MESO-SCALE CIRCULATION 1. Mountain and Valley Winds (1). Mountain winds A. Winds blow from the mountain top to the valley. B. Nighttime winds C. The mountain top cools more rapidly than does the valley. (2). Valley winds A. Wind blows from the valley along the mountain slope upward. B. Daytime winds (A). At the same elevations, air pressure is higher over the slope areas than over the valley because of the warmer air column over the slope areas (the hydrostatic equation). (B). The upper level winds from both slopes converge toward the center over the valley causing the subsidences toward the valley floor. The air currents then flow toward the slopes and upward completing two circulation circles. 2. Land and Sea Breezes (1). Land breezes A. Surface winds blow from land to sea (offshore winds). B. Nighttime winds. (A). Land: Relatively cool and a weak High. Land cools and warms more rapidly than the nearby sea due to a lower specific heat capacity for soil (0.2 cal/g) than for water (1 cal/g). (B). Sea: Relatively warm and a weak Low (land to sea pressure gradient is about 1 to 2 mb or higher). C. The isobaric surface near the ground slopes downward from land to sea. D. Upper level: Return flow (winds blow from sea to land, frequently bringing stratus clouds from sea to land at night in southern California). (2). Sea breezes A. Surface winds blow from sea to land (onshore winds). B. Daytime winds (A). Land: relatively warm and a weak Low (thermal low). (B). sea: relatively cool and a weak High (thermal High). C. The isobaric surface slopes downward from sea to land. D. Upper level (a few hundred to a few thousnad feet high): Return flow from land to sea pushes stratus clouds seaward in addition to the burnoff (evaporation) of clouds by solar heating in southern California. One usually oberves stratus clouds off the coast on the daytime. 3. Foehn Wind (1). Definition: The warm and dry wind over the Foehn Village on the north slope of the Alps Mountains. The south wind blows from Italy crossing the mountains and descends on the lee side (north side) of mountain where the Foehn village is located. (2). The warm and dry winds that descend on the lee side of a mountain in any part of the world (Chinook wind and Santa Ana wind, for example). (3). Caused by the cpmpressional heating of descending air: warm and dry. (4). A type of katabatic wind (downslope wind) Anabatic wind: upslope wind. 4. Chinook wind (Snow-eater) (1). The warm and dry wind on the eastern slopes of the Rocky Mountains (Montana, Colorado). (2). Air temperature may rise more than 20 oC in a few minutes. (3). Synoptic patterns A. The occurrence of a Great Basin High. B. The occurrence of an Alberta or a Montana Low. C. The west winds (SW, W, NW) from the the Rocky Mountains downward to the Alberta Low. 5. Santa Ana Wind (1). Definition: A. The warm,dry, and strong wind that blows over the southern California from the Great Basin as a north, northeast or east wind (offshore wind). B. wind speed exceeds 20 mph. C. relative humidity is about 20% or lower. (2). Cold Santa Ana wind: A. The original air temperature is very cold. B. The air temperature may not be warm enough even after the air descends(sinks). (3). Synoptic Patterns: A. A cold Great Basin High The surface High decreases its intensity with the increasing height, becoming a Low at the 500 mb level. B. After the passage of a cold front over California. The High behind a cold front moves to the Great Basin. C. A surface Low to the south of California (in Mexico or off the Mexico coast) creating a large pressure gradient. D. A 500 mb ridge to the north of California in a northeast to souuthwest orientation. E. A 500 mb trough extends from the Great Basin to southern California. (4). Strong wind A. Large pressure gradient. B. Canyon effect: Wind speed accelerates when passing through the canyon. (A). Newhall pass. (B). Cajon Pass. (5). Diurnal variation A. Stronger Santa Ana wind at night Santa Ana wind is accelerated by land breeze. B. Weaker Santa Ana wind on the day Santa Ana wind blows against sea breeze. (6). Seasonal Variation A. Occurs most frequently in winter (September to May) peaking in December through April. B. Accounts for the warmest temperature in winter: 90oF+ (7). Types A. Weak Santa Ana wind (A). Santa Ana wind blows above sea breeze. (B). Smoggy weather. B. Strong Santa Ana wind (A). Santa Ana wind extends to ground surface. (B). Less smoggy but may be dusty. (8). Santa Ana Condition Same synoptic patterns as Santa Ana winds but lack of strong winds (heat wave in summer). (9). Fire weather Santa Ana wind provides favorable conditions (dry, warm, windy) for the occurrence of fire. 6. Sundowner (1). A downslope warm and dry wind over the Santa Barbara coastal areas along the lee of the east-west oriented Ynez Mountains: N or NW wind, 40-50 mph. may exceed 80 mph. (2). June 17, 1959: A. Temperature reaching 132oF at 2 p.m. in Goleta Harbor (meausred by a US survey ship). B. Santa Barbara City: 136oF (57.8oC): tied record high temperature for the earth if validated. (3). Painted Cave fire: June 27, 1990 Winds gusting at 40-70 mph, temperature reaching 112oF. (4). Much more local nature than the Santa Ana wind. Most vulnerable during the months of June and July. (5). Causes: A. Lee waves. B. Large pressure gradient. C. Canyon effect. 7. Mono Wind: A type of Foehn wind over Mono county of California. 8. Northers (in Texas) and Nortes (in central America). (1). A cold strong wind sweeps southward over the Great Plains of the United States behind a cyclone that has moved toward east. (2). The Rocky Mountain barrier helps to channel the flow of cold air. (3). In Kansas and Texas, temperature can drop as much as 10 oC in 3 to 4 hours. 9. Blizzard (1). Cold strong wind accompanied by snow.
(2). Blizzard warning
wind speeds: at leat 60 km/h (35 mph)
temperatures: less than -6oC(20oF)
(3). Severe blizzard warning
Winds are expected to be at least 75 km/h (45 mph)
Temperature are expected to drop below -12oC(10oF)