decreases in a southerly direction. The maximum southerly range of extratropical cyclones 

 (with winds of > 1 8 m/sec) is about 3 1 °N. 



Winds 



Tropical and extratropical cyclone intensities are reflected in surface-wind velocities. 

 In the northern portion of the survey area maximal surface winds occur in November and 

 December and generally blow from the southwest. Figures D.4 through D.7 give wind data 

 for the northern portion of the survey area and show percent frequency of wind speed and 

 direction for 5-degree squares for four months representative of seasonal wind characteris- 

 tics; January, April, July, and November, respectively. During November and December in 

 the area between 40° and 50°N the frequency of gale-force winds (> 14 m/sec) can be as 

 high as 30 to 35 percent. Figure 28 shows the percent of surface winds of Beaufort force 

 <3 (5 m/sec) and >8 (17 m/sec) in the northern portion of the survey area. 



For November there is a broad region of maximum frequency of strong winds where 

 greater than 5 percent of the time winds of >18 m/sec were observed north of approximate- 

 ly 40°N. These frequencies decrease to the south. Conversely, light winds (<5 m/sec) are 

 most frequent nearshore with decreasing percentage frequencies to the north and west. The 

 area of maximum frequency of light winds and minimum frequency of gale-force winds is 

 off southern Cahfornia and northern Baja Cahfornia (30°-35°N), where light winds prevail 

 50-60 percent of the time (Fig. 28). Light winds also prevail along the coast of Canada 

 throughout most of the year. 



Mid- and late-winter bring a gradual reduction in mean surface wind speed and 

 cyclone frequency. By February, average wind speeds are generally lower (9-10 m/sec) in 

 the northern and central portion of Fig. 28. There is a general decline in average speeds 

 toward the annual minimum in July or August. Minimum average speeds in most of the 

 quadrangles vary between 4.9 and 6 m/sec during the summer months, with some increase 

 occurring in the southeast section, south of 40°N. The reversal of climatic trends initiated 

 over the northern part of the area during the latter part of August spreads rapidly and causes 

 a marked increase of wind speeds and frequency of gales to nearly all of the area except the 

 southeast corner. 



In the southern portion of the survey area, wind speeds are generally less than those 

 in the northern portion. The trade winds derived from the north and south Pacific subtropi- 

 cal high-pressure centers are the predominant factor in the area 30°N to the equator. Fig- 

 ures D.8 to D.l 1 give the percent frequency of wind speed and direction for the southern 

 portion of the survey area. The trades are exceptionally steady winds, come from the north- 

 east in all seasons, and overlie the region extending from 30°N to the intertropical con- 

 vergence zone (ICZ) at approximately 10°N. Mean trade wind speed is nearly 8 m/sec. 

 South of this region, the southeast trade winds predominate, intersecting with the nortiieast 

 trades at the ICZ. Figure 29 gives the percent frequency of winds Beaufort force <3 (5 

 m/sec) and >8 (18 m/sec) for February and August. 



In the coastal areas north of 10°N, intrusions of air from the Caribbean and the Gulf 

 of Mexico are common. The more tiian 1 percent frequency of gales (Beaufort force 8) off 



22 



