connected line segments graphically showing the pressure change over those three hours, as
indicated on the right-hand side of Figure 2-2.
LARGE SCALE WIND PATTERNS
Now that we have discussed station models, it may be easier to understand how pressure and
wind fit together by imagining how a surface analysis chart is constructed (Figure 2-3). While
most of these are built automatically by computer, picture a meteorologist at the National
Weather Service starting with a U.S. map covered only with station models. The first thing
he/she would do is to start playing "connect the dots" by finding stations with the same
pressures, and drawing isobars between them (as discussed in Chapter One). These isobars are
drawn, as a standard, with four millibars of space between each line, and they are labeled
accordingly. At this point, it would become clear where the low and high-pressure systems are
located, and he/she could draw either a big red "L" or a blue "H" to signify these locations.
Finally, he/she could draw other symbols, such as fronts and troughs, as needed, depending on
the chart type. We now have enough of a picture to move onward in the discussion of winds.
Notice the winds in Figure 2-3 are moving in generally the same direction in the areas between
each of the pressure systems. If you look closely, you may even notice the winds are moving
almost parallel to the isobars, in most situations. After enough observation, you may also
recognize a pattern of air circulation around high- and low-pressure systems. In fact, each of
these characteristics is a result of pressure differences causing the air to circulate in a consistent
pattern: parallel (or almost parallel) to isobars, clockwise around high pressure, and
counterclockwise around low pressure. Next, we will discuss why winds blow in this fashion.
Atmospheric Mechanics of Winds, Clouds and Moisture, and Atmospheric Stability