For now, it suffices to say that upper-air troughs produce _____________ at high altitudes, leading to _____________ in air pressure at the earth's surface (see Figure 7.1). In response, there is convergence of air at the surface and upward motion, paving the way for clouds and precipitation.
constant pressure surfaces
Meteorologists don't draw these upper-air weather maps at constant heights (for example, 2000 m, or 10, 000 m). Instead, they plot weather data on _______________ _____________ _______________, which are surfaces on which the PRESSURE (NOT the altitude) is CONSTANT.
As you know, constant pressure surfaces are not ________.
mandatory pressure levels
The constant pressure surfaces that you'll deal with most frequently are called _____________ _____________ ___________. They are the ones at which radiosondes always take observations: 1000mb, 925mb, 850mb, 700mb, 500mb, 400mb, 300mb, 250mb, 200mb, 150mb, and 100mb. Think of a standard height of a mandatory pressure level as a representative, or average, height at which that particular pressure would be found.
pressure, warm, cold
In Figure 7.7, although the ____________ at the bottoms and tops of the three columns are all the same, the air columns vary in height.
The colorful shadings in the three air columns represent a graduated scale for temperature (dark red indicates the warmest air and dark blue corresponds to the coldest air). From this figure, it makes sense that ________ tropical air columns are typically the tallest and _________ air columns that lie at high latitudes are the shortest.
Look at Figure 7.7, pressure decreases more ___________ with increasing altitude in cold air columns and more ___________ with altitude in warm air columns.
height contours, contours
As in Figure 7.10a, we formally call these isopleths of equal elevation ___________ ______________, or more succinctly, just _____________. Again, the lowest heights on this 500-mb pressure surface lie to the NORTH where air columns are coldest; the highest heights of the 500-mb surface lie to the SOUTH where air columns are warmest.
For this text, we'll talk primarily about three constant pressure maps, each representative of one level of the troposphere: 850 ____ (lower troposphere), 500 _____ (middle), 300 _____ (upper). The standard height and interval at 850 _____ are, respectively, 1500 m and 30 m (see Figure 7.12a). At 300 _____, the standard height and interval are, respectively, 9000 m and 120 m (see Figure 7.12b).
Upper-air data can then be isoplethed with height contours, ___________ (lines of equal wind speed), and ____________ (and other variables as well) to identify important weather patterns at high altitudes.
In a nutshell, relatively low heights typically lie at __________ latitudes, where tropospheric air columns are relatively cold. And relatively high heights typically are found at __________ latitudes, where tropospheric air columns are relatively warm.
Note that, as a rule, 300-mb wind arrows are nearly parallel to the local ___________ ____________, resulting in a general west-southwesterly average 300-mb wind direction over the North Atlantic region during these three winter months. Figure 7.14.
This core of the fastest 300-mb winds marks a portion of the _______ ___________, a narrow channel of relatively speedy upper tropospheric winds that typically extends across the Atlantic during winter, (an observation that commercial airlines try to exploit on trans-Atlantic flights to Europe).
The first thing to remember is that PRESSURE decreases more slowly with height in a _________ air column than it does in a cold air column. Or, alternatively, pressure decreases faster with height in ____________ air than it does in warm air.
On any constant pressure map, a point that marks a region's lowest height corresponds to a center of ______ pressure on a flat surface at an altitude equal to (or approximately equal to) that height. Here's the working interpretation of this principle: You may treat a center of low heights on any constant pressure surface as if it were a center of low pressure on a nearby flat (constant height) surface (such as 5000 meters).
We can mimic the argument for high heights and warm columns of air, yielding a second result: On any constant pressure map, a point that marks a region's __________ height corresponds to a center of high pressure on a flat surface at an altitude equal to (or approximately equal to) that height.
On constant pressure maps, heights packed tightly together represent a large height ____________. Given the close connection between patterns of height and pressure that we just developed, it follows that a large height ____________ translates to a large (strong) pressure gradient. Conversely, a small height gradient translates to a small (weak) pressure gradient.
Note the tip, or ____________, in heights over the East in figure 7.16a, associated with southward-building, relatively short columns of cold air, and the bulge (called a __________) in heights over the West, associated with northward-building, relatively tall, warm air columns.
(a) A map of 500-mb heights shows a trough in the East and a ridge in the West; (b) A map of pressure on the 5500-m constant height surface, a representative altitude for a pressure of 500 mb, looks similar to the map of 500 mb heights in part (a), suggesting that high heights (low heights) on a constant pressure surface can be treated as if they were high pressures (low pressures) on a nearby constant height surface; (c) Highs and lows and ridges and troughs of heigh can thus be analyzed on a constant pressure surface just a highs and lows and ridges and troughs of pressure would be on a constant height surface.