First, visit the University of Wyoming Upper-Air Map site and retrieve a 500 mb map centered over North America. To do so, select "Type of output: GIF; Type of Map: Analysis". Next, verify that the level selected is "500 hPa" ("hPa" stands for hectopascals which is an equivalent unit to millibars), the region is set to "North America", and the time/date is no more than 12 hours old. Click "Get Map" and save this image to your local computer.
Next you will need a national infrared satellite image. Select "Infrared" and the time and date ("End Date" & "End Time") that correspond to the 500-mb map that you collected above. Finally, click on the "Contiguous U. S." label on the map. You should then see a national infrared satellite image that looks like this sample image. Save this image to your local computer.
Once you have saved the 500 mb map and infrared satellite image, you are ready to proceed with your lab write-up. Paste your images at the top of your working document and answer the following questions:

Analyze the Data

Using your 500-mb map, identify a region of upper-level divergence within the Continental United States. Provide both its geographical location (for example, central Illinois) and its position relative to a key feature on the 500-mb map.
In no more that 2-3 sentences, explain why you identified this region as having upper-level divergence. Your explanation must include a description of the behavior of air parcels traversing the upper-level flow.
Looking at the infrared satellite image, how would you characterize the state of the sky in the region that you identified? Is there evidence of clouds?
In no more than 2-3 sentences, explain why your area of interest was clear or cloudy. You must specifically address the role that upper-level divergence plays in creating any clouds that you observed.

The 500 mb height pattern and surface weather

This page reviews and expands upon what we have already covered. Some of it corresponds to some common weather terminology used by forecasters. The height pattern on a 500 mb upper air map can often be used to estimate the the large scale weather conditions at the surface. The simple analysis presented here does not explain everything that may be going on with the weather, especially at small spatial scales, but is very good at providing a snap shot or picture of the general weather conditions over large areas. It will work best in the wintertime in the middle and high latitude regions of the Earth. Thus analysis of a 500 mb height map provides a nice way to "see" the large-scale weather pattern over the United States, or even the entire Northern Hemisphere in winter. Two figures are shown below. The left figure is a simple instructional figure of a typical 500 mb height pattern over the Northern Hemisphere. On the left is the northern hemispheric 500 mb height pattern at 18Z on January 6, 2013, which corresponds with the extreme cold weather over the eastern United States.

First, recall that the height of the 500 mb surface is related to the (density averaged) 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 figure 13.6 above. 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, i.e., the larger the amplitude of a 500 mb wave pattern, the greater the temperature contrast between trough and ridge.

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. Divergence occurs when horizontal winds cause a net outflow of air from a region (more air leaving a vertical column of air than entering), while convergence occurs when horizontal winds cause a net inflow of air into a region (more air entering a vertical column than leaving it). Take a look at this document on dynamical lifting of air to help you to understand what is meant by divergence and convergence, and why divergence in the upper troposphere forces air in a vertical column to rise, while convergence in the upper troposphere forces air in a vertical column to sink. You are not expected to understand why upper level divergence and convergence occur downstream of upper level troughs and ridges respectively, just that it does.

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 the figure below. Thus if a forecaster says that an upper level trough will be "digging" into the local area, you can expect that it will get colder with time and that precipitation may occur just beyond the trough. If a forecaster says that an upper level ridge is building into the local area, you can expect that it will get warmer with time and precipitation is unlikely. If a forecaster says the US is under a zonal pattern, it generally means that about average temperatures are found everywhere and that strong areas of precipitation are unlikely. If a forecaster says the pattern over the US is amplified (or meridional), then more extreme weather is expected, i.e., cold under deep troughs, warm under large ridges, with strong areas of precipitation possible just beyond the positions of troughs.

From the given problem statement,I identified this region as upper level divergence and by looking at satellite images my area of interest is clear.