Introduction to 500 mb maps

Where to get Maps

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. In this class we will mainly be viewing what are called 500 mb height maps (mb stands for millibars, which is a unit for measuring air pressure). These are very good for getting a large-scale picture of the "weather pattern" over the United States, North America, or even the Northern Hemisphere. As we go through this material, you will better understand what is plotted on the map and why the maps look like they do. You should go through this exercise of looking at maps. I will expect you to be able to answer simple questions about how to do this on the exam.

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 the following link in another web browser. (This will allow you to read and follow the instructions below. You can do this by clicking on the link with the your right mouse button and selecting open link in new window.) University of Wyoming's weather model page

1. Select one of the six forecast models listed to go to the map page
2. Set the Initial time to the latest time and date available
   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 always used on these weather maps. 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 either the United States or North America
4. Set the Forecast to 0 hours to see the conditions at the initial time, 24 hours to see the model forecast one day into the future, 48 hours to see the model forecast two days into the future, etc. If you select "loop", you will see a movie of the forecast.
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

Estimating temperature from 500 mb pattern

The height contours on the map are actually the height of the 500 mb pressure surface 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. Notice that the height contours generally fall into the range 4600 - 6000 meters.

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 directly related to the temperature of the atmosphere below 500 mb -- the higher the temerature, the higher the height of the 500 mb level. We already discussed this when we were going over hurricanes. If you warm a column of air, it expands, therefore air pressure decreases more slowly as you ascend through a warm column of air, compared to a cold column of air. We will review this during lecture.

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. 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 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. Sample maps and diagrams will be used in lecture to help you understand what is meant by this.

To be a little more precise in estimating expected temperature compared to average, we should compare the actual 500 mb heights from a 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.

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 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.

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 5750 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

Finally, here is a link to the latest 500 mb map:
Latest 500 mb map