frequency in early summer. The light or moderate winds during the summer are 

 also favorable for the maintenance of fog. 



In winter the air from the land is heated from below and therefore this type 

 of advection will not produce fog, except extreme cases of steam fog. Due to the 

 cold coastal water near Newfoundland, fog will be formed when air is brought into 

 this region from the neighboring Atlantic. This is a less frequent occurrence in 

 winter than in summer. Fog due to radiational cooling of the maritime air is found 

 over the land regions of northwestern Europe. In general, the stronger winds during 

 the winter will result in low clouds rather than surface fog. 



-"&• 



12. Upper Winds. 



An attempt has been made to accumulate all available upper air data in order 

 to show the upper air circulation in the various seasons. The sources of data'^'^'^" 

 for Figures 23 and 24 are given in the Bibliography. The wind roses have been 

 constructed so that the wind direction frequency is indicated by the length of the 

 arrow and the average velocity is given in miles per hour by the number at the end 

 of the arrow. 



It is evident from the wind roses in Figure 23 that the upper winds vary not 

 only in direction but also in velocity. At 1)'^ km. we find a decided maximum of 

 west to northwest winds over the northeastern United States and Canada, with 

 velocities averaging from 30 to 40 m.p.h. In the southern United States the winds 

 are more westerly and the average velocity is about 25 m.p.h. Because the prevail- 

 ing surface winds south of 55° N are westerly in the Atlantic, we should expect to 

 find general westerly winds at 13^2 km. with velocities higher than those at the 

 surface. Consequently, in the middle Atlantic general west winds of about 40 m.p.h. 

 would be expected. Since the Icelandic low is to the west, we find more south- 

 westerly winds in the vicinity of the British Isles. Over Iceland the wind direction 

 is more variable, but, consistent with the mean position of the low pressure to the 

 west, southerly winds are predominant. Likewise, frequent southerly winds are 

 observed on the west coast of Greenland when there is a low in the Baffin or Hudson 

 Bay regions. It is only in low latitudes that consistent east winds are observed — 

 the Trade Wind belt. The 3 km. winds differ only slightly in direction from the 

 13^2 km. winds. Summarizing the main features of the winter upper air circulation, 

 we can say: 



(i) From 30° N to 55° N the prevailing winds are northwesterly over the 

 western Atlantic, westerly over the mid-Atlantic, and southwesterly along north- 

 western Europe. These west winds increase in velocity with elevation and are prob- 

 ably strongest about longitude 60° W. 



(S) South of 25° N the upper winds are easterly and the average velocity 

 diminishes with elevation. 



(3) Over Greenland the winds are more predominantly southerly, while 

 from Iceland northward there is a considerable variation in wind direction. The 

 direction at any time will depend on the location of the deep cyclones which frequent 

 this region. However, it is evident that when the winds are westerly they increase 

 in velocity with elevation, and when they are easterly the velocity decreases with 

 elevation. 



13. Mean Icing Charts. 



An attempt has been made to indicate the lower level at which aircraft 

 icing might be expected in winter and summer. Figures 25-28 show the height of 

 the 0° C isotherm; this temperature has been considered as the upper temperature 



11 



