The Synoptic Weather Map

FIGURE 4-21 A surface weather chart for 1200 GMT (700 EST), March 5, 1964, showing information available at each observation station. The chart also shows high- and low-pressure areas, fronts, and isobars constructed by interpolation between stations. (900 or 1000 millibars need to be added as appropriate to values see text. Adapted from FAA, 1965). 

Cloud cover, surface wind direction and speed, temperature, dew point, and precipitation are indicated by symbols at each weather reporting station. Interpretation of the weather in a region is facilitated by the identification of high-pressure and low-pressure areas, the delineation of frontal systems, and the construction of isobars. Isobars are lines of constant atmospheric pressure that are interpolated between observation points, and are so named because pressure often is presented using the unit of the bar, which is equivalent to 106 dyn/cm2 or 0.9869 atm. On some maps, such as shown in Fig. 4-21, pressure is given in millibars as a shorthand notation, only the last two digits are presented. The user must add either 900 or 1000 millibars to the value shown, whichever brings the value closest to 1 bar. 

The data presented in weather maps are relevant to chemical transport both at the local scale, where local weather governs atmospheric mixing, and at the regional scale, where synoptic-scale circulation governs the long-range transport of chemicals across state and national boundaries. 

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