Air mass - a body of air with characteristic properties of temperature and humidity. Classified by temperature & by surface (land or water) over which they form. cP – Continental Polar cT – Continental Tropical, mP – Maritime Polar, mT – Maritime Tropical, Air Masses

Properties depend on where mass originated – air takes on characteristics of area over which it has lingered. When it moves, it takes those characteristics with it.

Continental Polar Air Masses Cool/cold and dry with little precipitation EXCEPT after crossing the Great Lakes, especially late fall early winter. Picks up heat & humidity from lakes to become unstable and produce lake effect snow on leeward side of lakes.

Maritime Tropical Air Masses Warm, humid, unstable Responsible for most precipitation in eastern 2/3 of U.S. Cause oppressive heat waves in summer.

Maritime Polar Air Masses Cool/cold & humid Mostly affect Pacific Northwest and Canadian Maritime provinces. Orographic lifting in Oregon & Washington produces heavy rain & snow on windward slopes of Cascades, rain shadow on leeward slopes.

Continental Tropical Air Masses Least influence on weather in North America. Warm, dry, unstable Usually form in summer and stay in southwest. Can bring drought to midwest & Indian Summer to northeast.

Air Masses Change as They Move Over a different area, air mass gradually takes on the characteristics of the new region. Accompanying weather also changes.

IX. How do fronts influence the weather? Boundary between air masses is called a front http://earth.usc.edu/~stott/Catalina/WeatherPatterns.html

Cold front - Frontal Wedging Frontal Wedging 2 1. Cold air mass meets warm air mass 2. Cold air forces warm air up along steep front 3. Sudden showers or thunderstorms result

Warm front Warm Front Summary 1. Warm air mass meets cold air mass 2. Warm air slides up over cold air in long wedge 3. Nimbostratus clouds form and may cause precipitation over long period

Stationary front Cool air & warm air battle to a standstill. Prolongs bad weather. Stationary Front http://www.usatoday.com/weather/tg/wsfront/wsfront.htm

Occluded Front A cold front catches up to a warm front. The two cool air masses trap & lift warm air between them, causing high winds and varied precipitation Occluded Front 1 Occluded Front 2

Formation of Middle-Latitude Cyclones Stationary front develops w/ 2 air masses moving in opposite, parallel directions. Wave develops w/ warmer air pushing into colder air toward poles, cold air sweeping toward equator.

Formation of Middle-Latitude Cyclones Low pressure develops at center of flow. Air movement forms warm & cold fronts. Cold front moves faster, overtakes warm front, forming occlusion. Pressure decreases, winds & precipitation increase. ML Cyclone 1

Airflow Aloft Airflow aloft feeds cyclones & anticyclones. Cyclones & Anticyclones feed each other.

Severe Storms Lightning Thunderstorms form when unstable warm, humid air rises in thermals or along a cold front & builds into cumulus, then cumulonimbus clouds. May occur individually or in a line ahead of a cold front. TS2 TS3 Produce lightning & often gusty winds, heavy rains, hail. Lightning

Precipitation Hail - forms in cumulonimbus clouds Ice pellets grow by colliding with supercooled water droplets Strong updrafts carry them back up to repeat the cycle Develop one “growth ring” for each cycle through cloud before falling. Precipitation http://www.classzone.com/books/earth_science/terc/content/visualizations/es1805/es1805page01.cfm?chapter_no=visualization

Tornado Formation TF2 TF3 Vortex – rotating column of air extending from cumulonimbus cloud. Strong winds aloft cause lower winds to roll horizontally. Strong updrafts in thunderstorm tilt vertically – mesocyclone, which may strengthen into a tornado, w/ extremely low pressure at center. Tornado Formation TF2 TF3

Hurricane Formation A.k.a. tropical cyclones or typhoons, winds of 74+ mph Starts at tropical (between 5-20 degrees N & S latitude) depression (low pressure system) over water. Heat engine - energy released from condensation feeds storm. Form in late summer. Severe Storms Hurricane

Hurricanes, Typhoons, Tropical Cyclones Inward rush of warm air turns upward, forms ring of cumulonimus clouds, the eye wall, where winds are strongest & precipitation heaviest. Storm surge – dome of water pushed ahead of hurricane. Hurricanes, Typhoons, Tropical Cyclones

X. How is weather portrayed on weather maps? Local collecting stations contribute data in form of station models http://teach.fcps.net/immex/preview/Weather_Station/case3/lib_weather_station_model.htm

Calculating Relative Humidity from Temperature and Dew Point

Precipitation Cold cloud precipitation depends on supercooled water (< 0oC, pure water in clouds won’t freeze solid until -40oC) and supersaturated air (saturated air, 100% relative humidity, is supersaturated with respect to ice crystals). Excess water vapor becomes ice, which lowers relative humidity around cloud droplets, so they evaporate, providing more water vapor for growth of ice crystals, (a.k.a. snowflakes). http://www.atmosedu.com/meteor/Animations/IceCrystalProcess.mov

Precipitation Cold Cloud Precipitation

Precipitation Warm Cloud Precipitation Collision-coalescence process – Larger cloud droplets move fast through cloud, colliding and joining together with smaller droplets.

Rain – often begins high in troposphere as snow, melts as it falls through warmer air. Snow – in low temp., low humidity air, snow is light and fluffy. In temps. >-5oC, flakes clump together to make heavy, wet snow. Precipitation

Precipitation Virga – precipitation evaporates before it reaches the ground.

Precipitation Glaze (a.k.a. Freezing Rain) – raindrops become supercooled as they fall through cold air and impact cold surfaces.

Sleet – rain falls through cold air and partially freezes into clear ice pellets. Precipitation