Meteorologists use specialized charts to visualize the three-dimensional structure of the atmosphere. While many people are familiar with surface weather maps that plot conditions at ground level, an isobaric map provides a unique view by holding one variable constant: atmospheric pressure. By focusing on a constant pressure level, scientists can simplify the complex fluid dynamics of the atmosphere, making it easier to analyze the flow and movement of weather systems far above the surface.
Defining Constant Pressure Surfaces
An isobaric map differs fundamentally from a constant height chart, such as a sea-level pressure map. On an isobaric map, the pressure is fixed, and the corresponding altitude varies across the map, whereas on a constant height chart, the elevation is fixed and pressure changes. These charts use standard pressure levels like 850 millibars (mb), 700 mb, or the commonly used 500 mb, which represents the approximate middle of the atmosphere. The 500 mb level averages around 5,500 meters (18,000 feet) above sea level, but its actual altitude changes depending on the air mass above it.
This framework is highly useful because the atmosphere’s large-scale flow aloft tends to follow the height contours of these constant pressure surfaces. Air parcels at these altitudes are not significantly affected by surface friction, allowing the wind to blow nearly parallel to the lines drawn on the map. Analyzing the conditions on these standard pressure surfaces helps forecasters determine the overall steering mechanism for surface-level weather phenomena, such as low-pressure systems and fronts.
The Primary Reading: Geopotential Height
The Geopotential Height represents the actual altitude above sea level where the specified pressure level occurs. These heights are drawn as contour lines and are measured in meters or decameters. Because warmer air is less dense, the constant pressure surface will be found at a higher altitude in warm air masses and a lower altitude in cold air masses.
The pattern of these height contours reveals the large-scale undulations of the upper atmosphere. Elongated areas where the height contours curve poleward are called ridges, which generally indicate warm, stable air and fair weather. Conversely, areas where the contours dip equatorward are called troughs, marking regions of relatively cold air and often unsettled weather. The spacing of these contours directly correlates to wind speed: tightly packed contours show a steep pressure gradient, indicating stronger winds, while widely spaced lines suggest lighter winds. This pattern of troughs and ridges determines the overall track of surface weather systems.
Additional Meteorological Variables
While the height contours provide the atmospheric steering information, other variables are overlaid on the isobaric map. Temperature is commonly plotted using isotherms, which are lines connecting points of equal temperature at that specific pressure level. By observing where the isotherms cross the height contours, meteorologists can identify advection—the transport of warm or cold air—which influences the development of weather systems.
Wind information is displayed using wind barbs, which show both the wind direction and speed at various points on the map. The shaft of the barb points in the direction the wind is blowing from, and the attached feathers or flags indicate the speed in knots. For a generalized view of wind speed, some maps include isotachs, which are lines connecting areas of equal wind speed, often used to locate the core of the jet stream. Moisture is often shown through isodrosotherms, which connect points of equal dew point temperature, or by shading areas of high relative humidity. This information is critical for predicting cloud cover and the potential for precipitation.