The atmosphere and the Weather

Relationships between the 500 mb height pattern and surface weather

Much of this page is a review of what we have already covered. Some of it corresponds to an in-class handout that describes some common weather terminology.

The height pattern on a 500 mb upper air map can often be used to estimate the weather conditions at the surface. The simple analysis presented here does not explain everything that may be going on with the weather. It will work best in the wintertime in the middle and high latitude regions of the Earth. It cannot be used to understand or predict tropical or summertime weather conditions. However, it does provide a nice way to "see" the large-scale weather pattern over the United States in winter.

First, recall that the height of the 500 mb surface is directly related to the temperature of the atmosphere below 500 mb -- the higher the temerature, the higher the height of the 500 mb level. Consider what the 500 mb pattern would look like if temperatures decreased steadily from the equator toward the north pole. In that case the height contours would be concentric circles around the north pole with the highest heights to the south (toward the equator). While this is generally true, the actual pattern at any given time is wavy. See the in-class handout labeled Figure 13.6. Where the height lines bow northward (a ridge), warm air has moved north; and where the height lines bow southward (a trough), cold air has moved south. Therefore, in general warmer than average temperatures can be expected underneath ridges and colder than average temperatures can be expected underneath troughs. The more pronounced the ridge (or trough), the more above (or below) average the temperatures will be.

The 500 mb pattern can also be used to locate where surface storms and precipitation are most likely to be occurring. Surface storms and precipitation are most often found over areas downstream of troughs (following the horizontal wind direction from a trough to a ridge). The reason for this is that rising air motion is forced in this part of the flow pattern. Rising motion means that surface air is forced to move upward toward the tropopause. Later we will see that clouds and precipitation develop where air rises. Conversely, sinking air motion is forced over areas downstream of ridges. Clouds do not develop where air is sinking. Underneath these areas fair weather is most likely. By looking at a 500 mb map, you should be able to distinguish where storms are most likely and where fair weather is most likely. The reason that rising motion occurs just downstream of 500 mb troughs is that in this region divergence of air occurs in the upper troposphere, while just downstream of ridges, sinking motion occurs as a result of convergence of air in the upper troposphere. You are not expected to understand why upper level divergence and convergence are forced.

Because the 500 mb pattern is often a good indicator of what is going on at the surface, much common weather terminology has arisen to describe it. Some of this terminology is depicted on an in-class handout. A few additional comments will be given here. In general, a sharply curving (or amplified) wave pattern provides more energy for storm development than a flat wave pattern. Basically, this means that stronger vertical motions are forced near sharply curving waves as compared with flat waves. The more powerfully air is forced to rise, the more violent the weather. You will notice that the caption for figure 13.6 refers to the waves in the 500 mb pattern as longwaves. These are the waves that define the large-scale weather pattern. However, there are often smaller wiggles or waves that are superimposed on the longwave pattern. These are called shortwaves. There are shortwave troughs and shortwave ridges. These indicate smaller regions of warm/cold temperature contrasts and forced rising or sinking vertical air motions. Shortwaves typically flow through the longwave pattern following the general wind direction but at a slower speed. Shortwaves can indicate the position of a strong weather system especially if it is sharply curved. Shortwaves tend to strengthen as they move into the region just downstream of a longwave trough and weaken as they move into the region just downstream of a longwave ridge.