CHAPTER 8 REVIEW 1 REVIEW Air masses continental polar cP continental tropical cT maritime polar mP maritime tropical mT What properties Where do they come from a Continental Polar i Properties Continental Polar Winds come from the Land 1 Cold dry stable 2 As the cP air mass moves over the warmer Gulf of Mexico and Gulf Stream waters surface warmed air becomes unstable rises and forms extensive rows of cumulus cloud streets 3 Western mountains such as the Rocky and Sierra Nevada ranges normally protect the Pacific Northwest from cP air 4 Strong highs however can create NE winds that cause cold outbreaks along the western coast ii Where Comes from the Rocky Mountains and Sierra Nevada Ranges 1 Summer cP Summer cP air over the US brings relief from heat but also triggers steeper environmental lapse rates due to ongoing surface heating and will lead to cumulus cloud development 2 Winter cP Cold surfaces during the winter create temperature inversions b Continental Tropical i Mexican Air c Maritime Polar i Where 1 Dry hot air from the Mexican desert can cause low level instability in the U S interior during summer and may trigger dust devils 2 An upper level ridge of high pressure may add compressionally heated air to the region enhancing the dry hot conditions 1 Cold air passing over the ocean south of the Aleutian L will pick up warmth and moisture and reaches the Pacific Coast as cool moist and unstable bringing rain and snow 2 East Coast mP Air a A strong anticyclone in eastern Canada creates NE winds that may bring cold unstable Atlantic mP air and storms into New England and the middle Atlantic States 3 These storms are known as Northeasters d Maritime Tropical i Properties 1 Warm and moist maritime air from the tropical Pacific ii Where 1 Gulf and Caribbean Air a Gulf of Mexico and Caribbean Sea warmth and moisture flows into the East Coast by a strong anticyclone b When mT air rises above dense cP air heavy precipitation can result 2 Warm fronts cold fronts occluded fronts What meteorological variables change during a frontal passage and how What specific properties e g precipitation where are warm and cold air masses a Warm Front is a warm front i From the vantage point of the ground if warm air replaces colder air the front ii Typically moving from southwest SW to northeast NE iii Air behind the front is much warmer and more moist than air ahead of front iv Precipitation occurs in front of the surface warm front v Warm Front Transition 1 Unique clouds and precipitation patterns with a broader range of showers than in a cold front Clouds well in advance of the surface front 2 The cross sectional view shows gentle slope 1km in 300km of overrunning warm air a typical temperature inversion shifting winds b Cold Front i If cold air replaces warmer air the front is a cold front ii Typically move from northwest NW to southeast SE iii Air behind the front is noticeably colder and drier than air ahead iv precipitation occurs at and behind the surface front v Cold Front Transition 1 The front rises steeply 1km in 50km and cirri form clouds protrude c Occluded Front ahead i Fast moving cold fronts will eventually overtake the slower moving warm front and push the warm air aloft ii Temperature differences at the occluded front will be moderate 3 Cyclogenesis When can surface low pressure systems intensify What is a barotropic what is a baroclinic atmosphere b Cyclogenesis a A low pressure system may develop into a cyclone cyclogenesis if the low pressure system intensifies pressure drops in the center of the low i When upper air divergence air going out is stronger than lowerlevel convergence air coming in more air is going out at the top than is brought in at the bottom 1 2 surface pressure drops the low intensifies or deepens c Barotropic conditions d baroclinic state i where isobars and isotherms are parallel i where the lines cross and cold or warm air is advected downwind 4 Thunderstorms Prerequisites Stages What clouds Lightning a occur when warm moist air rises CHAPTER 10 11 in an unstable environment i ii near weather fronts iii iv over mountain slopes in advance of upper level troughs b Stages i Cumulus ii Mature iii Dissipation c Lightning 1 An unstable atmosphere and vertical updrafts keep precipitation suspended Cu cloud grows to Cb cloud 1 Entrainment of dry air that causes cooler air from evaporation triggering downdrafts and falling precipitation and gust fronts 1 Weakening of updrafts Downdrafts and evaporation begin to dominate i Charge layers in the cloud are formed by the transfer of positive ions from warmer hailstones to colder ice crystals ii Rapid heating of air 30 000 C creates an explosive shock wave thunder which requires approximately 3 s to travel 1 km iii Nearly 90 of lightning is the negative cloud to ground type iv But positive cloud to ground lightning can generate more current and more damage v Distant unseen lightning is often called heat lightning 5 Tornadoes Hurricanes Prerequisites Properties Where do they occur Where are strongest winds a Tornado i A rapidly rotating column of air often evolve through a series of stages from dust whirl to organizing and mature stages and ending with the shrinking and decay stages ii Occurrence 1 Tornadoes from all 50 states of the U S add up to more than 1000 tornadoes annually but the highest frequency is observed in tornado alley of the Central Plains 2 Nearly 75 of tornadoes form from March to July 3 more likely when warm humid air is overlain by cooler dryer air causes strong vertical lift iii Strongest Winds 1 Less wind speed occurs on the left side of the tornado s path trajectory 2 Maximum wind speed occurs on the right side of the tornado s path b Hurricanes i Structure of a Hurricane 1 Circulate counter clockwise around their centers in NH 2 Diameter range from 100 to 1500 km average 600 km 3 Hurricane season normally last from June to November ii Development 1 2 3 tropical disturbance tropical depression a winds 20 knots tropical storm a Winds 35 knots 4 Hurricane a Winds 64 knots CHAPTER 12 6 REMEMBER THESE Factors which control climate intensity of sunshine latitude altitude distribution of land and ocean ocean currents Intensity of Sunshine a Latitude b c Altitude d Distribution of land and ocean e Ocean currents 7 Regions of sinking and rising air masses ITCZ subtropical high polar front polar high 8 Global Climates tropical wet climate tropical wet and dry climate arid climate semi arid climate