Chapter 11 — WATCH ROUTINES (CONTINUED) 



The approximate heights of the constant pres- 

 sure surfaces are as follows: 



Millibar chart 



Maters 



Feet 



1,000 



110 



370 



850 



1,460 



4,780 



700 



3,010 



9,880 



500 



5,570 



18,280 



400 



7,180 



23,560 



300 



9,160 



30,050 



200 



11,790 



38,660 



150 



13,620 



44,680 



100 



16,210 



53,170 



As previously mentioned, constant pressure 

 charts are primarily used as an aid in weather 

 forecasting. They are used in conjunction with 

 surface synoptic charts to accomplish the fol- 

 lowing: 



1. Determine movements of weather sys- 

 tems. 



2. Determine cyclonic and anticyclonic wind- 

 flow. 



3. Help define air masses. 



4. Locate moist and dry areas. 



5. Aid in forecasting the formation, inten- 

 sity, and dissipation of pressure systems. 



. 6. Determine the actual slopes of fronts. 



7. Determine the vertical extent of pres- 

 sure systems. 



8. Forecast the Jetstream. 



Plotted Data 



The meteorological data entered are the 

 height of the standard pressure level above 

 sea level, the temperature and dewpoint depres- 

 sion at the standard pressure level, and the 

 wind speed and direction at the standard pres- 

 sure level. 



The meteorological plotting chart used for 

 plotting constant pressure data is the same as 

 that used for surface synoptic data. It is prop- 

 erly labeled for whatever pressure value the 



chart is being constructed (850 mb, 700 mb, 

 etc.); it is also marked with date and time. 



Tha information plotted on a constant pres- 

 sure chart is obtained from the radiosonde and 

 rawinsonde reports received by teletype, radio, 

 or computer. The coded formats of these mes- 

 sages were described and discussed in an earlier 

 section of this chapter. These charts are usually 

 plotted and analyzed every 12 hours; they rep- 

 resent the data obtained from the 0000 and 

 1200 GMT radiosonde and rawinsonde releases. 

 Figure 11-14 is a representative entry of data 

 on a constant pressure chart (700 mb). 



Some stations require that the dewpoint de- 

 pression be plotted. In this case, the depres- 

 sion is subtracted from the temperature to 

 obtain the dewpoint. 



Winds aloft reports may be plotted on sepa- 

 rate charts or may be used to fill in stations 

 on the constant pressure charts which do not 

 take or transmit radiosonde data. Extreme care 

 should be taken when plotting the wind direc- 

 tion and speeds on these charts, as many areas 

 of the world are using streamline analyses al- 

 most exclusively on upper air charts. A mis- 

 plotted direction or an incorrect speed could 

 cause an entirely erroneous analysis in a crucial 

 area. 



When wind speed and direction are critical 

 in the analysis of an upper air chart, the fol- 

 lowing procedure is recommended: 



1. Plot all wind directions with a protractor, 

 using the latitude and longitude lines for orien- 

 tation. Draw the wind shaft to the station circle. 

 (See fig. 11-14.) 



WIND SPEED 

 (OPTIONAL) 



WIND 

 DIRECTION 



/ TEMPERATURE 



.4) r 



(54) 



8.2 160. 



-o \ 



13.3 HEIGHT 



/ 



DEWPOINT 

 DEPRESSION 



Figure 11-14.- 



209.288 

 •Constant pressure chart plotted 

 report. 



211 



