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Slide 1 Visualizing Physical Geography by Alan Strahler and Zeeya Merali Chapter 6 Weather Systems ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 2 ___________________________________ Chapter Overview Air Masses ___________________________________ Fronts Midlatitude Cyclones Tropical Cyclones ___________________________________ Thunderstorms and Tornadoes ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 3 ___________________________________ Air Masses Weather • Weather refers to the state of the atmosphere at one location and at one time • Weather variables include: • Temperature • Moisture (Dew Point and RH) • Precipitation • Winds • Weather conditions change from day to day or even from hour to hour ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 4 ___________________________________ Air Masses Weather • Weather systems are patterns of atmospheric circulation that lead to distinctive weather events, such as cyclones or thunderstorms. ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 5 ___________________________________ Air Masses Weather • Air mass = An extensive body of air in which temperature and moisture characteristics are fairly uniform over a large area ___________________________________ ___________________________________ 1) What is the current high temperature, wind, moisture (dew point), and precipitation forecast for today? 2) Based on today’s weather, where do you think the air mass came from? ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 6 ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 7 ___________________________________ Air Masses Source Regions of Air Masses •Source region: region where an air mass acquires its characteristics •Moisture characterized by whether it came from the: • Ocean = moist = m • Continent (land) = drier = c •Also classified by latitude: • Maritime equatorial (mE) • Maritime tropical (mT) • Continental tropical (cT) • Maritime polar (mT) • Continental Arctic (cA) • Continental Antarctic (cAA) ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 8 ___________________________________ Air Masses Source Regions of Air Masses •Maritime equatorial (mE) •Maritime tropical (mT) •Continental tropical (cT) ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 9 ___________________________________ Air Masses Source Regions of Air Masses •Maritime polar (mT) •Continental Arctic (cA) •Continental Antarctic (cAA) ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 10 ___________________________________ Air Masses Movement of Air Masses (Air mass modification) • When an air mass moves to a new area, its properties change due to the influence of the new surface environment What air mass is impacting your region today? ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 11 Fronts ___________________________________ Front = The surface or boundary of contact between two different air masses associated with a midlatitude cyclone (MLC) • Four types of fronts: • Cold ___________________________________ • Warm • Stationary • Occluded ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 12 ___________________________________ Fronts Cold Front • A moving weather front along which a cold air mass moves underneath a warm air mass, causing the warm air mass to lift rapidly • Air rises steeply • Cumulonimbus clouds and thunderstorms may form ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 13 ___________________________________ Fronts Warm Front • A moving weather front along which a warm air mass slides over a cold air mass, producing stratiform clouds and precipitation •Air rises gradually •Nimbostratus clouds may form Which type of clouds first indicates an approaching warm front? a. nimbostratus b. stratus c. altostratus d. cirrus ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 14 ___________________________________ Fronts Occluded front • A weather front along which a fast-moving cold front overtakes a warm front, forcing the warm air mass aloft • Triangles and semicircles pointing in the same direction ___________________________________ ___________________________________ Stationary Front • Little to no relative motion • Blue triangles and red semicircles pointing in opposite directions ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 15 ___________________________________ Fronts 1. Why was the warm air forced upward? 2. The clouds mark an advancing front. What type of front might this be? ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 16 ___________________________________ Midlatitude Cyclones Cyclones • Air spirals inward, converges and moves upward • As the air rises, it cools adiabatically • If the air becomes saturated at the LCL, condensation or deposition occurs, forming clouds and possibly precipitation • Low pressure • Three types: midlatitude cyclone (MLC), tropical cyclone, and mesocyclone ___________________________________ ___________________________________ Anticyclone • • • • Air spirals outward and diverges as it sinks The air warms as it sinks Fair skies High pressure ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 17 ___________________________________ Midlatitude Cyclones Cyclones and Anticyclones C B ___________________________________ A ___________________________________ 1) Is a cyclone or anticyclone impacting the region shown in Figure A? 2) Is a cyclone or anticyclone impacting letter C? 3) Is a cyclone or anticyclone impacting letter A? ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 18 Midlatitude Cyclones (MLC) ___________________________________ Evolution of a midlatitude cyclone • Midlatitude cyclone also known as a wave cyclone • Dominant in middle and higher latitudes • Large inspiral of air that repeatedly forms, intensifies, and dissolves along the polar front • In the northern hemisphere, a cyclone normally forms at a polar front and moves eastward as it develops, propelled by prevailing westerly winds aloft ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 19 Midlatitude Cyclones (MLC) ___________________________________ Evolution of a midlatitude cyclone • Initial conditions • Cyclogenesis • Open stage • Occluded stage • Dissolving stage ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 20 Midlatitude Cyclones (MLC) ___________________________________ ___________________________________ ___________________________________ In the open stage of the cyclone depicted in step 3, a ____ air mass could be behind the warm front, and a ___ air mass could be behind the cold front. a. maritime tropical; continental polar b. continental polar; maritime tropical c. maritime polar; continental tropical d. continental polar; continental tropical ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 21 Midlatitude Cyclones (MLC) ___________________________________ Weather changes within a midlatitude cyclone ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 22 Midlatitude Cyclones (MLC) ___________________________________ Weather changes within a midlatitude cyclone • Cyclonic storm = An intense weather disturbance within a moving cyclone that generates strong winds, cloudiness, and precipitation • Nor’easter ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 23 Tropical Cyclones ___________________________________ Tropical Cyclone • An intense traveling cyclone of tropical and subtropical latitudes, accompanied by high winds and heavy rainfall • Naming: • Hurricanes in western hemisphere • Typhoons in the western Pacific • Cyclones in the Indian Ocean • Develop over oceans approximately between 8 to 20oN/S • Winds greater than 74 mph ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 24 Tropical Cyclones ___________________________________ Tropical Cyclone • Characteristics • Eye = relatively calm, sinking air at its center • Eyewall = strongest section, with intense rising air and wind • Spiral rain bands ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 25 Tropical Cyclones ___________________________________ Formation of Tropical Cyclones • Four conditions need to be in place: • Low pressure • Weak Coriolis force • High humidity • Warm sea surface temperatures ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 26 Tropical Cyclones Formation and Movement of Tropical Cyclones • Tropical cyclone originates as a slow-moving band of low pressure, which then intensifies and grows into a deep, circular low • Easterly wave = a slowly moving trough of low pressure within the belt of tropical easterlies (trades) that grows over the warm ocean • Does not form over the equator since there is no coriolis force ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 27 Tropical Cyclones ___________________________________ Formation and Movement of Tropical Cyclones • High humidity = Always form over the warm ocean with high water vapor content. • Warm sea surface temperatures (SST) greater than 81oF • Season = peaks in late summer/early autumn due to warmest SST ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 28 Tropical Cyclones Movement of Tropical Cyclones • Typically, clockwise movement in the N. hemisphere • West with trade winds • Then moves northwest, north, and eventually northeast following the winds from the subtropical high and the upper level westerlies ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 29 Tropical Cyclones ___________________________________ ___________________________________ ___________________________________ Tropical cyclones do not form in the southern Atlantic Ocean because _____. a. the water is too warm b. the water is not warm enough c. cyclones rotate clockwise in the southern hemisphere d. cyclone tracks would hit land ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 30 Tropical Cyclones ___________________________________ Movement of Tropical Cyclones • Hurricane tracks for 1985–1994 (blue) and 1995–2004 (red) • The intensity of storms increases when sea surface temperatures increase ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 31 Tropical Cyclones ___________________________________ Significant Tropical Cyclones • Naming of Tropical Cyclones • Hurricane Andrew (1992) ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 32 Tropical Cyclones ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 33 Tropical Cyclones Tropical Cyclone Intensity and Damage • Saffir-Simpson Scale ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 34 Tropical Cyclones Tropical Cyclone Intensity and Damage • Precipitation • Earthflow ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 35 Tropical Cyclones Tropical Cyclone Intensity and Damage • Storm Surge • A rapid rise of coastal water level accompanying the onshore arrival of a tropical cyclone • Deadliest aspect of tropical cyclone ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 36 Tropical Cyclones Tropical Cyclone Intensity and Damage • Hurricane Katrina Flooding (2005) in New Orleans ___________________________________ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 37 Thunderstorms and Tornadoes ___________________________________ Thunderstorms and Unstable Air • Thunderstorms may form as a result of intense heating of the ground, resulting in the heated parcel of air warmer (and less dense) than the surrounding air • Air cools to the dew point, clouds form (cumulus) and may grow into cumulonimbus (thunderstorm) clouds • Clouds and storm may dissipate with the loss of heating ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 38 Thunderstorms and Tornadoes ___________________________________ Thunderstorm • An intense local storm associated with a tall, dense cumulonimbus cloud in which there are very strong updrafts of air ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 39 Thunderstorms and Tornadoes ___________________________________ Severe Thunderstorm • Intense winds, precipitation, and hail • Strong updrafts and downdrafts • Anvil shape • Wind shear • Change in wind velocity with height • Required in a severe thunderstorm • May result in rotation (mesocyclone) ___________________________________ ___________________________________ In which direction is this thunderstorm moving, and how do you know? Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. ___________________________________ Slide 40 Thunderstorms and Tornadoes ___________________________________ Lightning • Upward and downward motions in a thunderstorm create areas of positive and negative static charge within the cloud that are discharged by lightning ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 41 Thunderstorms and Tornadoes ___________________________________ Forecasting Thunderstorms • Hot, summer weather in central and SE US • National Oceanic and Atmospheric Administration (NOAA) and National Weather Service (NWS) • For a thunderstorm outlook, go to: www.spc.noaa.gov/products/exper/enhtstm/ ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 42 Thunderstorms and Tornadoes ___________________________________ Tornadoes • A small, very intense wind vortex with extremely low air pressure in the center, formed below a dense cumulonimbus cloud as apart of a cyclone, typically an MLC • Mesocyclone (rotating thunderstorm) ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 43 Thunderstorms and Tornadoes ___________________________________ Tornadoes • Enhanced Fujita Scale ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 44 Thunderstorms and Tornadoes ___________________________________ Tornadoes • Most Common in central (tornado alley) and SE United States where cold and warm air clash ___________________________________ ___________________________________ ___________________________________ Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. Slide 45 Thunderstorms and Tornadoes ___________________________________ ___________________________________ ___________________________________ Which stage of a thunderstorm leads to tornado formation? a. cumulus b. mature c. dissipating Visualizing Physical Geography Copyright © 2012 John Wiley & Sons, Inc. ___________________________________
