Part 5: Weather Patterns

Air Masses Large bodies of air Movement of them causes changes in weather Cover thousands of square kilometers Properties fairly uniform Classified by where they form Continental – form over continents; relatively dry Maritime – form over oceans; relatively humid Polar – cold Tropical - warm

4 Major Air Mass Types in USA 1. Maritime tropical – Forms over ocean near the equator Warm, moist air Most commonly affects Eastern states In summer brings hot, humid weather In winter brings rain or snow if it encounters a cold air mass

4 Major Air Mass Types in USA 2. Maritime polar – Forms over northern Atlantic and northern Pacific oceans Cool, moist air Bring cloudy, damp weather to Northeast and Pacific Northwest

4 Major Air Mass Types in USA 3. Continental tropical – Hot and very dry Forms over the desert southwest and Mexico usually during summer Affects southwest states, plains, and Mississippi valley and can bring record high temperatures

4 Major Air Mass Types in USA 4. Continental polar – Cold and dry Forms over Canada Often dominant weather in winter Brings very cold weather Can also bring clear, pleasant weather to the North in summer

Major Air Mass Types mT = Maritime Tropical mP = Maritime Polar cT = Continental Tropical cP = Continental Polar cA = Continental Arctic (5th type) Brings extremely cold temperatures with very little moisture, originate north of the arctic circle in winter

Fronts A front is a boundary that forms when two air masses with different properties meet Weather is usually unsettled and stormy along a front. 4 types of fronts: Cold Warm Occluded Stationary

Cold Front Cold air mass meets and pushes under a warm air mass Pushes warm air up Can cause violent storms Fair, cool weather usually follows a cold front Generally move northwest to southeast Can cause a rapid drop in temperature of more than 15° in one hour

Cold Front Weather symbol: blue triangles point in the direction the cold front is moving http://commons.wikimedia.org/wiki/File:Example_of_a_cold_front.svg

Warm Front Warm air overtakes cold air and goes over it Showers followed by hot, humid weather Noticeably warmer after the front passes Generally move southwest to northeast

Warm Front Weather symbol: Red, semicircles pointing in the direction the warm front is moving http://commons.wikimedia.org/wiki/File:Example_of_a_warm_front.svg

Occluded Front A cold front moves faster than a warm front When a cold front overtakes a warm front an occluded front forms Usually form around low pressure areas At the occluded front the cold air mass meets the cool air mass that was ahead of the warm front Warm air rises to form cumulonimbus or nimbostratus clouds bringing precipitation

Occluded Front Weather symbol: purple, alternating triangles and semicircles pointing in the direction the front is moving https://commons.wikimedia.org/wiki/File:Occluded_cyclone.svg http://commons.wikimedia.org/wiki/File:Occlusiefront.png

Stationary Front When warm air meets cold air and no movement occurs Rain may fall for many days Weather Symbol: Alternating blue triangles pointing towards the warmer air and red semicircles pointing towards the colder air