10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld Chapter 8 Weather I Atmospheric Lifting Mechanisms a Convergent Lifting i When air masses meet each other the only place to go is up which causes cooling condensation and cloud formation ii Examples low pressure systems and ITCZ b Convectional Lifting i Happens when there is a heat differential on the surface dark surfaces absorb more heat and then the air above the surface will warm up and then rise cool condense and form clouds ii Occurs on fair weather days cumulus clouds iii Examples dark surfaces urban heat island and fires c Orographic Lifting i Air is forced upward over a mountain ii Windward side precipitation iii Leeward side dry rain shadow and chinook winds dry warm winds 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld iv Orographic Precipitation MAR Lifting Condensation Level DAR DAR d Frontal Lifting i Lifting along fronts cold and warm fronts Cirrus clouds are an indicator of an incoming front ii Cold fronts ushering in of cold air mass 1 Cold air forces warm air up 2 400 km wide 250 miles 3 Rapid and severe thunderstorms squall line 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld iii Warm fronts ushering in of a warm air mass 1 Warm air moves up and over cold air 2 1000 km wide 600 miles 3 Gradually lift light drizzle II Air Masses a Homogeneous body of air that has taken on the temperature and moisture characteristics of its source region i Source Region large flat homogeneous ii Air masses form in low latitudes and high latitudes bit not Midlatitudes too unstable b Classification i Humidity 1 m maritime moist 2 c continental dry ii Temperature 1 A Artic cold very cold 2 P Polar cool cold 3 T Tropical warm hot 4 E Equatorial hot very hot 5 AA Antarctic cold very cold c Major Air Masses Affecting the U S i cA and cP dry cool stable high pressure 1 Alaska and Canada ii mP moist cool unstable 1 Pacific Aleutian low and Atlantic Icelandic low iii mT humid warm 1 Gulf and Atlantic unstable winter to very unstable Summer 2 Pacific Stable to conditionally unstable cold ocean currents make the surface temperatures cooler makes the air masses more stable iv cT dry hot 1 Only in the summer in the U S 2 Form over Northern Mexico 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld III Midlatitude Cyclones a Polar Front i The leading edge of the polar air as it approaches Midlatitudes ii Forms a stationary front 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld b Cyclone area of low pressure with converging and rising air c A K A wave cyclones because they travel westerly across the U S in waves d Develop in four stages i Cyclogenesis either the polar or Midlatitude air mass moves forward and one overpowers the other air rises creates low pressure zone ii Open Stage Fronts will continue to move cold front will advance on the warm front cold air moves faster than warm air it s denser Not quite a cyclone yet iii Occluded Stage Once the cold front catches up with the warm front an occluded front will form cold air in front of cold front and behind cold front iv Dissolving Stage An occluded front means no opposing air masses so the cyclone dissipates 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld e Happen during the winter and early spring Causes blizzards in the NE U S i Storm Tracks In Winter pattern is higher because the Polar Front is higher than in the summer IV Severe Weather a Thunderstorms i Three places they develop In warm moist areas 1 2 Along cold fronts 3 On the windward side of orographic lifting upslope winds ii Frequency of thunderstorms 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld iii Three stages of formation 1 Developing Cumulus Stage Uplift and adiabatic cooling 2 Mature Stage Precipitation starts with lightning and thunder strong updrafts and strong downdrafts cumulonimbus cloud 3 Dissipating Stage When downdrafts dominate iv Hailstones 1 Found if the thunderstorms are really severe 2 Made up of concentric rings because hail is blown up and down within the cumulonimbus clouds and is melted and refrozen multiple times before it becomes so heavy that it drops b Tornadoes i Form in severe thunderstorms ii General Characteristics 1 Few meters to hundreds of meters in diameter 2 Last seconds to tens of minutes 3 Winds up to 300 mph 485 kph 4 Average ground speed is 44 55 mph 5 Average path length is 7 km 4 3496 miles 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld iii Global Frequency of Tornadoes Most frequent in North America because there are north south mountain ranges Rockies and Appalachians which funnel cP and mT air masses together while in other parts of the world the mountain ranges run East to West which keeps the cP and mT air masses from meeting iv Location of Tornadoes in the U S Most frequent in Tornado Valley in the plains because that is where the cP and mT air masses are most able to meet 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld v Seasonal Distribution of Tornadoes Peak in late spring early summer because the temperature gradient is the largest and mT and cP air masses are BOTH around in the transitional months Increase in Tornadoes over the Years vi Increase is due to better technology easier to measure and increase in population density more people to report increased tornadic activity HAS NOT been tied to Global Climate Change 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld vii Enhanced Fujita Scale Weak 69 Strong 29 Violent 2 viii Formation Mesocyclone to Tornado 1 Wind Shear Weak winds on the surface and stronger winds above creates horizontal spinning air mass 2 Thunderstorm forms has updrafts tilts the horizontal spinning air vertical mesocyclone 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld 3 Mesocyclone touches down on the surface tornado if it touch down on the surface funnel cloud c Tropical Cyclones i Different from Midlatitude cyclones 1 No Fronts 2 No differing air masses warm year round ii Form from Easterly Waves Trade Winds 1 Occur between 5 and 30 not at the equator because no Coriolis Force 2 Many form over North Africa that affect the U S iii Easterly Wave bump caused by differential heating either land edge ocean currents meeting etc 10 21 15 10 23 15 10 26 15 10 18 15 Dr Frauenfeld 1 Convergence in upstream of wave a Rising air b Thunderstorms 2 Divergence in downstream of wave 3 a Sinking Air b Dry If convergence continues the wave will form a …