Transcript Slide 1
Air masses An air mass is a relatively homogenous large mass of air in terms of temperature and moisture characteristics. Four air masses commonly affect our region: cP: continental polar cT: continental tropical mP: maritime polar mT: maritime tropical What do these regions have in common? Why? Air mass modification Continental polar (cP) Only occurs in the northern hemisphere at middle and high latitudes More dominant in the winter Can be extremely cold and dry Generally stable (cold days, bright blue skies) – However, where cP air masses meet moist air masses, the weather conditions can become severe Maritime polar (mP) Source areas are over the Pacific and Atlantic Oceans in the mid and high latitudes Generally produce cool and unstable conditions Maritime tropical (mT) Originates in the tropics in the Atlantic and Pacific Oceans Pacific Ocean mT air mass contains less moisture than the Atlantic mT air mass – Also more stable Air mass modification Since air masses move, they can migrate over areas and inherit characteristics from those areas – e.g. the Great Lakes In winter cold cP or cA air can move over the lakes and pick up moisture The moisture is precipitated in the lee of the lakes as snow – Lake effect snow Types of fronts WARM STATIONARY COLD OCCLUDED A front separates two different air masses. Cold fronts Occurs between advancing cP or mP air and warmer air masses Warm air is forced to rise upwards against the colder denser air, forming cumulus and cumulonimbus clouds Cold front symbol Cloud types with a cold front Stratocumulus These cloud types Cirrus usually occur ahead of a cold front. In which order would you expect them to occur as a front approaches? Cumulonimbus Altocumulus Identifying a cold front’s passage Precipitation generally falls in the vicinity of the cold front. Before Temperature Warm Dew Point High Winds SSW Pressure Fall During Sharp fall Sharp fall Shift Lowest After Slight fall Slight fall W to N Rise Warm fronts Warm front symbol Occurs when warm air advances over colder air Cloud types with a warm front Nimbostratus These cloud types usually occur ahead of a warm front. Cirrus In which order would you expect them to occur as a front approaches? Altostratus Cirrostratus Identifying a warm front’s passage Precipitation generally falls ahead of the warm front. Before Temperature Rising Dew Point Rising Winds E to SE Pressure Fall During Rising Rising Shift Lowest After Warm High S to SW Rise Cold Fronts and Warm Fronts Dry Line Mid-latitude cyclones Migrating low-pressure cells Contain converging, rising air that spirals towards the center of the system, in a counterclockwise direction Also called wave cyclones L Life cycle of a mid-latitude cyclone Takes 3-10 days Goes through 4 stages – – – – Cyclogenesis Open stage Occluded stage Dissolving stage Cyclogenesis Occurs where cold air masses and warmer air masses collide Where air converges at ground level it must diverge aloft Common at the polar front, the gulf coast, eastern areas of mountain Open stage Air to the south of the cold air forces it’s way northward along a warm front Air from the north/west forces its way southward/ eastward along a cold front Occluded stage Cold air “catches up on itself” When this happens it forces warm air aloft with only cold air below Most occluded fronts run from the north to the south Dissolving stage Usually all warm air is now aloft and all cold air is at ground level This will eventually dissipate causing air to mix Open stage cyclogenesis Occluded stage