Ch.29 -Air Masses and Fronts 29.1 Origin Air mass is a huge section of lower troposphere with the same weather throughout. T. and Humidity are horizontally uniform. Originate in parts of the world where winds are light(polar & subtropical Hi pressure belts.

29.2 Kinds and Sources T. depends on where from(polar or trop.) and Humidity depends on from over land or sea. Maritime trop.-(mT) Wm. And Humid. Comes to US from Pacific and Atlantic or Gulf of Mexico. Continental trop.-(cT) from trop. Land areas- hot and dry Maritime polar (mP) from cold ocean water cold and humid.

29.2 continued Continental polar (cP) from land areas at high latitudes - cold and dry (from canada) Continental arctic - (cA) Very cold and dry from ice covered polar regions. There are NO MARITIME ARCTIC because arctic air is so dry.

29.6 What is a Front? Boundary between any 2 air masses is a FRONT. FRONTS BRING PRECIPITATION !!!!! Where temp. and humidity change, and approaching front = a change in weather. On weather maps, fronts are drawn in regions of great change in T & wind direction. Less dense, warm air is forced to rise over heavier more dense cold air. Resulting shape is a wedge.

29.7 Kinds of Fronts Cold Front - cold air is advancing & displacing wm air. Triangles point to where air is moving toward.(steep slope) Warm Front- Wm air is pushing & displacing cold air. Semi-circles point to where air is headed toward.(gentle slope) Stationary Front - Front doesn’t move Occluded Front - When cold front overtakes a warm front. SEE ESRT FOR SYMBOLS

29.7 CONTINUED Fronts control weather only when 1 air mass displaces another. Weather is mostly controlled by air mass movement.

29.8 Mid-latitude lows (N. Hemis) Winds blow W-->E. Trop. Air pushes polar air N. between cold & warm. Wm bulges into & over heavy polar air. To form low pressure. Lowest pressure formed when wm & coldmeet. A CYCLONE or LOW is formed. Turns COUNTER-CLOCKWISE and IN. The whole system moves E but cold front moves faster than wm.

29.9 Wind & Weather in a Low Air converges & air rises to form clouds & precip. The air is condensed & releases heat energy. Low pressure forms at the surface & low becomes stronger. (increasing pressure gradient = stronger winds)

29.10 Warm Front Weather Wm front effects weather long before arrival of rain. High cirrus clouds appear then cirrostratus then low, thick stratiform. Clouds form in air sliding up frontal surface. Air is usually stable and form vast stratiform clouds, then finally steady rain or snow. *** a warming follows the passage of the front at the surface.

29.11 Cold Front Weather. Steep and fast moving. Air is forced up QUICKLY!!! Air is unstable & cumuliform clouds grow around the cold front. Heavy Precip. Falls. T-storms common. Due to steepness, Precip. Before and after front passes. When it passes, sharp change in weather. Temp. drops & wind speed increases. Rain ends shortly after front passes.

29.12 Weather in a High Between Lows and their fronts lie a high pressure area. “Anticyclone”appear as smooth circular isobars & represent a single air mass. ***Since air pressure is highest at center, winds blow outward *** Winds blow clockwise in N. Hemisphere. Highs bring bright, clear weather due to sinking dry air at the center