Transcript Chapter
Chapter 8 Air Masses The Atmosphere 10e Lutgens & Tarbuck Air Masses • Immense bodies of air • ~1600 kilometers (1000 miles) across on average, can be much wider • Several kilometers thick • Similar properties at a given altitude, such as temperature or moisture content Movement of a Cold and Dry Air Mass Brings Winter Weather The Siberian Express Source Regions • The areas where air masses originate • The location of the source region largely determines the characteristics of the air mass • A source region should have a relatively uniform topography, and… • Should have relatively stagnant atmospheric motion for a period of time A Hot and Dry Air Mass Source-region Air Mass Source Regions Classifying Air Masses Temperature (latitude) P-Polar A-Arctic T-Tropical Humidity m-Maritime-wet (generally over water) c-Continental-dry (generally over land) A Maritime Air Mass Source-region Air Mass Modification k-cold w-warm •As airmasses move away from source regions, they change slowly •May get warmer, cooler, more or less humid •cP or cA airmasses that move over water in winter can convert to mP airmasses •If the airmass is colder than the surface it passes over, we add a k; we add a w in the reverse •Consider the stability implications More Modification • We just discussed temperature modification, with some associated humidity aspects • Basically associated with temperature differences between airmass and ground below • There are also mechanical modifications caused by airflow around weather systems and/or topography such as mountains cP and cA • Air masses formed north of about 50°N latitude • cA is a little colder than cP, but many meteorologists do not differentiate between them • Generally very dry, except when passing over warmer bodies of water • The cause of our cold snaps in fall and winter More cP and cA • The source region for these air masses is stable • Can be modified to cPk in winter due to passage over the Great Lakes • This can result in “Lake Effect Snow” on the lee side of the lakes (side opposite wind flow) Lake Effect Snow Atmospheric Hazard Extraordinary Lake-Effect Snowstorm cP Records from cP air masses • Fastest temperature drop – On January 23-24, 1916, temps dropped 55.5°C (100°F) in 24 hours at Browning, Montana • Most lake effect snow in Buffalo, NY – Between December 24, 2001 and January 1, 2002, the city received 207.3 cm (81.6 in) of snow mP air masses • Forms over oceans at high latitudes • Cool to cold • mP can come from – The North Pacific • Source region is mild (cool) and humid all year • Source is unstable in winter, stable in summer – The Northwestern Atlantic • Source region is cold and humid in winter, cool and humid in summer • Source is unstable in winter, stable in summer More mP • mP from the North Pacific brings showers in winter, heavy orographic (caused by mountains) precipitation in winter, and low stratus/fog in winter • mP from the NW Atlantic brings occasional “nor’easters” in winter (giant mid-latitude cyclones along the east coast), occasional periods of clear, cool weather in summer Pacific mP air masses • Control more of our weather than NW Atlantic mP • Usually begin as cP in Siberia • Travel across the Pacific where they play a large role in the weather experienced along the West Coast of the U.S. • Become more mild as they move across the Pacific Modification of cP Air Mass NW Atlantic mP • Only occasionally affect the U.S. • Usually associated with “nor’easters” which act to funnel the airmass back behind a low pressure system into the U.S. • “Nor’easters” are a wintertime event with strong winds, cold to freezing temps, and abundant precipitation • In the summer, they bring pleasant cool air as a break from the typical hot, humid weather in the northeast U.S. Nor’easters Images: NOAA, USCG mT air masses • mT air masses originate in the GOM, Caribbean, western Atlantic • mT air masses also originate in the subtropical Pacific • While both source regions are warm and humid all year, the Atlantic side is unstable all year with the reverse occurring in the Pacific side More mT • mT Atlantic can move north in the winter, bringing precip and fog • It dominates the southern U.S. in summer with hot and humid conditions, as well as plenty of rain • mT Pacific occasionally brings drizzle and light rain to the SW U.S. in winter • It sometimes reaches the west U.S. in summer, causing some thunderstorms mT Air Moves over Heated Land Resulting in Cumulus Development and Showers ISOHYETS Desert Rainfall from mT Air Mass cT airmasses • Sometimes in summer, cT air forms in Mexico and the interior southwestern U.S. • The air is, as expected, quite hot, but there is very little humidity • Therefore the summer weather is generally hot and dry, with large daily temperature changes • Rarely, cT air masses move north into the Great Plains Chapter 8 END