METEOROLOGICAL FACTORS 

 IN THE FIRE RUN 



The account of the Sundance Fire in the 

 text of this paper has emphasized the influ- 

 ence of the weather conditions, particularly 

 those that produced the unusually strong 

 winds that were a determining factor in the 

 fire's phenomenal run. In this section we will 

 first look at the general weather situation that 

 existed over North America and the eastern 

 Pacific Ocean, and then pay special attention 

 to the relation of upper-air and surface winds 

 in the vicinity of the fire. Comments on the 

 relative humidity are also included. 



The Large-scale Weather Situation 



For the general weather picture, we will re- 

 fer to a series of maps depicting atmospheric 

 conditions at about 10,000 ft. m.s.l. (figs. 18 

 through 22). Maps at this level represent the 

 general airflow and temperature pattern in the 

 lower troposphere, largely free of irregularities 

 induced by local terrain effects in this area. 



The solid lines on these maps are height 

 contours; these connect points of equal height 

 (m.s.l.) at which an atmospheric pressure of 

 700 millibars (mb.) is reached, in analogy to 

 contours on topographic maps (which give 

 heights of the ground surface). The patterns 

 shown by this form of depiction are nearly 

 identical to the isobaric (equal-pressure line) 

 patterns that would be found at a fixed height 

 of 10,000 ft. m.s.l. The contours are drawn 

 for 30-meter intervals and are labeled in tens 

 of meters. The dashed lines are isotherms, 

 drawn at intervals of 5 degrees Celsius. In ad- 

 dition, the winds observed at several individ- 

 ual stations are plotted in conventional 

 symbolic form. That is, the windspeed is pro- 

 portional to the number of barbs on the tail 

 of an arrow (head not shown). Each full barb 

 represents 10 knots (or 11.5 m.p.h.); a half 

 barb indicates 5 knots. The tail of the arrow 

 points in the direction from which the wind is 

 blowing. 



From the wind arrows in these maps, the 

 airflow at 700 mb. is seen to be approxi- 



mately parallel to the adjacent height con- 

 tours. The windspeed, at a given latitude, is 

 approximately inversely proportional to the 

 contour spacing. Thus, the steeper the height 

 gradient, the greater the windspeed in that 

 area. In the present discussion, we will substi- 

 tute the term "pressure gradient," which may 

 be more dynamically descriptive. 



The first map (fig. 18) depicts conditions 

 on the morning of August 30 and illustrates a 

 rather abnormal pattern, characteristic of 

 much of the summer of 1967. The dominant 

 pressure features seen here are (a) a strong, 

 extensive warm ridge over the Western United 

 States and Canada and (b) a deep trough and 

 low center in the eastern Pacific area. This 

 trough has been stationary for 3 days. Be- 

 tween the ridge and trough there is a strong 

 pressure gradient. Over the interior North- 

 western United States, winds are very light; 

 the map shows a tendency for slow northward 

 movement of warmer'air. 



Figure 18. — Analysis of 700-mb, conditions at 0500 

 m-s.t. August 30, 1967, For explanation of 

 symbols in flguxes 18-22, see text. Small circle 

 in northern Idaho is approximate location of 

 Sundance Fire. 



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