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  1. 1. Weather
  2. 2. Overview Air Masses mP, cP, mT, cT Lifting Mechanisms convectional, convergent, orographic, frontal Mid-Latitude Cyclonic Systems Severe Weather Thunderstorms Tornadoes Tropical Cyclones
  3. 3. Weather Weather vs. Climate Weather refers to atmospheric conditions at any given place and time Climate is a long term average of rainfall and temperature patterns Weather is the product of different air masses coming into contact through pressure induced movement, such that atmospheric instability is produced through lifting. Air masses Lifting mechanisms Pressure systems
  4. 4. Air Masses The characteristics of air are influenced by the region of origin latitude (temperature) A, AA – Arctic, Antarctic P – Polar Mid-latitudes receive air masses from higher and lower latitudes T- Tropical E- Equatorial surface, either oceans or continents (moisture) m – maritime c- continental
  5. 5. For North American weather, important air masses include: cP (continental polar): cold, dry, stable air mP (maritime polar): cold, moist, unstable mT (maritime tropical): warm, moist, variable stability mT Gulf/Atlantic – warmer, very unstable (Gulf Stream) mT Pacific – less moist, stable to conditionally unstable (cold ocean currents)
  6. 6. Influence of mT Gulf/Atlantic air mass on US rainfall
  7. 7. Lifting Mechanisms For instability, adiabatic cooling, saturation, condensation, cloud formation and precipitation to occur, air must rise. Convectional Lifting Convergent Lifting Orographic Lifting Frontal Lifting
  8. 8. Convectional Lifting Land masses heat up more quickly than oceans when maritime air masses cross onto land, they experience surface heating creates low pressure, and air rises Convectional lifting common over Urban heat islands surfaces with low albedo
  9. 9. Convergent Lifting Winds converge on an area of low pressure Collision of winds force air upwards Common in the ITCZ Produces a high amount of precipitation
  10. 10. Orographic Lifting If a pressure gradient crosses a mountain range, wind flow is forced over the mountains Wind travels upslope (windward side), then drops back downslope (leeward side) Cooling, saturation, condensation, cloud formation and precipitation on windward side Removes moisture from the air Dry wind drops down leeward side, warming as it drops Rain shadow
  11. 11. For islands, orographic rainfall is the primary source of rainfall. During times of high prevailing winds, a definite rain shadow effect is present. One side of the island can experience a tropical rainforest climate while the other experiences tropical monsoon patterns. Atolls suffer especially, since the highest point of elevation rarely exceeds 5 m above sea level.
  12. 12. Frontal Lifting The leading edge of an advancing air mass is called a front. Cold Fronts Cold, stable air advances into warm air mass, causing abrupt and rapid uplifting and lowering of pressure Vertically developed clouds, thunderstorms Warm Fronts Warm air advances over cold air, less abrupt lifting Form a variety of stratiform clouds at various altitudes Warm air creates an inversion layer for cold air beneath With cyclonic systems: Stationary fronts: warm and cold fronts meet head-on Occluded fronts: cold front overtakes warm front in cyclonic system
  13. 13. Midlatitude Cyclonic Systems The movement and collision of air masses create moving centers of low pressure Tend to move from West to East, creating paths called storm tracks Cyclonic rotation of air around the low brings cold air masses from the north and west and warm air masses from the south Creates a rotation of warm and cold fronts around the low pressure center Cold front tends to move faster than warm front, so the fronts eventually become occluded This pattern is called a Midlatitude Cyclone or a Wave Cyclone
  14. 14. Cycle Storm tracks Higher latitudes during the summer Greatest frontal activity during the Spring Cyclogenesis Form on polar front, eastern slopes of mountains Intensification of low pressure, commencement of rotation Open Stage Cold air masses drawn from North and West, Warm air from the South Occluded Stage Cold front overtakes warm front, forming an occluded front Dissolving Stage Lifting energy is spent, weather patterns dissipate
  15. 15. Severe Weather Thunderstorms Occur wherever there is pronounced lifting by any of the mechanisms Common along cold fronts Rapid condensation liberates large quantities of heat energy Thunderstorms produce Lightning charge separation due to ice crystals at top layer of cloud Positive charges migrate to top of cloud, electrons to bottom and ground Hail Strong updrafts hold rain drops aloft, recirculating them and freezing them Atmospheric Turbulence Incoming gusts from convergent winds Outgoing downdrafts pulled along by falling precipitation
  16. 16. Tornadoes As faster upper level winds overtake slower surface winds, horizontal-axis turbulence develops This horizontal rotating air mass can be lifted by the updrafts of thunderstorms into a vertical orientation Accelerated by the cyclonic uplift of the storm (positive feedback) Forms a mesocyclone Smaller turbulent rotations within the mesocyclone form Funnel Clouds A tornado occurs when a funnel cloud reaches the ground Fujita Scale (F scale)
  17. 17. Tropical Cyclones Form in the Tropics Coriolis force is insufficient near the equator for cyclone formation Energy comes from hot ocean water Low pressure centers migrate across oceans Winds converge, uplift, reach tropopause, diverge Divergence aloft accelerates updraft, decreases pressure If a jet stream is overhead, further acceleration occurs Heat energy released by condensation further fuels process Extreme drop in pressure causes a steep pressure gradient, converging winds accelerate to high speeds, and a tight, spiral structure is formed
  18. 18. Structure Eye An area of calm at the center Eye wall Band of intense thunderstorms surrounding the eye Most intense activity Rain bands alternating zones of rainfall in spiral structure Storm surge Low pressure raises sea level below center