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Air Masses and Fronts Definition Air mass- huge section of the lower troposphere that have the same kind of weather humidity) throughout. Properties of Air Masses cause changes in the weather are classified by where they form form over oceans and land amount of moisture (humidity) depends on where the mass develops. named for source region or places of origin Source Regions Source regions for cold air masses are polar areas and are labeled P Source regions for very cold air are arctic areas and are labeled A Source regions for warm air are masses are tropical areas and are labeled T Air masses over oceans are called martime and are labeled m Air masses over land are called continental and are labeled c Types of air masses 5 major types of air masses mP- maritime polar is cold and humid brings cooler temperatures to Eastern US Brings fog to California and western states Maritime tropical mT maritime tropical holds warm moist air in summer- brings very hot, humid weather in winter- if this air meets with cold air mass, rain or snow will fall (temperature and Continental tropical cT continental tropical originates in tropical land areas and desert areas of SW US are hot and dry Continental polar cP continental polar originates over land in N Canada are cold and dry causes very cold temp in the US in the winter Continental arctic cA continental arctic very cold and dry come from ice covered Arctic regions Air masses change as they move from their source region. they diverge from areas of high pressure and converge in areas of low pressure are very large and take several days to pass through a region. Fast moving air masses spend less time over any one area and bring more extreme weather Rawinsonde balloon carried package of instruments, used to measure humidity, temp., and winds measurements made twice daily very detailed Fronts boundary between air masses that have different temp. and humidity. when two air masses meet a front forms an approaching front means a change in weather almost always brings ppt. four types of fronts forceful lifting- when two contrasting air masses meet, the lighter warmer of the two is lifted or rises above the cooler, denser air mass. MAJOR CAUSE OF PPT in the US. Cold Front forms when a mass of cold air meets and replaces a mass of warm air cold air forces the warm air mass upward violent storms are associated with cold front has a steep slope often forms a line of thunderstorms called a squall line Warm Front forms when a mass of warm air overtakes a cold air mass and moves over it Drizzle may accompany a warm front hot humid weather follows gentle slope Occluded Front forms when cold front overtakes a warm front (cold front travels faster then a warm front) warm air is pushed upward and the cold air meets cool air weather is complex and extreme Occluded Front forms when cold front overtakes a warm front (cold front travels faster then a warm front) warm air is pushed upward and the warm air meets cool air weather is complex and extreme Stationary Front warm air mass meets a cold air mass no movement occurs rain may fall for days Cyclone Cyclone Development A wavelike kink develops- warm air is pushing N, (warm front) cool air is pushing S (cold front) Cold front moves more rapidly and overtakes warm front producing unstable air and cloud development Occluded front is produced. Cyclone Anticyclone high pressure area containing cold dry air clockwise direction clear dry weather isobars usually farther apart , winds are weaker Tornadoes • A violently rotating column of air in contact with the ground • • associated with cumulonimbus clouds and intense low pressure categorized as weak, strong or violent Types of tornadoes • • Weak- ropelike appearance, windspeeds no greater than 110 mph Strong- funnel shaped cloud with speeds between 110-200mph. 3 in ten tornadoes are classified as strong Violent tornadoes • violent tornadoes- 2 % of all tornadoes reach this strength. Wind speeds are greater than 200mph Important terms • • • • • Wall cloud- large, rotating, dark cloud used as an indication of tornodic development Tornado Alley- Kansas, Missouri, Texas and Oklahoma most tornadoes occur here waterspout- a tornado over water mesocyclone- rising, spinning column of air funnel cloud- a tornado that has not reached the ground Weather Alerts • • watch- issued when there is a potential for tornado development warning- issued when a tornado has been spotted. Wall Cloud Tornado safety • • • • • Have a safety plan Stay away from windows and doors take shelter in an interior room or basement if you are in an open area look for a ravine or ditch or other low lying area Have an emergency kit- What should it include? Other tornado information • • • • • • • • • • Most occur from 5-9 PM June is the month in which most tornadoes occur Tornadoes are associated with hail, heavy rain and lightning. Fujita Scale is used to measure tornado intensity The US has more tornadoes than any where else in the world. HURRICANES!!! Hurricanes Tropical cyclones with winds that exceed 64 knots blow in a counterclockwise direction also called a cyclone or typhoon develop on the western side of oceans most common in late summer and early fall • do not last long after they reach land because source of moisture is cut off Development • • • • • • • • • • • • Start as a Tropical depression - 20-34 mph gain speed and circulation to become a Tropical Storm- 34-64 mph once wind speeds reach 75 mph classified as a hurricane. The center has extremely low pressure which causes a strong pressure gradient to form. Form when cold air mass of unstable air is over water with temp. greater then 81 oF Important terms Eye- the center of sinking air. It is a calm, clear, warm area eye wall- intense thunderstorms that surround the eye. Storm surge- unusually high seas, cause of greatest hurricane damage Saffir-Simpson Scale- rates the intensity of a hurricane Naming system List alternate between male and female names, ex. The next two names on the list for 2000 Atlantic storms are Alberto and Beryl A storm is named once it reaches tropical storm status. A name is retired if the hurricane is an exceptional one. Watches and Warnings • • • • • • Hurricane watch- hurricane conditions possible Hurricane warning- hurricane is expected within 24 hours. Hurricane Safety Have an emergency plan and an emergency kit which should include…. Board and tape windows remove damaged trees move outdoor items inside. Weather Forecasts • • • • Daily forecasts are based on weather maps and computer models Data is collected by rawinsonde, satellites, and commercial aircraft Using data collected, forecasts are made at Weather Service Forecast Offices four times a day. May be issued more frequently during severe weather. Computer models are produced twice a day. Satellites and Radar • • • • • Satellites are used more and more to fill in data-poor regions geostationary satellites are over the same area at all times and keep time with the earth rotation. Radar- the different colors of radar gives an indication of the amount of rain and severity of the associated weather. Station Model 20 different weather observations may be plotted next to each station on a weather map. This information is expressed in symbols and called a station model. • The symbols are always located in the same location. Symbols • Highlighted portion represents • Highlighted portion represents dew point • Highlighted portion represents Sea level pressure % cloud cover Symbols • Highlighted portion represents • Highlighted portion represents • Highlighted portion represents temperature Weather Symbol wind speed and direction