One of the main points of a surface weather map is that it displays a considerable amount of information about the weather from a large area at the same time. Sometimes the word "synoptic" is used to describe a weather map. This term is derived from the Greek words "syn" = same or together; "optic" = visible; hence, seen together. Since weather systems move and evolve, "snapshots" in time are needed. As a result, synoptic weather analysis requires the simultaneous and systematic observation of various weather elements at many widely located sites using standardized instruments and observation techniques.
The first weather map was drawn in 1819 showing a powerful European storm in 1783. Only after the electric telegraph became widely available in the mid 19th century could current daily weather maps become a reality. So when you display a DataStreme map, say for 00Z, all the information that appears on that chart was observed at midnight Greenwich time, or 7:00 PM EST (8:00 PM EDT). Only the plotted positions of the fronts may be several hours old. You should always check the legend to find the observation times and the time when the fronts were plotted.
Since weather elements are interrelated, the modern surface weather chart typically incorporates several observed weather elements plotted simultaneously at each station. These plots would include air temperature, dewpoint temperature, air pressure, sky cover and wind information (wind speed and direction). To display all the observed weather information for many locations at one given time would be difficult unless a uniform system of plotting were adopted. The pictorial presentation and weather data together with an analysis can be determined at a glance. The location of each reporting station has been printed on the base maps as a small circle. The data submitted by each reporting station are plotted around these circles on these base maps in a particular systematic fashion called a "station model". While data are collected hourly from hundreds of weather stations, only a very limited number of station plots appear on the DataStreme maps for ease of interpretation.
Weather maps used in the DataStreme Project contain abridged station models, where the following conventions are used.
Using a clock analogy, the arrangement of observed weather elements plotted around the station model would include the air temperature at the 10 o'clock position; the sea level corrected air pressure would be located at the 2 o'clock position and the dewpoint temperature (an indicator of the water vapor in the atmosphere) is at the 8 o'clock position. If some significant weather phenomenon were observed, such as precipitation or some obstruction to horizontal visibility (fog or blowing snow), then a special symbol would be plotted in the 9 o'clock position. A list of symbols appears in the Homepage Users Guide or you can click on the highlighted "DataStreme Weather Map Symbols" entry on the DataStreme Homepage. Inside the circle of the station model, the sky or cloud cover is indicated by the amount of shading. No shading would mean clear conditions, while a circle completely shaded indicates overcast conditions.
The wind direction is provided by the orientation of the plotted wind arrow on the map. The wind arrow with feathers can be thought of as the back portion of an arrow that would "fly with the wind". In other words, the tail is on the upwind side of the station, while the small circle at the head of the arrow is located at the station. Thus, the orientation of these wind arrows on the map indicates the wind direction to the nearest 10 degrees, measured clockwise from true north (defined as 360 degrees, and located at the top of the chart). By meteorological convention, the winds are named for the direction from which they are blowing. Hence, a south wind is from the south. A concentric circle drawn around the station model with no wind arrow indicates calm conditions (no perceptible wind).
The number and length of the barbs on the tail of the arrow indicate the wind speed in knots (nautical miles per hour, which are 15% larger than the familiar statute miles per hour). Each half barb portrays the wind to the nearest 5 knots, while each full barb is an increment of 10 knots; a pennant (rare for surface maps) represents 50 knots. By convention, the wind barbs and pennants are plotted on the side of the shaft of the wind arrow pointing toward lower pressure (or to the left of the wind direction in the northern hemisphere). This convention is useful when performing an isobar analysis.
By convention, the lead "9" or "10" is dropped from the reported value and the decimal point omitted. A sea level pressure report of 995.8 mb would be plotted as "958", a report of 1002.8 mb would be plotted as "028", and 1025.8 mb would be "258". Since the sea level pressure usually ranges between 980 and 1040 mb, you should have no problem in determining whether the plotted value is preceded by a "9" or "10". If in doubt, check the pressure values at neighboring reporting stations.
| . | rain | * | snow | , | drizzle |
| Two symbols
Three symbols Four symbols |
Light
Moderate Heavy |
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0/8 of sky covered by clouds
1/8 through 2/8 of sky covered by clouds 3/8 through 4/8 of sky is covered by clouds 5/8 through 7/8 of sky is cloud covered 8/8 of sky is cloud covered |
You may wonder about the distinction between a "plot" and an "analysis". A plot is a chart that contains only the observed data plotted at the station. An analysis on the other hand, represents a map product that contains a set of lines, either drawn by hand or by machine-essentially a computer program that uses mathematical formulae to interpolate. These lines, variously called isolines, isopleths or contours, are used to help visualize patterns in the geographical distribution of a given weather element more effectively than can be obtained from a plot of numbers across the map. Last week you gained some experience drawing isobars (lines of equal air pressure). Fronts are often placed upon the surface maps when applicable. The frontal symbols represent the intersection of the boundary between dissimilar air masses at the earth's surface. Frontal analysis involves inspection of the isotherm (lines of equal temperature) patterns, how the winds change direction over an area, and cloud and precipitation patterns.
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Prepared by Edward J. Hopkins, Ph.D., email
hopkins@meteor.wisc.edu