United States 

 Department of 

 Agriculture 



Forest Service 



Intermountain 

 Forest and Range 

 Experiment Station 



Research Paper 

 INT-276 



July 1981 



Why Windspeeds 

 Increase on High 

 Mountain Slopes 

 at Night 



Robert G. Baughman 



INTRODUCTION 



The diurnal change of wind velocity on high mountain 

 slopes and ridges differs from that of lower levels. Usually, 

 the wind velocity increases on the higher slopes and may 

 reach a maximum value at night and a minimum during 

 the daytime (Yoshino 1975). The occurrence of these 

 stronger winds blowing on a ridgetop fire at night can prove 

 very frustrating to fire control crews who often count on 

 a nighttime lull in fire activity. Such winds could not 

 only fan a fire, but could also carry firebrands much 

 further than expected and widely disperse smoke and 

 pollution. 



The existence of this particular weather phenomenon 

 in mountainous areas has been reported in literature over 

 the past 40 years. Gisborne (1941) declared that on 

 mountaintops the maximum wind velocity is often reached 

 during the night. Barrows (1951) states, "On the upper 

 slopes of the mountains the peak velocities may be reached 

 much later in the day, and frequently at night." Byram 

 (1954) related a low-level jet current to extreme fire 

 behavior. Several of his case-history fires were located 

 in mountainous country. According to Schroeder and 

 Buck (1970), low-level jet winds have not been studied in 

 rough mountain topography; however, the higher peaks 

 and ridges above lowland night inversions may 

 occasionally be subjected to them. Brown and Davis (1 973) 

 say, "While low-level surface winds are generally weakest 

 at night, the reverse is noted on mountain ridges; that is, 

 ridge winds tend to be stronger at night and weaker during 

 the daytime." 



The cause of these nocturnal mountain winds has not 

 been fully explained. A few researchers have related the 

 cause to the occurrence of low-level jet winds, but 

 details are not given (Schroeder and Buck 1970; Lee'). 

 A more complete explanation of the windspeed increase on 

 high mountain slopes at night is given here. The key to the 

 explanation is the common occurrence of nocturnal 

 temperature inversions in mountain valleys. The top of 

 the inversion layer provides a smooth fluid interface 

 that enhances the formation of a low-level jet (Hoecker 

 1965). The mountain slopes and ridges that extend above 

 the surface inversions are exposed to the jet wind and are 

 thus subject to strong surface winds. 



Included in the explanation given here is a brief dis- 

 cussion of a particular low-level jet wind theory (by 

 Blackadar 1957) that can be used to predict the strength 

 of these mountain winds during the course of an entire 

 night. Also discussed is the type of synoptic situation most 

 likely to produce these winds. Finally, shown here is an 

 example of a low-level jet wind that produced strong winds 

 on a ridgetop fire at night. 



The primary purpose of this report is to offer a reason for 

 stronger windspeeds on high mountain slopes at night. 

 Additional work is required to describe the extent of the 

 problem, at least in terms of fire behavior. This requires 

 studies to determine: the frequency of these winds, the area 

 affected, the season variation, and the effect on fire 

 behavior. Also, additional studies are needed to develop 

 the type of procedures required to forecast these 

 nocturnal mountain winds. 



'Lee. Glenn M. 1959. Observations of ridge-top winds in northeastern 

 Oregon. Unpubl. rep. 3 p. On file at Northern Forest Fire Laboratory, 

 Missoula, Mont. 



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