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Chapter 9 Lecture Understanding Weather and Climate Seventh Edition Air Masses and Fronts Frode Stordal, University of Oslo Redina L. Herman Western Illinois University © 2015 Pearson Education, Inc. Air Masses and Their Source Regions • Introduction – Air masses contain uniform temperature and humidity characteristics. • They affect vast areas. – Fronts are boundaries between different air masses. • Fronts are spatially limited and usually linked to midlatitude cyclones. © 2015 Pearson Education, Inc. Air Masses and Their Source Regions • Source Regions – The areas of the globe where air masses form are called source regions. – Long-term heating or cooling of large bodies of air must remain over a source region for a substantial length of time for an air mass to form. – The air mass will reflect the temperature and humidity characteristics of the source region. • These characteristics are categorized using a lowercase and uppercase letter scheming. © 2015 Pearson Education, Inc. Air Masses and Their Source Regions • Source Regions © 2015 Pearson Education, Inc. Air Masses and Their Source Regions • Source Regions © 2015 Pearson Education, Inc. Air Mass Formation • Continental Polar (cP) Air Mass – Continental polar (cP) air masses form over large, high-latitude landmasses, such as N Canada and Siberia. – In winter, these locations have short days, low sun angle, and are usually snow-covered (reflecting more energy). – The cold, dry air associated with this air mass is typically stable. © 2015 Pearson Education, Inc. Air Mass Formation • Continental Arctic (cA) Air Mass – Continental arctic (cA) air masses are characterized by extremely cold and dry conditions, as it contains very little water vapor. – The boundary between cA and cP air is the arctic front. – Some meteorologists believe that the distinction between cA and cP is minor. © 2015 Pearson Education, Inc. Air Mass Formation • Continental Polar (cP) and Continental Arctic (cA) Air Masses © 2015 Pearson Education, Inc. Air Mass Formation • Continental Polar (cP) • Continental Arctic (cA) Air Masses © 2015 Pearson Education, Inc. Air Mass Formation • Maritime Polar (mP) Air Masses – Maritime polar (mP) air masses form over the North Pacific and are more moderate than cP. – mP air masses migrate to the east, crossing the Gulf of Alaska, west coast of N America. – Impacts N America primarily in winter. – mP air masses affect the east cost of N America in the form of northeasters, bringing cold winds and heavy snowfall to the US northeast. © 2015 Pearson Education, Inc. Air Mass Formation • Continental Tropical (cT) Air Masses – Continental tropical (cT) air masses form during the summer over hot, low-latitude areas (southwestern US and Mexico). – cT air masses are hot and dry. – These air masses are very unstable but due to low availability of water vapor, do not usually produce precipitation. – When water vapor is present, thunderstorms form within these air masses. © 2015 Pearson Education, Inc. Air Mass Formation • Maritime Tropical (mT) Air Masses – Maritime tropical (mT) air masses form over warm tropical waters, influencing southeastern US primarily in the summer. – mT air masses form over the Atlantic and Gulf of Mexico and migrate toward N America. – High moisture content and instability create opportunities for thunderstorms and heavy rains. – Ideal for precipitation and clouds. © 2015 Pearson Education, Inc. Fronts • Midlatitude Cyclone © 2015 Pearson Education, Inc. Fronts • Fronts – Fronts separate air masses and bring about changes in temperature and humidity as one air mass is replaced by another. – There are four general types of fronts associated with midlatitude cyclones. • • • • Cold fronts Warm fronts Stationary fronts Occluded fronts © 2015 Pearson Education, Inc. Fronts • Cold Fronts – A cold front is a mass of cold air advancing toward warm air. – Typically associated with heavy precipitation, rain, or snow, combined with rapid temperature drops. © 2015 Pearson Education, Inc. Fronts • Cold Fronts © 2015 Pearson Education, Inc. Fronts • Warm Fronts – Warm fronts are warm air moving toward cold air. – This overrunning process produces large amounts of warm, moist air over cooler, drier air. – Shallow stratus clouds dominate and bring light precipitation to affected regions. – Stable regions above the warmer air create vertically limited clouds and light precipitation. – Frontal fogs may occur as rain evaporates in the colder air near the surface. © 2015 Pearson Education, Inc. Fronts • Warm Fronts © 2015 Pearson Education, Inc. Fronts • Warm Fronts © 2015 Pearson Education, Inc.