Meteorological Charts: Interpretation
These maps provide information on weather conditions at the surface.
For certain variables like air pressure, surface maps are plotted at a constant elevation: sea level. Changes in atmospheric pressure are much larger in the vertical than in the horizontal. If pressure were plotted without correction weather patterns would not be visible as the pressure distribution would simply reflect the terrain. For this reason, the pressure is reduced to sea level so the effects of elevation are eliminated, allowing for a picture of high and low weather patterns at the surface. On surface charts, lines connecting areas of equal pressure are called isobars.
Fig. 3: Surface chart interpretation: pressure reduction to sea level.
Source: The Atmosphere, 8th edition, Lutgens and Tarbuck, 8th edition, 2001.
Upper-air maps provide information on the weather above the surface. These maps are plotted on surfaces of constant pressure (e.g. 200 hPa, 500 hPa, 700 hPa, etc.). Lines on these maps are drawn to show the altitude of the pressure surface, which tends to be lower when the air is colder and higher when the air is warmer (Fig. 2). These lines are called lines of equal height or isoheights. Pressure surfaces tilt down toward colder air. Since cold air is denser, it is more compact, reducing the thickness of the atmosphere below the pressure surface and causing it to slope down.
Wind speeds on upper air and surface charts are displayed in knots (kts), where one knot equals 0.51 m/s, 1.15 mph, and 1.85 km/h. Wind barbs, shown on the right, depict the wind direction and wind speed (rounded to the nearest 5 knots). The longest line (shaft) points in the direction FROM which the wind is blowing.
The shorter lines, called barbs, indicate the wind speed in knots (kt). The speed of the wind is determined by the length and number of barbs (Fig. 1).
Each full-barb represents 10 kts while each half-barb represents 5 kts. A pennant (flag) is used to represent each 50 kt. Added together the barbs and pennants sum to provide the total wind speed at that location.