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LECTURE 8. AIR MASSES, FRONTS AND STORMS AIR MASSES PRESSURE SYSTEMS WEATHER Air masses are large bodies of air with uniform moisture, temperature and stability characteristics. Often, day-to-day weather in the mid-latitudes is the result of a single air mass or the interaction of 2 or more air masses E.g.1: DOMINANCE by 1 air mass -> weather reflecting air mass characteristics (warm-moist; cold-dry; etc.) E.g.2: INTERACTION of several air masses -> weather system; involves air masses brought together by atmospheric circulations; found especially in the mid-latitudes (35-600), where warm tropical air meets cold polar air. Harry Williams, Earth Science 1 Harry Williams, Earth Science 2 An air masses alone can have a dominant influence on midlatitude weather; for example, an outbrust of cP can bring cold temperatures to the south central U.S. (Notice that the air itself is modified (warmed) as it moves into warmer latitudes). Harry Williams, Earth Science 3 Fronts A front is the boundary between a warm air mass and a cold air mass. When these air masses meet, the warm air always rises up over the cold air, because it is less dense. Although fronts can be stationary, more often they are moving - either a warm air mass pushes along and overrides a cold air mass, forming a gently sloping (1:200) WARM FRONT; or a cold air mass pushes along and under a warm air mass, forming a steeper (1:100) COLD FRONT. Harry Williams, Earth Science 4 Harry Williams, Earth Science 5 Along the gentle warm front clouds are produced which are extensive and usually thin/layered; precipitation can occur some distance ahead of the front and is often light/showery. Above the steeper cold front, clouds of greater vertical extent are produced (this is aided by the fact that cold fronts often travel faster than warm fronts - causing more rapid uplift); rain is often heavier and occurs immediately behind the front. A well-developed cold front can form an abrupt squall line. Harry Williams, Earth Science 6 Harry Williams, Earth Science 7 Sometimes a cold front overtakes a warm front forming an OCCLUDED FRONT. Harry Williams, Earth Science 8 These various types of front are shown on weather maps by standard symbols (symbols face direction of movement). Harry Williams, Earth Science 9 Midlatitude Cyclones The mid-latitudes are the principle zone of air mass conflict. Probably the most important result of this conflict is the midlatitude cyclone (or called a low or depression in Europe), which is responsible for much of the day-to-day weather in the mid-latitudes. It is called a cyclone because it is characterized by low pressure at the center, which causes cyclonic circulation (counterclockwise in the northern hemisphere). Because these cyclones occur in the mid-latitudes they are "embedded" in the westerlies of the global circulation and usually travel from west to east at about 20-30 miles per hour, crossing the U.S. in 3-4 days. Harry Williams, Earth Science 10 Harry Williams, Earth Science 11 These cyclones usually begin as a small disturbance in the Polar Front (where cP meets cT air). This small wave develops into the WARM SECTOR of the cyclone; cold air begins to curve around in the COLD SECTOR, beginning cyclonic circulation. As air rises up the fronts it produces low pressure at the cyclone's center. Cyclones are more numerous and better developed in winter because that is when polar air pushes down more frequently into the mid-latitudes. Harry Williams, Earth Science 12 System moving east 5 4 3 2 Harry Williams, Earth Science 1 13 1. In advance of warm front: prolonged widespread lowintensity precipitation; steadily falling pressure. 2. At the warm front: abrupt wind change east to south; temperature increase. 3. Between fronts: showery convective conditions, unstable warm air; pressure steadily falls as center gets closer, then rises as center passes. 4. At/immediately after cold front: shorted-lived intense precipitation; abrupt temperature drop; abrupt wind change south to west/northwest. 5. After cold front: skies clearing; temperature falling steadily; pressure rising steadily; wind shifting to more northerly. Harry Williams, Earth Science 14 Anticyclones (High) The other major disturbance to the general westerly circulation of the mid-latitudes is the anticyclone or high. This highpressure center involves no conflict of air masses and so no fronts. The air above is subsiding and diverging at the surface so skies are clear. This gives rise to hot dry weather in the summer and cold dry weather in the winter. Harry Williams, Earth Science 15 Harry Williams, Earth Science 16 Thunderstorms Thunderstorms are associated with towering cumulus clouds, undergoing rapid uplift resulting in adiabatic cooling and pronounced cloud formation. In the mid-latitudes, rapid uplift is often aided by 1. the ascent of warm air up a cold front of a cyclone. 2. the release of latent heat during condensation, which warms the air causing more vigorous uplift. After the cumulus stage, ice crystals grow at the top of the thundercloud and begin to fall through the cloud creating a downdraft - this is the mature stage of the thunderstorm, with heavy rain, lightening and hail stones common. The hail results from ice pellets being carried up and down within the cloud, gaining a new ice layer each time. Formation of hail may also be involved in the production of lightening - shattered ice pellets may carry positive charges upwards (by updrafts), while hail pellets carry negative charges downwards - thus the cloud becomes electrically charged. Eventually, downdrafts dominate the cloud and the thunderstorm dissipates. Harry Williams, Earth Science 17 Harry Williams, Earth Science 18 Thunderstorms are most common along the intertropical convergence zone, due to the combination of high temperatures, high humidity and high instability. In the U.S. they are most common in the southeast, where warm humid air from the Gulf of Mexico penetrates inland. Harry Williams, Earth Science 19 Tornadoes: A moving mass of air often moves faster aloft than at the surface – a rotation or “forward-rolling” effect can result. If this air encounters strong uplift e.g. a front or developing thunderstorm, the rotating air can be lifted upright. It becomes a vertical rotation. Harry Williams, Earth Science 20 Strong updrafts can stretch the rotating air vertically, so that it narrows horizontally and spins faster – a funnel cloud. If the funnel cloud extends to earth, it is a tornado. Rapid uplift of air within the tornado lowers the air pressure – often 10% lower than surroundings. This causes strong winds at the surface (pgf) – sometimes exceeding 300 mph – these winds cause much of the destruction. Harry Williams, Earth Science 21 Many tornadoes are associated with thunderstorms, so they occur in thunderstorm season – May and June – and they occur where thunderstroms are common e.g. Texas, the southeast, the mid-west. Harry Williams, Earth Science 22 Tropical Cyclones Cyclonic circulations that develop in the tropics can be very powerful because the warm tropical oceans provide an abundant supply of energy (in the form of latent heat released by rising tropical air). Unlike mid-latitude cyclones, they are no contrasting air masses or fronts involved in the growth of tropical cyclones. Harry Williams, Earth Science 23 They probably develop along the lee of easterly waves of low pressure. Surface winds converging on the wave are deflected counterclockwise and cause rising air (convergent uplift). The uplift lowers pressure even more and the system intensifies into a cyclonic circulation. Harry Williams, Earth Science 24 Once developed, these cyclonic systems move generally to the west. The central low pressure can intensify (e.g. < 900 mb), causing very large PGF and high wind speeds. If the winds reach 74 mph (and sometimes exceeding 200 mph), they are hurricanes (or cyclones or typhoons). Harry Williams, Earth Science 25 Whether or not a hurricane is damaging to the U.S. depends on where it matures – if it matures early (mid-Atlantic 400 west) it will usually curve to the north and miss the U.S. coast. Hurricanes that mature further west (after 700 west – Dominican Republic), have a greater chance of striking the U.S. Like tornadoes, high speed winds cause much of the damage, although flooding from heavy rain and storm surges (sea water pushed onland by strong winds) can also be very destructive and deadly, especially in coastal areas e.g. 300,000 dead in Bangladesh in 1970 (many drowned). Harry Williams, Earth Science 26