Appendix B:
Explanation and Decoding
of the Daily Weather Map

Weather maps showing the development and movement of weather systems are among the most important tools used by the meteorologist. Some maps portray conditions near Earth’s surface, and others depict conditions at various heights in the atmosphere. Some cover the entire Northern Hemisphere, and others cover only local areas as required for special purposes.

Principal Surface Weather Map

To prepare the surface map and present the information quickly and pictorially, two actions are necessary: (1) Weather observers and automated observing stations must send data to the offices where the maps are prepared; and (2) the information must be quickly transcribed to the maps. To achieve the necessary speed and economy of space and transmission time, codes have been devised for sending the information and for plotting it on the maps.

Codes and Map Plotting

A great deal of information is contained in a brief coded weather message. If each item were named and described in plain language, a very lengthy message would be required, one confusing to read and difficult to transfer to a map. A code permits the message to be condensed to a few five-figure numeral groups, each figure of which has a meaning, depending on its position in the message. People trained in the use of the code can read the message as easily as plain language (Table B.1).

Table B.1
Explanation of Station Symbols and Map Entries

The location of the reporting station is printed on the map as a small circle (the station circle). A definite arrangement of the data around the station circle, called the station model, is used. When the report is plotted in these fixed positions around the station circle on the weather map, many code figures are transcribed exactly as sent. Entries in the station model that are not made in code figures or actual values found in the message are usually in the form of symbols that graphically represent the element concerned. In some cases, certain of the data may or may not be reported by the observer, depending on local weather conditions. Precipitation and clouds are examples. In such cases, the absence of an entry on the map is interpreted as nonoccurrence or nonobservance of the phenomena. The letter M is entered where data are normally observed but not received.

Both the code and the station model are based on international agreements. These standardized numerals and symbols enable a meteorologist of one country to use the weather reports and weather maps of another country even though that person does not understand the language. Weather codes are, in effect, an international language that permits complete interchange and use of worldwide weather reports so essential in present-day activities.

The boundary between two different air masses is called a front. Important changes in weather, temperature, wind direction, and clouds often occur with the passage of a front. Half circles or triangular symbols or both are placed on the lines representing fronts to indicate the kind of front. The side on which the symbols are placed indicates the direction of frontal movement. The boundary of relatively cold air of polar origin advancing into an area occupied by warmer air, often of tropical origin, is called a cold front. The boundary of relatively warm air advancing into an area occupied by colder air is called a warm front. The line along which a cold front has overtaken a warm front at the ground is called an occluded front. A boundary between two air masses, which shows at the time of observation little tendency to advance into either the warm or cold areas, is called a stationary front. Air-mass boundaries are known as surface fronts when they intersect the ground and as upper-air fronts when they do not. Surface fronts are drawn in solid black; fronts aloft are drawn in outline only. Front symbols are given in Table B.2.

Table B.2
Weather Map Symbols

A front that is disappearing or weak and decreasing in intensity is labeled frontolysis. A front that is forming is labeled frontogenesis. A squall line is a line of thunderstorms or squalls usually accompanied by heavy showers and shifting winds (Table B.2).

The paths followed by individual disturbances are called storm tracks and are shown by arrows (Table B.1). A symbol (a box containing an X) indicates past positions of a low-pressure center at 6-hour intervals. HIGH (H) and LOW (L) indicate the centers of high and low barometric pressure. Solid lines are isobars and connect points of equal sea-level barometric pressure. The spacing and orientation of these lines on weather maps are indications of speed and direction of wind flow. In general, wind direction is parallel to these lines with low pressure to the left of an observer looking downwind. Speed is directly proportional to the closeness of the lines (called pressure gradient). Isobars are labeled in millibars.

Isotherms are lines connecting points of equal temperature. Two isotherms are frequently drawn on large surface weather maps when applicable. The freezing, or 32°F, isotherm is drawn as a dashed line, and the 0°F isotherm is drawn as a dash–dot line (Table B.2). Areas where precipitation is occurring at the time of observation are shaded.

Auxiliary Maps

500-Millibar Map

Contour lines, isotherms, and wind arrows are shown on the 500-millibar contour level. Solid lines are drawn to show height above sea level and are labeled in feet. Dashed lines are drawn at 5° intervals of temperature and are labeled in degrees Celsius. True wind direction is shown by “arrows” that indicate the direction from which the wind is blowing. The wind speed is shown by flags and feathers. Each flag represents 50 knots, each full feather represents 10 knots, and each half feather represents 5 knots.

Temperature Map (Highest and Lowest)

Temperature data are entered from selected weather stations in the United States. The figure entered above the station dot shows the maximum temperature for the 12-hour period ending 7:00 p.m. EST of the previous day. The figure entered below the station dot shows the minimum temperature during the 12 hours ending at 7:00 a.m. EST. The letter M denotes missing data.

Precipitation Map

Precipitation data are entered from selected weather stations in the United States. When precipitation has occurred at any of these stations in the 24-hour period ending at 7:00 a.m. EST, the total amount, in inches and hundredths, is entered above the station dot. When the figures for total precipitation have been compiled from incomplete data and entered on the map, the amount is underlined. T indicates a trace of precipitation (less than 0.01 inch) and the letter M denotes missing data. The geographical areas where precipitation has fallen during the 24 hours ending at 7:00 a.m. EST are shaded. Dashed lines show depth of snow on ground in inches as of 7:00 a.m. EST.

Table B.3
Air Pressure Tendency

Table B.4
Cloud Abbreviations

Table B.5
Cloud Types

Table B.6
Height of Base of Lowest Cloud

Table B.7
Cloud Cover

Table B.8
Wind Speed

Table B.9
Weather Conditions