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4/24/2012 Air Masses Classified by latitude and source regions • Maritime equatorial (mE) tropical (mT) • Continental tropical (cT) • Maritime polar (mP) • Continental Arctic (cA) • Continental Antarctic (cAA) • Maritime Weather Systems Geo210 Moisture An Introduction to Physical Geography Continental (c) Air Mass – dry Maritime (m) Air Mass - moist Michael Paluzzi, ABD Weather Weather – short-term, day-to-day expression of atmospheric processes Ex. - Today is clear, cold and sunny Climate – long-term, average conditions Usually at least 30 years of daily weather data (temperatures and precipitation) Ex. - CT – Humid Continental, warm summer Climate regions – boundaries drawn around areas with similar average climate conditions Meteorology – the scientific study of the atmosphere Air Masses Air mass – homogeneous body of air that has taken on the moisture and temperature characteristics of it’s source region Earth’s surface get it’s temperature and moisture characteristics from these air masses. Regional air masses has differing temperature, humidity and stability Air Mass Types Continental Polar (cP) Occur only in northern hemisphere Responsible for weather in mid and high latitude locations Cold dense air lifts warm moist air Producing lifting, cooling, condensation Greater difference in cold and warm air masses, more dramatic weather produced Lack of landmasses in southern hemisphere prevent the occurrence Lake-Effect Snow cP air masses move south and east Cold air passes over warmer Great Lakes Air masses are warmed and water vapor added As the air masses move over cold land, they cool and reach the dew point, where the condensation falls as heavy snow Their interaction produces weather patterns 1 4/24/2012 Lake-Effect Snow Atmospheric Lifting Mechanisms Convectional Lifting FL – landmass warmer than Gulf of Mexico Local heating lifts warm, moist from the Gulf of Mexico, as it passes over the warmer land Causing frequent afternoon showers Atmospheric Lifting Mechanisms Atmospheric Lifting Mechanisms Convergent Lifting - air along the surface flows toward a low pressure center and rises Orographic Lifting - air is forced over a barrier (like a mountain range) Common in the tropics NE & SE trade winds converge Forming large cumulonimbus clouds High average annual temperatures Windward side – wet Leeward side – dry Rain Shadow Atmospheric Lifting Mechanisms Convectional Lifting - stimulated by local surface heating Maritime air mass passes over continental Heating from land causes lifting Atmospheric Lifting Mechanisms Orographic rainfall it occurs in the Western Ghats and Himalayas in South Asia – resulting in Rain-shadow effect: the area of low rainfall found on the leeward (or downwind side) of a mountain range Plowed fields Local heating due to dark soil 2 4/24/2012 Atmospheric Lifting Mechanisms Cold Fronts Orographic rainfall Atmospheric Lifting Mechanisms Frontal Lifting (Cold and Warm Fronts) along the leading edges of contrasting air masses. Front – a place of atmospheric discontinuity, a narrow zone forming a line of conflict between two air masses Cold Front – Squall Line Squall line – fast moving cold front that forms a line of severe storms along a cold front Large cumulonimbus clouds form, producing heavy precipitation, hail, frequent lightning, strong straight line winds, and possibly tornadoes Leading edge of cold air mass is a cold front Leading edge of warm air mass is a warm front Frontal Lifting - Cold Front Cold Fronts Denser, advancing cold air forces warm, moist air to lift abruptly. As the air is lifted, it cools by expansion, cooling to the dew - point as it rises it condenses and forms clouds. Cumulonimbus clouds may produce large raindrops, heavy showers, lightning and thunder, and hail. Frontal Lifting – Warm Front Warm front: moving weather front along which a warm air mass slides over a cold air mass Gentle lifting of the warm, moist air produces nimbostratus and stratus clouds and drizzly rain showers – sometimes a thunderstorm 3 4/24/2012 Warm Fronts Dangerous Weather Sleet – frozen raindrops or partially refrozen snowflakes that bounce when they reach the ground Stationary Front Stationary Front: two air masses are in contact, but there is little or no relative motion Once this boundary resumes its forward motion, it once again becomes a warm front or cold front Rain Dangerous Weather Freezing Rain – precipitation that starts as snow at high altitudes, melts and freezes after it hits the ground Violent Weather Thunderstorms - condensation of large amounts of water vapor creates lots of energy, heating the air Causes up drafts Raindrops form and cause friction as they fall, causing downdrafts Giant cumulonimbus clouds cause dramatic weather Heavy rain, lightning, thunder, hail, heavy winds 4 4/24/2012 Violent Weather three months' NASA's Lightning Imaging Sensor (LIS) records all lightning strikes between 35° N and 35° S latitudes Hailstones Hail is a form of precipitation which consists of balls or irregular lumps of ice Form in strong thunderstorm clouds, particularly those with intense updrafts Water droplets accumulate as ice falls, updrafts carry it up, then falls again – creating another layer of ice. Once it’s too heavy for updraft, it falls to ground Stronger the updrafts – larger the hail stones Violent Weather Hailstones Thunder – produced by a sonic shock wave caused by the rapid expansion of the air surrounding a bolt of lightning. can range from a sharp, loud crack to a long, low rumble Figure 8.21 Violent Weather Lightning – flashes of light caused by atmospheric discharge of electricity Freak Hailstorm, Bogota, Colombia, November 3, 2007 These discharges heat the air to temperatures 15000 ° C – 30000° C 5 4/24/2012 Violent Weather Derecho – straight line wind associated with a thunderstorm winds associated with derechos blow straight ahead – not in circles like tornadoes Violent Weather Birth of a Tornado Mesocyclone forms as a rotating updraft within the thunderstorm. mesocyclone: a rapidly rotating air mass with a thunderstorm that often gives rise to a tornado If one forms, a tornado will descend from the lower portion of the mesocyclone. Violent Weather Tornadoes Birth of a Tornado Air at higher altitudes moves faster than surface air Creating rotation in the air parallel to the ground Violent Weather Violent Weather Birth of a Tornado Updrafts create a shift in the axis of rotation rotation in the air perpendicular to the ground 6 4/24/2012 Violent Weather Fujita Scale – scale for rating tornado intensity based on damage to human-made structures, vegetation Category Wind Speed F0 65-85 mph F1 86-110 mph 36% Moderate Damage: severe roof damage, mobile homes overturned, broken glass F2 111-135 mph 19% Considerable Damage: roofs torn off, foundations shifted, large trees snapped, cars lifted off ground F3 136 -165 mph Severe Damage: severe damages to large buildings (malls), trains overturned, structures with weak foundations blown great distances F4 166-200 mph 1% Devastating Damage: homes completely leveled, cars thrown, small missiles generated F5 >200 mph <0.1% Incredible Damage: houses swept away, cars carried 100 meters, structural damage to high-rises 39% 5% Potential Damage Light Damage: Minor roof damage, gutters, siding damaged Branches broken off trees Top Ten Deadliest Tornadoes Rank 1 States Date MO-IL-IN March 18, 1925 F-Scale F5 Dead Injured 695 2027 Murphysboro, DeSoto LA-MS May 7, 1840 317 109 Nachez 3 MO-IL May 27, 1896 255 1000 St. Louis, E. St. Louis 4 MS April 5, 1936 216 700 Tupelo 5 GA April 6, 1936 203 1600 Gainesville 181 970 Glazier, Higgins TX-OK-KS April 9, 1947 F5 F5 7 LA-MS April 24, 1908 8 WI June 12, 1899 9 MI June 8, 1953 F5 115 844 Flint 10 TX May 11, 1953 F5 114 597 Waco F5 Figure 8.23 Tornadoes Towns 2 6 Tornado Damage 143 770 Amite,. Pine, Purvis 117 200 New Richmond Figure 8.25 7 4/24/2012 F5 Tornado, Oklahoma City, May 3, 1999 Saffir-Simpson Hurricane Scale 1995 Atlantic Storm Season Tropical Cyclones Tropical cyclone: generic term for low pressure system over tropical waters with convection – thunderstorms rotation – cyclonic circulation. Air masses originate in tropics (doldrums – 10°) Fueled by warm moist air of the tropics Figure FS 8.1.1 Hurricane Andrew Tropical Cyclones Tropical Depression: maximum sustained winds of 39 mph Tropical Storm: maximum sustained winds of 74 mph Hurricane: minimum sustained winds of 75 mph (Northeast Pacific, North Atlantic, South Pacific Ocean) Typhoon: Northwest Pacific Ocean Severe Tropical Cyclone: Southwest Pacific Ocean, Southeast Indian Ocean Severe Cyclonic Storm: North Indian Ocean Tropical cyclone: Southwest Indian Ocean Figure 8.31 8 4/24/2012 9