METEOROLOGICAL FACTORS 



183 



lowest few hundred or thousand feet. Normally the 

 temperature decreases with height in the atmosphere 

 (at a rate of about 2°C per 1,000 ft), and the mois- 

 ture decreases gradually with height. Under these 

 conditions the propagation is of the standard type. 

 Temperature may sometimes increase with height 

 for a few hundred or thousand feet above ground 

 and then, at greater heights, begin to decrease again. 

 The vertical increase of temperature is called a tem- 

 perature inversion. Sometimes a layer of moist air 

 is found near the ground, and the air overlying it is 

 very dry. There is then a rapid decrease of moisture 

 over a short vertical distance; in other words there 

 is a pronounced moisture lapse (see Figure 7). A 



w 1500 - 



u 1000 



o 



K 



500 



AIR OVER AFRICA 



3000 



MIXING RATIO INCREASE 



Figure 7. Moisture variation aloft. 1. Moisture distri- 

 bution with height in standard moist atmosphere. 2. 

 Example of sharp moisture lapse (dry air overlying 

 moist air) conducive to guided propagation. Mixing 

 ratio is amount of moisture in a unit weight of dry air 

 expressed as grams of water per kilogram of dry air. 



moderate or strong moisture lapse almost always will 

 produce trapping, but a temperature inversion 

 (except at low temperatures) will lead to trapping 

 only if the moisture distribution is favorable. A 

 combination of both effects within the same layer 

 usually will produce trapping. 



The meteorological conditions to be found over sea 

 and over land are quite different and must be con- 

 sidered separately. 



Over Sea 



When warm, dry air flows over colder water, a 

 temperature inversion will be established, and there 

 will be evaporation into the lowest layers of the air, 

 thus creating conditions of pronounced trapping. 

 This weather condition is one of the most common 

 causes of guided propagation. An example in point 

 is the Mediterranean, which to the south, east, and 

 west is surrounded by dry land masses producing a 

 flow of dry, warm air over the water when the winds 



\ TEMPERATURE 

 \ DISTRIBUTION 



\ 

 I 



*LAND TEMP 



AIR FROM SAHARA 

 VERY DRY 



AFTER PASSAGE ACROSS THE MEDITERRANEAN 



3000- 



ui 



u. 



TEMPERATURE 



■UP MIXING RATIO —INCREASE 



Figure 8. Modification of air from Sahara Desert in 

 passing over the Mediterranean. 



blow from these directions (see Figure 8). Similar 

 conditions are often caused by westerly winds blow- 

 ing from land to sea across the eastern boundary of 

 a continent. Land and sea breezes may influence 

 radar operation along a coast line. The wind direc- 

 tion at a coast is often an important factor in deter- 

 mining propagation conditions and should be closely 

 watched. Whenever unusual propagation is observed 

 by coastal radar stations, a record of prevailing winds 

 at the time is very helpful in determination of future 

 expected performance. 



TEMPERATURE 



■UP 



Figure 9. Formation of temperature inversion over land 

 due to nocturnal cooling. 



Over Land 



Temperature inversions are produced mainly by 

 nocturnal or night cooling of the ground (see Figure 

 9) . Trapping may occur when the moisture distribu- 



