Introduction to 500 mb height maps

Introduction

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 cool season weather patterns in the middle latiutudes (between about 30° and 60° latitude). This is why we are introducing them at the beginning of the semester. 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 (predicted) forecast maps of the 500 mb height pattern, as it tells us what the computer model predicts the future weather to be. By the way all weather forecasting today relies on computer models. The next section explains how to get current and forecasted 500 mb maps via the University of Wyoming's Atmospheric Sciences webpages. You should go through the exercise of viewing some maps. Don't worry if you do not understand the first few 500 mb height maps that you look at. You are not expected to be able to interpret the weather based on the maps until after you study and read over the explanation provided in the first few reading pages.

After studying over the first couple of reading pages, you should:

• Be able to closely estimate the 500 mb height at all places on the map (for example at the location of Tucson)
• Understand the relationship between 500 mb height and air temperature
• Be able to identify common features on 500 mb height maps, which include troughs, ridges, closed highs and closed lows
• Be able to determine where it is relatively cold and where it is relatively warm
• Be able to determine the general wind pattern at 500 mb based on the 500 mb height pattern
• Be able to determine regions that are favorable and unfavorable for precipitation based on the 500 mb height pattern

A Quick Look at 500 mb Weather Forecast Maps

There are many web sites that provide 500 mb weather forecast maps. One will be introduced in this section, and we will use a few others during the first part of this semester. If you become interested in looking at 500 mb weather forecast maps, then you are encouraged to check out some other sites. The purpose of this exercise is simply for you to produce a set of 500 mb height maps that are forecast by a computer weather model. Other than very short term weather forecasting, all weather forecasting beyond a few hours into the future is based on computer model forecasts.

You should attempt to go through this exercise of paging through some 500 mb forecast maps. 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. The material on how to interpret the 500 mb height maps is not covered until the next section below. You should pay attention to the information on Greenwich Mean Time (GMT) given below. All weather products are standardized and labeled in GMT. You should realize that Tucson local time is always 7 hours EARLIER than GMT.

We will use the University of Wyoming's weather model plotting page for this exercise. Some directions on how to use the plotting software to generate 500 mb maps height maps are given below. You may wish to left click on the link below to open it in another tab or browser window so that you can follow the instructions below:
University of Wyoming's weather model page (http://weather.uwyo.edu/models/ ).

1. Select one of the four forecast maps listed. Most often in this class we will use the "GFS Model" or "Medium Range Forecast" (MRF) Model. These are the operational weather forecast models run by the United States. The GFS forecasts are shown every 6 hours out to 240 hours (10 days) into the future, while the MRF forecasts are shown every 12 hours from 192 hours (8 days) to 384 hours (16 days) into the future. The other two models make shorter term forecasts out to only 12 and 48 hours.
2. Set the Initial time to the latest time and date available using the drop down menu under "Initial Time"
   Weather observations from around the whole world are used in forecast models, so everything must be coordinated to a standard time. The standard is to use Greenwich Mean Time (also called Zulu time) in military format (00-24 hours each day). The Zulu time (labeled with a Z) is used on weather maps generated by the Univerisity of Wyoming. The Tucson local time is 7 hours earlier than Zulu time.
   Some examples:
   At 12Z, it is 0500 hours in Tucson (5AM).
   At 00Z, it is 1700 hours in Tucson (5PM) the day before. This is a little tricky, but 00Z is midnight Zulu time and 7 hours before midnight is 5PM the day before.
   
3. Set the Region to North America, which is the region we will most often use.
4. Set the Forecast to "loop". This will generate a movie loop of the forecast going forward into the future. If you select a specific forecast time, only a single map at specific time will be generated.
5. Set the Level to 500 mb
6. Until you are more familiar with the maps, you should make the following selections to see the 500 mb height pattern:
   First color fill --> None
   Second color fill --> None
   First contour --> Heights
   Second contour --> None
   Vector --> None
7. Once all selections are made, click GET DATA to produce the map or movie.

If you selected "loop" you can control the movie using the buttons at the bottom of the screen. Pause the movie on a single frame. The time label under the 500 mb height map gives information about the forecast. The first part of the label "TTT Hour 500 hPa Forecast" tells you how many hours into the future this forecast was made. For example 120 Hour Forecast means the forecast map is 120 hours (5 days) after the computer model began running its forecast. The 00 Hour Forecast map is not really a forecast at all, but shows the measured conditions at the time the model started. 500 hPa stands for 500 hectopascals, which is the internationally standard unit for air pressure, and is the same the millibar (mb) that is used in the United States and in this course. The second part of the label at the bottom of each map, "Valid HHZ date", gives the date and time in GMT for the forecast. Remember HHZ is Zulu time (same as GMT) in hours, from 00Z (midnight) to 23Z (11 PM). You need to realize that the local time in Tucson is 7 hours earlier than the time labeled on weather maps.

What is the 500 mb Height? Reading 500 mb Height from Maps

The contours on the map are actually the altitude of the 500 mb pressure surface in meters above sea level (mb stands for millibars, which is a unit for measuring pressure, just like meters is a unit for measuring distance). The average air pressure near the ground at an elevation of sea level is about 1000 mb (you should remember this number), and since air pressure decreases as one moves upward in the atmosphere above the ground (you should remember this as well), at some altitude the air pressure will fall to 500 mb. At the top of the atmophere the air pressure is zero. The height above sea level where the air pressure falls 500 mb 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 further explained in subsequent lectures, so don't worry if you don't fully 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 ground (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.

The air pressure at any point in the atmosphere is caused by the weight of the air above that point. Thus, the weight of the total atmosphere results in an average air pressure of 1000 mb at sea level. Since 500 mb is one half of 1000 mb, this means the weight of air above the 500 mb pressure level is one half of the total weight of the atmosphere. Another way to think about this is that the 500 mb height is a measure of the height of the lower half of the atmosphere, i.e., the lower half in terms of weight. The 500 mb height varies with both location on Earth and time. A given 500 mb height map is a snapshot of the 500 mb height pattern at the time specified on the time label on the map. Note there is not a time stamp label at the bottom of the sample 500 mb height map shown below. The pattern of the 500 mb heights can be used to interpret weather conditions at the surface.

Example 500 mb map showing height contours

The height contours on a 500 mb map will generally be in the range from 4600 - 6000 meters. 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. 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 above 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 inside the closed 5580 contour, which is called a closed low since the lowest heights in region near point D are inside the 5580 m contour.

The Relationship Between the 500 mb Height Pattern and Air Temperature

We will now use the 500 mb height contour pattern to estimate the pattern of air temperature. 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, which raises the 500 mb height (and increases the height of the bottom half of the atmosphere). Therefore air pressure decreases more slowly as you ascend through a warm column of air, compared to a cold column of air. (See Figure showing air expands when warmed and contracts when cooled). Conversely, when air is cooled, it contracts, and if the air in a vertical column gets colder, the column contracts downward, which lowers the 500 mb height (and decreases the height of the lower half of the atmosphere).

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.

Troughs, Ridges, and Closed Contours

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 contours. A closed contour line is one which closes in on itself, often making a circular or oval shape. There are both 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. The center of a closed low is a center of low 500 mb height, which is sometimes marked by an "L." A closed low indicates a pool of colder air surrounded by warmer air. Two closed lows are indicated by L's on this sample 500 mb map highlighting closed contours. Closed lows are most often associated with troughs as are the two marked in the example, but not always. Depending of the strength of the closed low there can be more than one closed contour line encircling the center of lowest height. 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. The center of a closed high is a center of high 500 mb height, which is sometimes marked by an "H." Closed highs are commonly associated with ridges, but not always. There is one closed high marked on the sample map linked above. Depending of the strength of the closed high there can be more than one closed contour line encircling the center of highest height. Closed highs generally indicate warm and fair conditions. A 500 mb map containing several closed highs and closed lows is shown below. There are two closed highs on the map: a 5820 meter closed high in the Pacific Ocean and a 5880 meter closed high centered over Cuba. There are several closed lows on the map below. The two most southerly of these are a 5340 meter closed low over Utah and 5220 meter closed low off the east coast of North America. Notice that the 5220 meter closed low also has closed contours at 5280, 5340, 5400, and 5460 meters.

Some students have trouble distinguishing closed lows from closed highs. Start at the center of a closed contour. As you move away from the center, determine if the 500 mb heights are increasing (a closed low) or decreasing (a closed high). If there are multiple closed contours around the center, then it is quite easy to tell if the heights are getting larger or smaller as you move outward. For a single closed contour, it can be more difficult to tell. Continue moving outward past the closed contour until you hit the next contour line. For closed lows, the adjacent contour lines will be the same and higher heights than the closed contour line, while for closed highs, the adjacent contour lines will be the same and lower heights than the closed line.

Example 500 mb map to showing several closed lows and closed highs

Monthly Average 500 mb Heights

Using 500 mb Height to Estimate Local Temperature

Large scale features like troughs and ridges provide a look at the general temperature pattern, i.e., cold in troughs and closed lows, warm in ridges and closed highs, and near average in flat height patterns without pronounced troughs and ridges. If you want to make a specific temperature forecast for a given location, like Tucson, you should compare the 500 mb heights from a current or forecast map to the long-term average or "climatological" 500 mb heights for that day. Long-term average maps for the months January - May were shown in the section above. For a given location and time of the year, 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. Generally speaking, when 500 mb heights are within about 20 meters of average expect near average temperature, when 500 mb heights are 30-60 meters above or below average, expect moderately above or below average temperature, and when 500 mb heights are 80 or more meters above or below average, expect well above or below average temperature. For example, if you compare the actual (measured) 500 mb height over Tucson for a given day (say January) to the average 500 mb height, which is about 5680 meters for Tucson based on the January map above, you can estimate whether or not the air temperature for the day will be above average, below average, or near average. Similarly, if you compare a forecast of the 500 mb height over Tucson (from a weather forecast model) to the average, you can determine if the model is predicting above or below average 500 mb height and temperature.

Keep in mind that this is a way to estimate the local temperature relative to the average air temperature for that location and time of year. The 500 mb height by itself does not tell us what the exact air temperature will be at the ground surface. This is because there are many local factors that go into determining the surface air temperature, such as the sun angle (which is more direct for locations closer to the equator), type of ground surface (desert rock and sand warms more quickly than wet soils), amount of water vapor in the air (dry desert air warms more easily during day and cools more easily at night compared with more humid air), and other factors. Thus the same 500 mb heights over two locations does not mean those two places are expected to have the same near ground air temperature. The best we can do is say whether or not the local temperature is expected to above or below the local average for that day. Returning to the example posed in the last paragraph. Suppose on January 15, 2016, the 500 mb map shows that the 500 mb height over Tucson is 5740 meters above sea level. Since this is higher than the average for Tucson in January, which is 5680 meters, then we expect the the air temperature in Tucson to be above average for the day. You could then look up the fact that the average high temperature for Tucson in mid January 66°F. Based on the 500 mb height, you expect a warm day, with a high temperature above 66°F.

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. It is 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 determine if the current 500 mb height is above or below the long-term average 500 mb height for this month.

Note of Caution:

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 the sun angle, 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. 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.

Examples:

Do you remember the extreme cold outbreak that took place over the eastern part of the United States in early January of 2014? The 500 mb height map for January 6 at 18 Z (11 AM local time in Tucson) is shown below. The map on the right side is the color-filled contour image. Notice the large trough covering much of the eastern part of the United States. The closed low centered over lake Michigan shows a 4980 meter contour. It is rare for the continental US to have 500 mb heights below 5000 meters. Based on the January 500 mb height climatology map discussed above, the average 500 mb height over lake Michigan is about 5400 meters. Thus, the 500 mb height over lake Michigan on January 6 was amazingly more than 400 meters below average! If you remember this trough was resonsible for record cold temperatures over much of the eastern United States. At the same time, there is a strong ridge over the western United States, which produced above average temperatures. I would estimate the 500 mb height over Tucson to be about 5780 meters. This is about 100 meters above average and thus much above average temperature would be expected in Tucson based on the map.

Below are two examples that were used in previous classes. 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.