US Weather map dated April 1, 1997. Time of map is 00:00 Greenwich time (March 31,19:00 EST). The circles are plotted at locations corresponding to weather stations. The degree of fill in the circle corresponds to the fraction of sky covered by clouds at each station. The arrow at each station is flying with the local measured wind. The length and number of flags on the arrow depict the wind speed (measured in knots where 1 knot = 0.5 m/sec). The numbers in green are: temperature (upper left) and dew point (lower left, both in °F). Barometric pressure (reduced to sea level, in 10*[mb -1000] if between 1000 and 1050 or 10*[mb - 900] if between 950 and 999). The blue thin lines are isobars. Note that winds are stronger (more flags on arrows) where the isobars are closer together. Thick blue and red lines are fronts. The map also shows radar echoes (see scale below).
Source: Perdue University, Department of Earth and Atmospheric Science - http://wxp.eas.purdue.edu/