Chapter 11 — WATCH ROUTINES (CONTINUED) 



this new point. Repeat the search, always down- 

 wind. Project the isobar to the new point. This 

 procedure is repeated until the isobar being 

 drawn runs off one edge of the chart, returns 

 to the point of origin (the ends join to form a 

 closed curve), or enters an area where there 

 are no reports and there is no justification 

 for continuing. 



Remembering that all points along an isobar 

 represent the same numerical pressure value, 

 move from the finished isobar, at any point, 

 outward to an adjacent station. Determine if 

 another isobar, of greater or lesser value, may 

 be placed between this station and the isobar 

 just completed. If so, proceed in the same 

 manner outlined above. If not, continue in the 

 same direction to the next adjacent station. 

 Eventually, an area between two stations is 

 encountered where the next standard isobar may 

 be drawn; then proceed in the manner outlined 

 above. 



This process is repeated until an isobar has 

 been drawn for all standard values insofar as 

 the plotted information permits. 



The next step in analysis is locating and 

 sketching frontal positions. Careful isobaric anal- 

 ysis aids significantly in locating the more ob- 

 vious fronts. 



Frontal Analysis 



The location of fronts is determined by past 

 history, air mass analysis, satellite data, and 

 reports on the present charts. Since fronts and 

 their accompanying weather move across various 

 areas of the earth in established directions 

 with somewhat definite speeds, it is possible 

 to follow their movement on previous charts 

 as an aid to locating them at any particular 

 time. N 



In locating fronts, properties of the air 

 masses have to be studied and the fronts have 

 to be located so that they separate unlike air 

 masses. 



COLD FRONTS. — Cold fronts are normally 

 located in well-defined pressure troughs when- 

 ever there is a marked temperature contrast 

 between two air masses. In most cases, a 

 careful analysis of the isobars indicates the 

 correct position of the pressure trough that 

 contains the front. This method of isobaric 

 analysis is frequently the only possible means 



of locating fronts over ocean areas or regions 

 of scanty surface reports. Other indications of 

 the presence of cold fronts can be classified 

 as prefrontal, frontal, or postfrontal indications 

 and are as follows: 



1. Pressure tendencies. In advance of cold 

 fronts, the tendency characteristic is usually 

 indicated by a steady or an tinsteady fall. The 

 isallobars of falling pressure in advance of 

 the front usually form an elongated pattern 

 approximately parallel to the front. After the 

 passage of the front, the tendency generally 

 shows a steady rise, and stations behind the 

 front show a tendency characteristic oi\^, ^/, 

 or /. The first two, showing a fall and then 

 a rise in pressure, indicate that the front passed 

 the station during the 3-hour period prior to 

 map time. 



2. Wind. With the approach of the front, 

 the wind is normally from the south or south- 

 west in the Northern Hemisphere, veering to 

 parallel the front. At the passage, the wind 

 generally shifts abruptly to the northwest. Very 

 gusty winds frequently occur at the frontal 

 passage and usually after passage. 



3. Cloud forms. In advance of cold fronts, 

 the cloud types are typical of the warm air. 

 Towering cumulus, cumulonimbus, stratocumu- 

 lus, and nimbostratus are associated with the 

 passage. After passage, these cloud forms may 

 prevail for several hundred miles with the 

 slow-moving cold front. Very rapid clearing 

 conditions are associated with the fast-mov- 

 ing cold front after passage. Well back in the 

 cold air in both types of cold fronts, the only 

 clouds normally found are fair weather cumulus. 



4. Precipitation. Showers and sometimes 

 thunderstorms occur with the cold-front pas- 

 sage. Continuous precipitation is observed for 

 some hours after the passage of a slow-moving 

 cold front. Showers and thunderstorm activity 

 of short duration will occur with the passage 

 of a fast-moving cold front, followed by very 

 rapid clearing conditions. 



5. Temperatures. Temperature is relatively 

 high prior to passage. After passage, the tem- 

 perature decreases very rapidly with slow- 

 moving fronts. Such a rapid temperature change 

 does not accompany the passage of fast-moving 

 cold fronts; the real temperature change is 

 usually observed some distance (as far as 50 

 to 100 miles) behind the front. 



201 



