Synoptic Weather Chart

(or weather map), a map on which data from synchronous weather observations are entered in the form of numbers and symbols. Many kinds of synoptic charts are used for weather forecasting and for studying atmospheric processes. The most common and important are surface charts, which show data from meteorological observations characterizing the weather at the earth’s surface, and upper-air, or upper-level, charts, which contain data from aerological observations characterizing the distribution of weather elements at certain levels or in certain layers of the free atmosphere.

The scale and projection used depend on the purpose of the chart. The most common scales used by the weather service of the USSR are as follows: 1:30,000,000 for the northern and southern hemispheres, 1:15,000,000 for the USSR and adjacent countries and ocean regions, and 1:5,000,000 and 1:2,500,000 for individual sections of the country.

Figure 1

The data from each weather station are entered on the synoptic chart in accordance with established rules, which are obligatory for all offices of the weather service. Figure l, A shows the way in which the individual weather elements and phenomena are arranged on a surface chart. The small circle indicates the location of the observation point. The interior of the circle (N) shows the total amount of clouds—for example, ● means the sky is completely overcast, and ◐ means half the sky is covered by clouds. The symbols C_H, C_M, and C_L indicate the high-level, middle-level, and low-level cloud types, respectively. N_h is the amount of low clouds, and h is the height of the low clouds. The air temperature in whole degrees Celsius is represented by TT, and T_d T_d is the dew point. The explanation of the other symbols is as follows: WW, the state of the weather at the time of observation; VV, the visibility; PPP, the barometric pressure in tenths of millibars (mb) (the initial 9 or 10 is omitted); pp, the change in pressure in the past three hours; a, the barometric tendency in the past three hours; dd, the wind direction, and ff, the wind speed.

Figure 1,B illustrates how the weather data for a station are shown on a synoptic chart. In this case, the sky was completely covered with clouds. The height of the low clouds (the cloud base) was 150 m, and the code number for cloud amount at this height was 7. The air temperature was - 7°C, and the dew point was - 9°C. Snow was falling at the time of observation and had been falling since the last time of observation. The visibility was 2 km. The air pressure was 1,008.5 mb and has changed by 2.6 mb in the past three hours. The wind was blowing from the north-northwest at a speed of 12 m/sec.

An upper-air chart portrays the weather on a constant-pressure surface. Figure 2,A shows how the observation data are arrayed on such a chart. Here, h_n h_n h_n is the height of the constant-pressure surface, T_n T_n is the air temperature, D_n D_n is the dew point deficit, d_n d_n is the wind direction, and f_nf_n is the wind speed. Figure 2B illustrates how the data from one upper-air station are shown on a chart for the 500-mb pressure level. In this case, the height of the constant-pressure surface was 525 geopotential decameters. The air temperature was - 34°C, and the dew point was -38°C. The wind was blowing from the south at 35 m/sec.

After the data are entered on the synoptic chart, the chart is analyzed. On surface maps, fronts and isobars are drawn. Other improvements the analyst may make in the chart include the marking of regions of precipitation by green shading and the marking of fog regions by yellow shading. On upper-air charts, isohypses (lines of constant geopotential height) and isotherms are drawn. In addition, lines may be drawn that connect places of equal change of height or temperature during a period of 24 hours.

Figure 2

Synoptic charts are also compiled that record the values of one or two meteorological elements— for example, the depth of the snow cover, the maximum and minimum air temperature, the amount of precipitation, the height of the tropopause and the temperature at this height, or the location of jet streams.

In addition to synoptic charts that show weather data for a certain period of observation, the weather service also prepares surface and upper-air prognostic charts, which show the expected pressure field, air temperature, precipitation, and other weather phenomena for 12, 24, 48, or more hours ahead. Prognostic charts are also made for periods ranging from several days to a season.