Midlatitude Weather Systems PDF

Midlatitude Weather Systems  What is a Weather System? – recurring circulation pattern and the weather associated with that pattern. Weather What is an Air mass? – large bodies of air with uniform/same temperature & moisture Systems &...

Midlatitude Weather Systems  What is a Weather System? – recurring circulation pattern and the weather associated with that pattern. Weather What is an Air mass? – large bodies of air with uniform/same temperature & moisture Systems & An air system: Air Masses Can be up to thousands of km across May extend up to top of troposphere Measured by: Surface temperature Environmental lapse rate Surface specific humidity  The area over which an air mass forms is what determines the characteristics of the air mass. The longer the air mass stays over its source region, the more likely it will acquire the properties of the surface below. Air masses are named after their source region and temperature and classified as: Air Masses Latitude (Arctic (A), polar (P), tropical (T), equatorial (E)) Indicate temperature Surface type (Maritime (m), continental (c)) Indicate moisture content Combined air mass labels (no cE, mA or mAA) mE (maritime equatorial), mT, cT, mP, cP, cA, cAA Fronts (this is review) ∙ Surface of contact between 2 distinct air masses E.g. boundary between polar & tropical air → polar front ∙ Cold Front – moving front → cold air mass moves under warm air mass Cold front stimulates cumulonimbus clouds & Thunderstorms Warm Front ∙ moving front → warm air slides over cold air Slower than cold front Warm front stimulates nimbostratus clouds & rain Occluded front ∙ cold front overtakes warm front Warm air forced aloft ∙ Stationary Front – 2 air masses in contact, little movement  High pressure (so air is sinking) Fair weather system Descending air warms adiabatically (air heats as it sinks) Anticyclones No condensation (clouds cant form when air is pushing down on water vapor) Little pressure gradient in center of high Winds light & variable  Converging, inspiraling air rises → condensation Cyclonic storm: Intense convection Cyclones Strong winds Heavy precipitation 3 types of traveling cyclones: Midlatitude cyclone (or extratropical cyclone) Tropical cyclone Tornado  Dominant weather systems in Form, intensify mid & high and dissolve along latitudes polar front Air converging Cyclone may last a along front can few days, moving create circulation west-to-east Midlatitude Cyclones May drag cold front S or SW of Warm front E or low SE of low Cold front overtakes warm front → occluded front Lifecycle of Midlatitude Cyclone  Closely related to midlatitude cyclone development at surface Upper-Air Upper-air convergence → high surface pressure (so air is sinking, no Disturbances rain) Upper-air divergence → low surface pressure (Air is rising, could have rain)  How fast a cyclone or anticyclone is spinning Counterclockwise spin → positive vorticity Clockwise spin → negative vorticity 3 types of vorticity in atmospheric motions: Vorticity Relative - speed & direction of spinning system Planetary – related to rotation of Earth Greatest near poles Absolute Vorticity = relative vorticity + planetary vorticity Must be maintained as cyclones move e.g. cyclone nears equator → less planetary vorticity Relative vorticity must increase → cyclone spins faster  Enhancing Cyclone Formation ∙ Cyclone formation is enhanced by greater relative vorticity (spin) of air ∙ Increasing relative vorticity: Developing cyclone dips toward equator High-level divergence above developing cyclone On lee side of mts as developing cyclone passes over On warm side of steep temp gradient developing cyclone passes Cyclone Tracks & Families ∙ Certain regions are likely to develop cyclones ∙ Upper-level winds consistently steer cyclones ∙ So, consistent cyclone tracks are well known ∙ Sets of tracks form cyclone families ∙ Wave cyclones: A cyclone that forms and moves along a front. Aka, a wave depression ∙ Cyclone families: A series of wave cyclones that follow one after the other along the same track. Cold Air Outbreaks ∙ Occasionally cold air from polar regions pushes far south ∙ This February image shows cold, clear air as far south as Florida ∙ Last year a similar storm hit Texas, causing major infrastructure damage and freezing temperatures ∙ White pattern E & SE of image is clouds from cold front ∙ White in NW of image is snow cover