20 



METEOROLOGY AND CLIMATOLOGY: 



nates from colder regions of the North Atlantic 

 Ocean. In spring to summer it brings cold to cool 

 conditions to the area east of the Appalachians, 

 conditions often counteracted by the Tropical At- 

 lantic Air Mass. In winter, Polar Atlantic brings 

 moist cold air, leading to severe, snowy weather. 



Tropical Continental consists of hot, dry air that 

 originates from interior Mexico. It produces the 

 deserts of southwestern North America. 



Tropical Maritime is composed of three air masses. 

 Tropical Gulf and Tropical Atlantic are much the same. 

 Both originate in the Gulf-Caribbean-Sargasso Sea 

 region and are warm, moist, air masses. In the sum- 

 mer, they bring heat, precipitation, and thunder- 

 storms to the Midwest and East. In winter, they still 

 bring moisture. When active, they curb the effects 

 of Polar Continental and Polar Atlantic air masses. 

 Tropical Pacific originates in the southeast Pacific 

 subtropics. In winter, it brings cool to warm, some- 

 what moist air to the Pacific Coast, especially Cali- 

 fornia, and to the Great Basin. During the summer, 

 its influence is south of the United States, and the 

 Pacific Coast is most often influenced by the Polar 

 Pacific. 



Tropical Superior is mostly an upper-air mass. In 

 summer, it generally causes hot and dry conditions 

 in the southwestern deserts and western plains. On 

 occasion, especially during the winter, it moves west- 

 ward, across the desert and over the mountains. This 

 brings the rare, very hot and dry, strong winds called 

 Santannas to coastal California, especially the Los 

 Angeles area. 



FOEHN WINDS 



Santanna and Chinook winds are of a general type 

 caWed foehns. They occur when a "High" is on one 

 side of a mountain and "Low" on the other side. As 

 the "High" moves up its side of the mountain, water 

 vapor condenses, releasing heat into the air mass. 

 Moreover, when the wind travels down the other side 

 of the moimtain water vaporization and air com- 

 pression further heat it. Therefore, a warm, dry wind, 

 or foehn, descends into the "Low." 



Santannas are especially severe because they are 

 already quite dry and hot in the "High." Therefore, 

 condensation and compression further intensify the 

 conditions. In the case of a Santanna, the difTerence 

 in pressures between the "Low" and the "High" 

 must be great for this foehn to develop. This pres- 



sure differential causes a wind of extremely high 

 velocity. For example, a Santanna is a high-velocity, 

 searing wind and may carry small pebbles which 

 break windows and damage plants. 



AIR MASS TEMPERATURE 



Air masses are called cold or warm in reference to 

 the surface area over which they are flowing. Also, 

 they are often related to "Highs" and "Lows." 

 Cold air masses are frequently "Highs" and have 

 clear skies except for scattered cumulus clouds. But 

 if a cold air mass moves over water, the "High" ac- 

 quires more moisture, the mass becomes warmer and 

 is carried upward, and cumulonimbus clouds may 

 form and produce showers. On the other hand, warm 

 air masses are often "Lows." This implies possible 

 rising and, therefore, precipitation. However, if they 

 are not affected by the inrush of air from "Highs," 

 warm "Lows" remain fairly stable and quite moist. 

 In fact, stability is intensified when such masses ap- 

 proach colder masses, because the lower layer of the 

 warm mass becomes cooler and heavier. Within such 

 cooled areas, clouds are of the stratus type; stratus 

 clouds in the form of fog may extend to the ground. 

 Although this description represents usual condi- 

 tions, a "High" need not start from cold air or a 

 "Low" from warm. An example of a warm "High" 

 is a foehn. 



FRONTS 



Fronts affect local conditions because a front is an 

 air mass phenomenon. A front is formed when cold 

 and warm air masses from widely separated places of 

 origin make contact. At first, the two air masses do 

 not mix freely; rather, they retain their identities. 

 Such a boundary often occurs near 40° north lati- 

 tude. Actually the boundary is a shifting, undulat- 

 ing, three-dimensional structure and is often as- 

 sociated with a jet stream. During the latter stages of 

 the life cycle of a front, the air masses mix and lose 

 their separate identities. This life cycle is character- 

 ized by very active weather processes, especially the 

 creation of various kinds of local fronts. 



Fronts are not rare aspects of weather. They oc- 

 cur frequently as a direct result of the movement of 

 air masses. If one considers the poleward movement 

 of air and the fact that the easterlies blow away 

 from the prevailing westerlies, people below latitude 



