WORLD MAPS OF EXTREME iV-GRADIENTS 13 



found in the same tropical and subtropical loca- 

 tions as superrefractive layers, because a small 

 change in relative humidity at high tempera- 

 tures produces a very noticeable change in ab- 

 solute humidity, and the iV-change (either posi- 

 tive or negative) with height is highly depend- 

 ent upon the variation of absolute humidity. 

 For instance, subrefractive gradients occur 

 quite often during the afternoon at stations 

 which experience superrefraction or ducting 

 during the night and early morning. Other sta- 

 tions may have nocturnal subrefraction during 

 winter and superrefraction during the same 

 hours in summer. However, subrefraction, un- 

 like superrefraction, rarely occurs at surface 

 temperatures below 10 °C (the only exception 

 would be locations greater than 1 km above sea 

 level) . 



The surface conditions conducive to subre- 

 fractive gradients are of two rather opposite 

 types : 



(a) temperature > 30°C; relative humidity 

 < 40 percent ; 



(b) temperature 10° to 30°C ; relative humid- 

 ity > 60 percent. 



Type (a) is usually found during the day- 

 light hours of months when intense solar heat- 

 ing occurs at warm, dry continental locations 

 and forms a very nearly homogeneous surface 

 layer (no decrease of density with height) 

 which may be several hundred meters thick. 

 Since a moist parcel of air is less dense than a 

 dry parcel at the same temperature and pres- 

 sure, the intense convection which occurs with- 

 in such a layer of absolutely unstable air tends 

 to concentrate the available water vapor near 

 the top of the layer, because a moist adiabatic 

 upper boundary is formed where the super- 

 adiabatic lapse rate changes abruptly to a sub- 

 adiabatic or very stable lapse rate. The result is 

 an increase (sometimes as large as 50 percent 

 of the surface value) with height of the wet 

 term through the ground-based layer. This in- 

 crease, coupled with no change in the dry term, 

 leads to a subrefractive (or positive) gradient. 



This layer may retain its subrefractive na- 

 ture throughout the early evening hours at sta- 

 tions where conditions are favorable for the 

 development of a temperature inversion. As the 

 ground cools rapidly, the air very near the 

 ground cools and becomes more dense, but the 

 water vapor which is trapped between the two 

 stable layers causes the positive wet-term gradi- 

 ent from surface to the top of the original layer 

 to remain large enough to overbalance the 

 slightly decreasing gradient of the dry term. 

 This evening subrefraction is an outgrowth of 

 type (a) ; however, it may resemble type (b) 

 at the surface because it can be found with a 

 temperature as low as 20 °C and a relative hu- 

 midity as high as 60 percent. 



Type (b) occurs most often during night and 

 early morning hours, and is characteristic of 

 coastal trade-wind and sea-breeze areas where 

 differential heating of land and sea results in 

 the advection of air which is warmer and more 

 humid than the normal surface layer. In this 

 type, both dry and wet terms may increase with 

 height, creating a surface layer of subrefrac- 

 tion which is generally more intense in gradient 

 than type (a) but not so thick. This form of 

 subrefraction might also be found for short 

 periods in any location where frontal passages 

 or other synoptic changes create the necessary 

 conditions. 



Type (a) subrefraction is hard to evaluate 

 from figures C-l through C-4 because its per- 

 centage occurrence at any specific location is so 

 dependent upon the time of day represented by 

 the radiosonde data at that location. For in- 

 stance, because the local radiosonde observa- 

 tion times in the southwestern U.S.A. were 

 0800 and 2000 for the data period used in this 

 atlas, only the subtype (a) of evening subre- 

 fraction is recorded. Because conditions are 

 more suitable for inversions in February and 

 May, these months appear to have surface- 

 based subrefractive layers more often than Au- 

 gust. However, a detailed check of midafter- 

 noon observations near White Sands, N. Mex., 

 reveals that midday subrefractive conditions are 

 quite prevalent during much of August and Sep- 

 tember. The same diurnal problem is found in 

 northern Africa and the desert region south and 

 east of the Caspian Sea, where many of the sta- 

 tions take observations between 0300 and 0600 

 LST. Furthermore, even at those stations 

 which do have midday data, the "motorboating" 

 problem (i.e., humidities too low to be mea- 

 sured by the radiosonde — see sec. 5.5) during 

 the warmest seasons at very dry locations prob- 

 ably masks out a large percentage of subrefrac- 

 tive occurrences ; e.g., the occurrence of subre- 

 fraction recorded in November and February 

 for the interior of Australia (where afternoon 

 observations are included) is probably too low. 



Figures C-l through C-4 reveal that type (b) 

 subrefraction can be expected 10 to 20 percent 

 of the time in the western Mediterranean Sea 

 and the Red Sea area, and also in the Indone- 

 sian-Southwest Pacific Ocean region. These lo- 

 cations seem to indicate a slight seasonal trend, 

 with a higher probability of occurrence during 

 winter months. Another region with a 10 to 20 

 percent level of subrefractive gradient occur- 

 rence is the Ivory Coast and Ghana lowlands of 

 Africa where onshore winds prevail all year. 



Occurrences of type (b) subrefraction exceed 

 5 percent at these locations and times of year : 



(1) Southeast coast of U.S.A. all months; 



