Tropopause Maps

Our survey of maps begins from the top down, with tropopause maps. The tropopause is the sudden change in air mass characteristics which occurs at a height of about 10 miles. Below this level, the atmosphere is being constantly disturbed and mixed by thunderstorms and midlatitude weather systems. Above this level, in the stratosphere, the air is relatively undisturbed.

You can see the "height" of the tropopause using the tropopause map. Pressure is shaded in gray, and the lower the pressure, the higher the altitude. Since the tropopause surface isn't flat, pressure gradients can't be converted directly to wind speeds. So winds are also plotted, in the conventional barb format (short barb = 5 kt, long barb = 10 kt, pennant = 50 kt), and are color-coded according to wind speed for ease of interpretation. Usually, winds are strongest near the tropopause, so this serves as a useful map for identifying jet streams.

In the example above, the tropopause extends all the way down to near 600 mb over the Pacific Northwest and over Ohio and the mid-Atlantic states. Meanwhile, over the central United States, the tropopause is much higher, closer to 200 mb. Notice that the wind tends to curve clockwise around high areas of the tropopause and counterclockwise around low areas of the tropopause.

Another depiction of what's going on at the tropopause is given by the tropopause potential temperature map, shown above. Potential temperature, which is the temperature air would have if brought to a common pressure, is sometimes known by the Greek symbol used to represent it, theta. Wind is shown in an alternative fashion here, with streamlines and isotachs (lines of constant wind speed). Compare the two images to convince yourself that the winds being depicted are identical.

The value of plotting potential temperature on the tropopause is that it is conserved, except in clouds. This means that the potential temperature changes only according to where the wind blows it. So with potential temperature and winds plotted, you can infer how the pattern is evolving and get a sense of what changes are taking place.

This is useful only insofar as the tropopause potential temperature pattern affects the weather. Well, it does, and rather strongly, according to the following principles:

1. Jet streams are located where there are strong potential temperature gradients on the tropopause.
2. Cyclonic (counterclockwise) rotation occurs where the tropopause potential temperature is relatively cold. The opposite rotation occurs where the potential temperature is relatively warm.
3. These wind patterns extend downward toward the surface, especially when the tropopause patterns are large in horizontal extent.
4. When colder tropopause potential temperatures are approaching, the air tends to ascend and produce clouds and precipitation. The opposite vertical motion occurs where warm potential temperatures are approaching.

Where would clouds and precipitation be most likely, according to the above maps?