8.1: Pressure Levels and Heights

Pressure Levels

Rather than looking at a constant elevation, Meteorologists (and aviators for that matter) focus on constant Pressure Levels – a surface in the atmosphere where air pressure remains constant. There are several heights of importance, but for this investigation, we are going to focus on two particular pressure levels: 500 milibar (mb) and 300 milibar (mb). The 500 mb height is great at locating waves in the large scale wind flow (such as troughs and ridges) while the 300 mb level is great at locating the jet stream. Figure 8.1.1 is a map of the 500 mb pressure level over the United States, taken at 1200 UTC on February 28, 2025.

Figure \(\PageIndex{1}\): 500 milibar air map for 1200 UTC on February 28th, 2025. (CC BY-NC 4.0; American Meteorological Society via Unidata) Alternative description of image.

 

1. Some similar features to this map, when compared to a surface map (you could view one here: surface maps) include: 
   1. Wind Direction/Wind Speed are indicated at individual stations
   2. Air Temperature is indicated at individual stations
   3. Dew Point Temperature is indicated at individual stations
   4. All of these are true
2. On the other hand, a few big differences between a surface map and this upper-level map include:
   1. The absence of cloud cover and precipitation information
   2. The absence of recorded pressure values (and instead, replaced with height contours)
   3. Temperature and Dew Point are indicated using Celsius instead of Fahrenheit (hint: Figure 8.1.2)
   4. All of these are true 

If you notice, many symbols should look somewhat familiar, and others that appear quite different. One thing that should stick out to you pretty quickly is the presence of Station Models. These station models are extremely similar to the ones we’ve seen at the surface, with only a few key differences. Figure 8.1.2 breaks down how to read these station models.

Figure \(\PageIndex{2}\): How to read an upper-air station model. (Public Domain; Alicia Mullens) Alternative description of image. 

The main differences to note are that temperature and dew point are in °C (rather than Fahrenheit) and the three-digit number on the right now represents an altitude (remember, everywhere on an upper air map is going to have the same air pressure, so the altitude represents at what height the pressure level is). 

3. According to Figure 8.1.1, Oakland, in the San Francisco Bay Area, had an air temperature _______________ at 500 mb:
   1. -37°C       
   2. -18°C   
   3. -571°C   
   4. -65°C
4. Oakland’s dew point temperature at 500 mb was: 
   1. -37°C       
   2. -18°C   
   3. -571°C   
   4. -65°C
5. The winds at Oakland were coming from the Southeast at:
   1. 10kts       
   2. 15kts   
   3. 30kts   
   4. 60kts. 
6. The three-digit number in the upper-right-hand side of Oakland’s station model represents an altitude in tens of meters (to convert to meters, simply place a “0” on the right end). The three-digit number says 590, representing an altitude of:
   1. 571 meters   
   2. 5710 meters       
   3. 571,000 meters   
   4. 710.0 meters

Height Lines

If the blue lines you see on Figure 8.1.1 have you thinking of isobars, you’re in the right direction! However, since this map is for a 500 mb surface, everywhere on the map has a pressure of 500 mb. However, not everywhere will have 500 MB present at the same height. As a result, the blue lines on this map function almost like a topographic map, providing us with elevations at the 500 mb pressure level.

7. The location with the highest 500 mb height on Figure 8.1.1 is Miami, Florida, a warm town in Southern Florida, with a 500 mb height of:
   1. 5840m       
   2. 5830m       
   3. 5850m   
   4. 5820m
8. The location with the lowest 500 mb height on Figure 8.1.1 is Caribou, in extreme northern Maine, with a 500 mb height of:
   1. 5120m       
   2. 5130m       
   3. 5140m   
   4. 5160m
9. In general, the pattern of the blue height lines indicate that the highest heights are located:
   1. At lower latitudes       
   2. At higher latitudes 

To answer the next two questions, consider Miami and Caribou, from the questions above. At the time of Figure 8.1.1 (1200 UTC on 2/28/2025), Miami had a temperature of 76°F, and Caribou had a temperature of 14°F 

10. Higher 500 mb heights are present in:
    1. Warmer Parts of the United States    
    2. Cooler Parts of the United States
    3. There is no relationship between temperature and 500 mb heights
11. Lower 500 mb heights are present in:
    1. Warmer Parts of the United States
    2. Cooler Parts of the United States
    3. There is no relationship between temperature and 500 mb heights
12. The relationship between Miami and Caribou’ surface temperature and 500 mb heights, combined with the general pattern found in Question 9, tells us that: 
    1. Warmer Surface Temperatures generally result in higher 500 mb heights
    2. Warmer Surface Temperature generally result in lower 500 mb heights