In this class we will be viewing and interpreting what are called 500 mb height maps (mb stands for millibars, which is a unit for measuring air pressure). These maps are very good for getting a large-scale picture of the "weather pattern" over the United States, North America, or even the Northern Hemisphere. 500 mb maps are probably most useful for studying winter time weather patterns in the middle latiutudes (between about 30° and 60° latitude). However, since I think it is important to look at current maps and this class begins during the summer season, we will first focus on using the maps to tell us something about the chances for monsoon season thunderstorms in Arizona. We will return to 500 mb maps again later in the semester to study cooler season weather patterns. As we go through the first part of this course, you will better understand what is plotted on the maps and why the maps look like they do. The purpose of this page is to begin to show you how to interpret the height patterns (contour lines) that are plotted on the maps (see sample 500 mb height map).
With experience one can easily visualize the large scale weather pattern by looking at the 500 mb height pattern. This is nice when looking at computer-generated forecast maps of the 500 mb height pattern predicted for some time into the future to get an idea of what the computer model predicts the future weather to be. By the way all weather forecasting today relies on computer models. This is the main purpose for including the next section. Don't worry, I do not expect you to be able to easily visualize the weather pattern based on 500 mb maps in the short time we are going to cover the topic at the beginning of this semester. After studying over the first couple of reading pages, I would like you to be able to understand the relationship between 500 mb height and temperature, determine the general wind pattern at 500 mb based on the 500 mb height pattern, describe how the 500 mb height pattern changes during the southwest monsoon season, and determine whether or not the daily 500 mb pattern is favorable or unfavorable for the development of thunderstorms during the monsoon season.
If you do a web search for weather maps, you will find hundreds (maybe thousands) of sites containing maps. If you are interested, you should check some out. You should attempt to go through this exercise of looking at maps. If you become interested in looking at these maps, you can continue to visit the site listed below outside of this class.
There is no need to panic if you have trouble following the instructions below and/or understanding the maps that you do see. You will not be tested on your ability to use the web site described below. You should pay attention to the information on Greenwich Mean Time (GMT) given below. I expect that if you are given a weather observation or forecast map with the time given in GMT that you can tell me the local time in Tucson, AZ. All maps and weather charts that we will study in this class will be labeled in GMT.
I suggest using the University of Wyoming's weather model plotting page.
I will give you some directions on how to use the plotting software to make
500 mb maps.
Open another browser window or tab and type in the following address, then follow the instructions:
(Or simply left click on the link at the bottom of this section to open another window)
After you are familiar with how to use the map page, follow the link below to directly connect: University of Wyoming's weather model page
The height contours on the map are actually the height of the 500 mb pressure surface in meters above sea level. The average air pressure near the ground is about 1000 mb, and since air pressure decreases as one moves upward, at some altitude the air pressure will fall to 500 mb. The height above sea level of the 500 mb pressure surface is measured at many locations around the globe by sending instrumented weather balloons upward. The data from around the world is collected and maps of the current 500 mb height are generated. Computer weather forecast models predict the future pattern of 500 mb heights. The actual pattern of the 500 mb heights changes (evolves) daily.
The details of air pressure will be explained in subsequent lectures, so don't worry if you don't understand it right now. A simple analogy with liquid water may help. Water pressure is the force per area exerted by water on any object submerged and surrounded by water. As you dive downward into water, the water pressure increases since there is an increasing weight of water above you (think about swimming downward in a pool of water). The same basic concept happens with air. The highest air pressure is found at the bottom of the atmosphere (at the ground surface) because the weight of all the air in the atmosphere sits above the surface (like at the bottom of a pool of water). Moving upward in the atmosphere, the air pressure decreases since there is less weight of air above (just like swimming upward from the bottom of a pool of water). At the top of the atmosphere, where it merges with outer space, the air pressure falls to zero since there is no longer any weight of air above the top, just like water pressure will fall to zero as you emerge from the top of a pool of water. So again, since the total weight of the air in the atmosphere results in about 1000 mb of air pressure at sea level, there will be some height above sea level where the air pressure is 500 mb.
Notice that the height contours on a 500 mb map are generally in the range 4600 - 6000 meters (see sample 500 mb height map). You may want to left click on the image link and choose to open the link in a new tab or a new window, so that you can view the image as you read the text below. Most commonly, the contour interval (height difference from one contour line to the next) on 500 mb height maps is 60 meters as in the figure above.
The contour maps of 500 mb height are interpreted in the same way as topographic maps of ground surface elevation. The line highlighted in pink on the sample map is the 5700 meter contour line. The height of the 500 mb surface is 5700 for all points along the line. Above (or generally north) of the line the 500 mb heights are lower than 5700 meters and below (or generally south) of the line the 500 mb heights are higher than 5700 meters. I expect that you can determine the 500 mb height at any point on a map like this. Four points A - D are marked on the map. Point A is located about halfway between the 5760 m line and the 5820 m line, so a good estimate of the 500 mb height at that point would be 5790 m. Point B marks the location of Tucson. All we can say here is that the height at point B is greater than 5820 m, but less than 5880 m. The contour lines on these plots are spaced every 60 m. Point B is definitely higher than 5820 m, but is not enclosed by a 5880 m line. Point C is located between the 5640 m and 5700 m lines, but much closer to the 5640 m line, so the height at point C is about 5650 m. Point D is located in the center of a closed low (described below). The 500 mb height at point D is certainly lower than 5580 m, but not at low as 5520 m, since that contour does not show up within the closed low.
For now, I want you to be able to estimate the pattern of air temperatures based on the pattern of height contours shown on the map. The height of the 500 mb surface is related to the temperature of the atmosphere below 500 mb -- the higher the temperature, the higher the height of the 500 mb level. In other words, the 500 mb height at any point on the map tells us about the average air temperature in the vertical column of air between the ground surface and the 500 mb height plotted at that point. The height pattern tells us where the air is relatively cold and where it is relatively warm (see 500 mb side view.) Another way to think about it is that as air is warmed, it expands, and if the air in a vertical column of air is warmed, the column expands upward. Therefore air pressure decreases more slowly as you ascend through a warm column of air, compared to a cold column of air. (See Figure).
Consider what the 500 mb pattern would look like if air temperatures decreased steadily from the equator toward the north pole. (Note this is what you might guess based on the fact that the Sun's heating is strongest toward the south and weakest toward the north.) 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. 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 (See Figure). The more pronounced the ridge (or trough), the more above (or below) average the temperatures will be.
The terminology "trough" and "ridge" is related to the fact that the contour lines often look like waves. A "ridge" is the high point of a wave, and a "trough" is the low point of a wave. A simple diagram is shown below.
One other feature in the 500 mb pattern worth pointing out are closed lows and closed highs. A closed low on a 500 mb height map is a region of low heights around which one or more closed height contours are drawn. A closed contour line is one which closed in on itself, often making a circular or oval shape. A closed low indicates a pool of colder air surrounded by warmer air. Two closed lows are indicated on this sample 500 mb map. Closed lows are most often found near the base of troughs as in the example. Depending of the strength of the closed low there can be more than one closed contour line encircling the center of lowest height, which is sometimes marked with and 'L' on the maps. Closed lows are often associated with precipitation and a change toward colder conditions, and thus are important features in the weather pattern. There are also closed highs, which are centers of high heights surrounded by one or more closed contours. Closed highs are commonly found near the apex of a ridge. Closed highs generally indicate warm and fair conditions.
Having said all that, in the summer months, there is generally much less structure in the 500 mb height pattern as compared with the colder months, thus features like troughs, rigdes, and closed highs and lows are often harder to pick out. The reason for this is simple. The tropics remain warm throughout the year, while there is a huge seasonal change in temperature for locations closer to the north pole. Therefore, in winter there is a very large difference in 500 mb heights between the warm tropics and very cold Arctic, while in summer, there is a much smaller difference in temperature and hence 500 mb heights between the warm tropics and the now relatively warm Arctic. For example compare the average summer 500 mb heights with the average January 500 mb heights given in the links below. Note there is little change between summer and winter in 500 mb height (and thus average temperature) in tropical regions, but a large change between summer and winter at higher latitudes.
Below are some links to the averge long-term 500 mb heights over the United States
for the months June through August. There is also a link for the month of January for
comparison with a winter month. Notice that the average 500 mb height in Tucson for January is about
5700 meters, while in the much warmer months of June and September it is about 5870 meters and in
August it is above 5880 meters. This is an indication that the average air temperature is much warmer
in the summer season compared with the winter season.
June 500 mb height climatology (long-term average) ; June climatology (color-filled image)
July 500 mb height climatology (long-term average) ; July climatology (color-filled image)
August 500 mb height climatology (long-term average) ; August climatology (color-filled image)
September 500 mb height climatology (long-term average) ; September climatology (color-filled image)
January 500 mb height climatology (long-term average) ; January climatology (color-filled image)
To be a little more precise in estimating expected temperature compared to average on any given day, we should compare the actual 500 mb heights from a daily map to the long-term average or "climatological" 500 mb heights. For a given location, if the 500 mb height on the map is close to average, then the temperature is expected to be about average. If the 500 mb height is lower than the average height, then lower than average temperatures are expected. If the 500 mb height is higher than the average height, then higher than average temperatures are expected. The further the 500 mb height is away from average the more the temperature is expected to be away from average. For example, if you compare the actual 500 mb height over Tucson for a given day (say August 20) to the average 500 mb height (from the link above), you can estimate whether or not the air temperature for the day will be above or below average. In fact one of the map products that we will look when disucussing cold seasaon weather are maps of 500 mb height anomalies, which is the difference between the actual 500 mb height on a given day and the average 500 mb for that day.
Here is a link to the current 500 mb height pattern. Notice that the 500 mb height contours are labled with 3 digits instead of 4 digits as seen on the other maps we have looked at. On some maps the last zero in the 500 mb height is not displayed on the contour lines (in other words the units on the contour lines are in decameters instead of meters). Just realize that 500 mb heights will be in thousands of meters above sea level, not hundreds of meters. I expect that you can estimate the 500 mb height over Tucson or any other point on the map based on the contour pattern. You should also be able to tell me if the current 500 mb height is above or below the long-term average 500 mb height for this month.
This is a simplistic method. The 500 mb height actually tells you about the average air temperature in the vertical column of air between the ground surface and 4.6 - 6.0 km (2.9 - 3.8 miles) above sea level. Often this provides a good estimate of how warm or cold the air temperature is near the ground where we live. However, the vertical column of air from ground to 3 miles above sea level does not have to be uniformly warm or cold. There can be smaller (in vertical extent) layers of relatively warm air and relatively cold air. Sometimes there will be shallow (small in vertical dimension) layers of warm or cold air just above the ground. In these cases, the corresondence between the 500 mb height and surface air temperature will not work as well. In addition, factors like cloud cover, precipitation, and the type of ground surface (dry desert, moist soil, snow cover, etc.) also influence the temperature of the air at the surface. A good example of this occurs in Tucson during the summer. The average high temperature in Tucson is higher in June than it is in August, even though the average 500 mb height is lower in June. The reason for this is that most June days are sunny and dry, while during August it is more common for there to be clouds and precipitation, which tend to reduce the high temperature. Thus, using the 500 mb heights to estimate surface temperatures is not exact. However, as you will see, the 500 mb maps often provide a very good overview of the pattern of warm and cold conditions near the ground surface.
The first example is a 500 mb map valid for 06Z (0600 GMT), Thursday, June 23, 2011. You should realize that the local time in Tucson is 7 hours earlier than GMT time, which is 2300 (11 PM local) on Wednesday, June 22. This map has several examples of closed highs and closed lows in the height pattern. Two closed highs are highlighted in yellow and three closed lows are highlighted in green. The height in Tucson is approximately 5930 meters. This can be compared with the average 500 mb height in Tucson of around 5870 meters as shown in the June 500 mb height climatology linked above. Thus, well above average temperatures would be expected in Tucson. The high temperature in Tucson for June 22 and June 23 2011 was 109°F. Concerning other features of interest for the United States. Looks rather warm through the Rocky Mountain states under a 500 mb ridge. Cool with a closed low and trough over the Pacific northwest. Cool also for the great lake states down the Mississippi River valley associated with another closed low and trough.
Below are two examples that were used in previous classes. Both of these examples were taken from the colder season. We are not going to study winter-time maps like this until later in the semester, but I include this information as it may help you better understand how to interpret the relationship between a 500 mb height map and expected temperature. The first is the 500 mb height map for the time 00Z on Monday, October 25, 2004 (This corresponds to a local Tucson time of 5 PM on Sunday, October 24). Next to the 500 mb map is the high temperature relative to average for the day Sunday, October 24. Notice that below average temperatures occured in the western US (associated with the trough over this region), while above average temperatures occured over the midwest southward to the southern great plains and lower Mississippi valley (associated with the ridge over this region), and below average temperatures over parts of the east coast (associated with the trough centered just offshore).
The next example is from January 14, 2007. The 500 mb map is valid for Sunday, January 14, 2007 at 18Z (this corresponds to 11 AM local Tucson time). Next to the 500 mb map is a map showing the surface temperatures across the United States at 20:15 GMT (or 20:15 Z, which is 13:15 (i.e., 1:15 PM) local time). Again, notice that temperatures are cool or cold near the trough in the western United States, for example 42°F in Tucson at 1:15 PM local time. In most of the southeastern United States, temperautures are much warmer in association with a broad ridge and higher 500 mb heights.