as that southerly arrow. From the northwest we have dry and cold air.

For contrast, consider a low over the northwestern corner of Colorado, as in Figure 2. Not one bit of circulation has close access to a lot of moisture. The low in Colorado might be characterized more by strong winds than by frog-strangling precipitation. Indeed, there is a sign in Gunnison, Colorado, proclaiming that the sun shines at least a little on every day of the year.

Low pressure areas dont often remain stationary, but when one does, the rising air is mostly in the center of the low. A low that remains stationary can become quite strong, with increasing circulation as it deepens. Everything eventually has to move, though, and when a low pressure area is on its way, the major ascent of air in the low is usually on the forward side, in the direction of motion, which is usually to the east or the northeast. Thus we would usually find clouds stacked high ahead of a moving low as warm, moist air moving up from the south at low levels is lifted over a wide area. And, a moving low can maintain strength as it feeds on the warm moist air moving northward ahead of it.