Review 500 mb maps

Introduction to cold season 500 mb maps

In this class we will mainly be viewing what are called 500 mb height maps (recall that mb stands for millibars, which is a unit for measuring air pressure). When you encounter the words "500 mb map" in the reading pages, you should say "500 millibar map." 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. The pattern of the 500 mb heights can be used to interpret weather conditions at the surface. We have already looked at 500 mb height maps in relation to the southwest monsoon season. Much of the information below is a review of what was covered at the beginning of the semester.

500 mb maps are best for studying cool season weather patterns in the middle latiutudes (between about 30° and 60° latitude). This is why I waited until mid to late October to revisit this topic. We are going to use 500 mb height maps to look at the current and forecasted weather both in Tucson and throughout the continental United States. This time through the material we will pay more attention to the trough and ridge patterns on 500 mb height maps.

To begin we review how to interpret the height patterns (contour lines) that are plotted on the maps. 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.

Review: 500 mb Height and 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. Thus the contour lines measure a length (or distance), which is height above sea level where the air pressure is 500 mb. 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. Twice per day (at 00Z and 12Z), helium-filled weather balloons are sent upward from many locations around the globe. These balloons carry instrument packages that measure things like air pressure, air temperature, and winds as the balloons float upward. The altitude where the measured air pressure is 500 mb is simultaneously collected at many locations around the globe. This data from around the world is gathered to produce contour maps of the current 500 mb height. The 500 mb height above any location on Earth will typically range from 4600 meters up to 6000 meters above sea level. Computer weather forecast models predict the future pattern of 500 mb heights. The actual pattern of the 500 mb heights changes (evolves) daily.

The first step is to be able to determine the 500 mb height at all points on a 500 mb height map. A 500 mb height map from 12Z on September 25, 2017 is shown below. The labeled contours on the map are in units of decameters (1 decameter = 10 meters). Just add a zero to the labeled contours for the 500 mb height in meters. The contour interval (height difference from one contour line to the next) on the 500 mb height map shown above is 3 decameters (abbreviated as 3 dam) or 30 meters (abbreviated as 30 m). Contour maps of 500 mb height are interpreted in the same way as topographic maps of ground surface elevation. Every point on the same contour line has the same 500 mb height. For example, locate the 570 dam contour line on the map below. This line has the numerical label '570' in the Pacific Ocean off the coast of Canada and another '570' label in the state of Colorado. Moving from west to east, this 570 dam line runs from the northern Pacific Ocean, through the state of Washington, bottoming out in northern Arizona. The 570 dam line then curves back toward the northeast, crossing back into Canada before exiting Canada in the northern Atlantic Ocean. Every location on this line has a 500 mb height of 5700 meters above sea level. Note that above (or generally north) of the 570 dam line, the 500 mb heights are lower than 5700 meters, and below (or generally south) of the 570 dam line, the 500 mb heights are higher than 5700 meters.

500 mb height map at 12Z on September 25, 2017. Height contours on this map are labeled in decameters (dam). Simply add a zero to the end of each number to get the 500 mb height in meters (m). For example 576 dam = 5760 m. The color shading shows the height anomaly in dam, which is the observed 500 mb height minus the average 500 mb height for each location. Four points, a, b, c, and d have been marked on the map and are discussed in the text.

Four points, a, b, c, and d are indicated on the map. Point a, which is the position of Tucson, sits right on the 576 dam contour line. Thus, the 500 mb height at point a is 5760 meters above sea level. Point b is located between the 579 dam and 582 dam contour lines, which means that the 500 mb height at point b is higher than 5790 meters, but lower than 5820 meters. Since it appears that point b is about halfway between the contour lines, we can estimate the 500 mb height as 5805 meters at point b, which is halfway between 5790 meters and 5820 meters. Points c and d are located within closed contours. Closed contour lines literally close onto themselves, often forming irregularly shaped circles or ovals. Point c is located within a closed 564 dam line. Notice that the 500 mb heights are getting lower as you move toward the 564 dam closed contour and point c. Thus, the 500 mb height at point c must be lower than 564 dam, but higher than 561 dam, which would be the next lower contour. A good estimate for the 500 mb height at point c would be 5630 meters above sea level. Point d is located within a 588 dam line. Notice that the 500 mb heights are getting higher as you move toward the 588 dam closed contour and point d. Thus, the 500 mb height at point d must be higher than 588 dam, but lower than 591 dam, which would be the next higher contour. A good estimate for the 500 mb height at point d would be 5890 meters above sea level.

Review: Estimating the temperature pattern from the 500 mb height pattern

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.) This basic concept was covered earlier in the semester. Warmer air is more spread out (less dense) compared with colder air, which is more compact (more dense). As a column of air is warmed, it expands, therefore air pressure decreases more slowly as you ascend through a warm column of air, compared to a cold column of air. 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" (pronounced trôf) 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 on the left. The figure on the right shows the 500 mb height map for September 25 with the positions of two ridges and one trough marked.

Idealized 500 mb height pattern. The 500 mb height pattern is commonly wave-like. High points in the waves are called ridges and low points in the wave are called troughs. Generally ridges correspond with relatively warm air temperature, while troughs correspond with relatively cold air.

Same 500 mb height map shown above for 12Z on September 25. The positions of two ridges and one trough are indicated in green. In addition, three closed lows and one closed high are labeled.

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. A closed low indicates a pool of colder air surrounded by warmer air. 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, but not always, associated with a trough. The map above on the right shows three closed lows, one over the Pacific Ocean near Hawaii (inner contour line 582 dam), one associated with the large trough over the western United States (inner contour line 564 dam), and one in the Atlantic Ocean, which is associated with the remnants of Hurricane Maria that previously devasted Puerto Rico.

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, all adjacent contour lines will be the same or higher heights than the closed contour line. If any adjacent contour line is lower than the closed contour, then it must be a closed high, not a closed low. Find the marked closed low located over the western United States centered over the state of Wyoming on the map above on the right. Start at the center of the closed contour and move outward. After crossing the closed 564 dam closed contour, the next adjancent contour lines are 564 dam to the north and 567 dam to the south. Becasue there are no adjacent contour lines that are lower than the closed contour, this is a closed low. You can also determine that it is a closed low because one of the adjacent contour lines is higher than the closed contour.

For closed highs, all adjacent contour lines will be the same or lower heights than the closed contour. If any adjacent contour line is higher than the closed contour, then it must be a closed low, not a closed high. Find the large 588 dam closed high on the map above on the right. Start at the center of the closed high and move outward. After crossing the 588 dam closed contour, the next adjacent contour lines are 585 dam. Since all adjacent contour lines are lower than the closed contour, this is a closed high. I realize that you are not experts in analyzing 500 mb height maps, so on exams, you will not be given maps with closed contours that are difficult to specify as closed highs or closed lows.

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 height from a current or forecast map to the long-term average or "climatological" 500 mb height for that day. Maps of the average 500 mb height for the months of October through December are linked below. 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.

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. One reason is because there are many local factors that go into determining the surface air temperature, such as the 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.

Let's use the 500 mb map from 12Z, September 25, 2017 as an example. The 500 mb height over Tucson is 5760 meters, while the average 500 mb height over Tucson during this time of year is about 5860 meters based on the September 500 mb height climatology (long-term average). Since 5760 meters is lower than the average of 5860 meters, below average temperature is expected in Tucson. In fact the high temperature on September 25, 2017 was 85°F and the low temperature was 56°F, which are lower than the average high and low temperatures for September 25, which are 92°F and 65°F respectively.

Below are some links to the averge long-term or climatological 500 mb heights for North America for the months October, November, and December.
October 500 mb height climatology (long-term average)
November 500 mb height climatology (long-term average)
December 500 mb height climatology (long-term average)

Notice (from looking at the links above) that the average 500 mb heights get lower as the winter season approaches. For example, over Tucson, the average heights fall from about 5800 m in October to about 5750 m in November to about 5700 m in December. This corresonds with the fact that the average air temperature decreases from October through December.

Here is a link to the current 500 mb height pattern over the United States with the height contours labeled in decameters. The contour spacing on this map is 6 dam (60 meters). It is expected 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 the corresponding month. Based on this information, you can make a prediction for below or above average temperature.

In order to make comparisons to the average easier, most of the 500 mb height maps that we will use in this class directly show the difference between the 500 mb height plotted on the map and the long-term average 500 mb height for that location. Scroll up to the first map shown on this page, which is for 12Z on September 25. The labeled contour lines show the 500 mb height pattern at the valid time marked on the map. The color shading, with the color key at the bottom, shows the 500 mb height anomaly, which is defined as the 500 mb height on the map minus the average 500 mb height at each location. Positive height anomalies, which are shown using red and orange colors, indicate locations where the 500 mb height is above average. In these areas, above average temperature is expected. The more above average the heights, the more above average is the expected temperature. Negative height anamolies, which are shown using blue and purple colors, indicate locations where the 500 mb height is below average. In these areas, below average temperature is expected. Notice that in general, below average heights are associated with troughs and closed lows and above average heights are associated with ridges and closed highs as expected. However, troughs and ridges are defined based on the shape of the contour pattern, not based on the height anamoly. Thus, it is possible for there to be above average heights in troughs and below average heights in ridges, though not common. If available, it is best to consider the height anamoly, rather than just the positions of troughs and ridges, when estimating the temperature relative to the average temperature for a given location. As an example, consider Tucson, which is marked by point a on first September 25 map shown above. Based on the color shading for height anomaly, the height in Tucson was about 10 decameters (100 meters) below average and below average temperature is expected. This can also be determined by comparing the 500 mb height over Tucson, which is about 5760 meters, with the average 500 mb height for the month of September, which is about 5860 meters based on the long-term average map for September.

Examples:

To give you an idea about how the 500 mb heights are related to temperature, we will look at the 500mb map for October 19, 1952. On that day the record high temperature for Tucson (for October 19) was recorded. It reached 99°F. The 500mb height over Tucson on October 19, 1952 was about 5900 m. This is well above the average of 5800 m for the month of October. The unusually high 500 mb height, corresponds with a very warm day.
500 mb map for October 19, 1952

A second example was a local cold weather event from fall 2011. The high temperature on campus Friday, October 7, 2011 was 72°F, which was 16°F below the average high temperature for the date. The map below shows the 500 mb height pattern at 00Z, October 8 (local time 5 PM October 7). The 500 mb height over Tucson was about 5640 meters, which is well below the average of 5800 meters for the month of October. The unusually low 500 mb height corresponded with a very cold day.
500 mb map for 00Z October 8, 2011

Below are two additional 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.

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 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. Some other reasons for this are described in the next section. However, as you will see, 500 mb height maps often provide a very good large-scale picture of the pattern of warm and cold conditions near the ground surface.

Thickness: A better estimate for air temperature near the ground

This section was included at the bottom of the page on the kinetic model for gases. It is repeated below. This section is included to provide a more complete discussion of the relationship between 500 mb heights and inferred surface air temperature. We will not use thickness maps in this class, so please don't be overly concerned if you are unable to follow the discussion in this section.

Although we will continue to use the 500 mb height to estimate the pattern of air temperature in the lower troposphere (just above the ground surface where we live), it is not exact. In this section, we will discuss a couple of reasons why the 500 mb height is not completely determined by the air temperature in the lower troposphere. In other words, we will point out why the 500 mb height can sometimes be misleading with regard to air temperature just above the ground.

One issue is that the 500 mb height does depend on the sea level air pressure, i.e., the air pressure at ground level. While the average air pressure at sea level is 1013 mb, the actual sea level pressure at a given location and time varies, typically within the range from 983 - 1043 mb. Thus relatively high sea level pressure tends to raise the 500 mb height since the pressure drop from sea level pressure to 500 mb is greater compared to a case where the sea level pressure is lower. To overcome this dependence on sea level pressure, meteorologists often use a measurement called 1000 mb to 500 mb thickness. This is a measure of the vertical distance between the 1000 mb pressure surface and the 500 mb pressure surface. The 1000 - 500 mb thickness is directly related to the average air temperature between 1000 mb and 500 mb without being influenced by variations in surface pressure (see 1000 - 500 mb thickness figure). Therefore, thickness is a better indicator of how warm or cold the air is in a vertical column above a given location.

Another potential problem for interpreting surface air temperature even if using the 1000 - 500 mb thickness is that the 1000 - 500 mb thickness is generally in the range from 4800 to 5700 meters (or about 3 to 3.5 miles). This entire vertical extent need not be uniformly warm or cold, i.e., there can be sublayers of smaller vertical extent that are both relatively warm and relatively cold. For surface temperature, we need to know how warm or cold it is at the bottom of the air column. A better estimate of surface temperature could be obtained by looking at the thickness of a shorter column of air just above the ground surface. For this purpose, meteorologists will look at the 1000 - 850 mb thickness, which is more closely related to the air temperature just above the ground (see 1000 - 850 mb thickness figure).

In general, the vertical height between any two pressure levels in the atmosphere is related to the temperature of the air between the two pressure levels. The warmer the air, the lower the air number density, and the greater the vertical distance between the two pressure levels.