Updated: May 1, 2019
Constant pressure charts like the 500 mb chart shown below may seem obvious to interpret. It has many features similar to what you might see on the surface analysis chart. There are H's and L's that represent highs and lows and there are lines that resemble lines of constant pressure or isobars. However, those H's and L's are not marking the position of high and low pressure centers and the lines are not lines of constant pressure or isobars. So why is this different from the surface analysis chart?
The surface analysis chart like the one shown below depicts the pressure that is recorded by barometers located at the surface. However, if we plotted the surface pressure strictly from these observations of pressure it would look a lot like a surface relief map or topographic map. That is, higher elevation terrain generally has a lower pressure. So you would simply be plotting the difference in pressure due to the height of the terrain and not due to the relative differences in the pressure.
Consequently, meteorologists adjust the observed pressure at the surface to a height we know as mean sea level. This depicts the surface pressure as if there were no terrain features higher or lower than sea level. In other words, the surface analysis chart depicts how pressure changes while holding the height constant. This is formally described as pressure depicted at a constant geopotential height of zero.
A constant pressure chart is just that...a chart where the pressure is held constant. Unlike the surface analysis chart, the pressure does not vary. What varies on this chart is the height of that specific pressure surface. For example, the solid lines on the constant pressure chart below depict the height of the 500 mb surface. So, why use constant pressure instead of "constant altitude" to depict the weather aloft? It would seem more intuitive to depict how the pressure varies at 10,000 feet instead of the how the height varies at 700 mb. For the last 50+ years, meteorologists have drawn weather maps describing upper air conditions using constant pressure surfaces. These charts are prepared twice a day at 0000 UTC and 1200 UTC for several mandatory pressure levels (925 mb, 850 mb, 700 mb, etc.) from the temperature, humidity and wind data provided by the operational radiosonde network (weather balloons), supplemented with data from other sources such as aircraft and satellites.
Meteorologists use these constant pressure charts rather than constant altitude charts for several reasons.
1. Since most aircraft of the time used pressure altimeters, most "constant altitude" flights were actually flown on constant pressure surfaces. Aircraft today still fly constant pressure surfaces at altitudes at or above 18,000 feet (also known as the flight levels). This is done by using a constant altimeter setting of 29.92. As the pressure changes while en route, the pilot does not adjust the altimeter setting. Instead, they adjust their true altitude up or down in order to fly a constant pressure surface.
2. The radiosonde telemetry data from which the charts are prepared are reported in terms of pressure. As the balloon ascends, temperature, dew point temperature and wind are measured and reported as a function of pressure, not altitude.
3. Use of pressure as the vertical coordinate simplifies many of the thermodynamic equations and computations used in numerical weather prediction models. In other words, when you hold pressure constant it makes these equations easier to solve. And that was real important when computing power wasn't as good as it is today. Therefore, in addition to the charts made from the radiosonde data, forecast data is also depicted on constant pressure charts.
Meanwhile, we're stuck seeing these charts since most forecasters have adapted to them over the years. In addition to the height of the pressure surface, many constant pressure charts available online will also depict the actual or forecast wind speed and direction. Therefore, many pilots do examine these charts for the winds. The Aviation Weather Center provides the winds on constant pressure surfaces without displaying the height contours as shown above. Even though the chart is labeled with a specific mean sea level height (such as 18,000 feet MSL), please be aware that this is only an approximation. This is still a constant pressure chart (note the pressure in parentheses following the altitude). As a result, the true altitude of the winds can vary by 2,000 feet or more. This Aviation Weather Center rendition is best used for aircraft flying at 18,000 feet and higher since they are truly flying a constant pressure surface.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS research meteorologist