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ATS/ESS 452: Synoptic Meteorology - Lab - Fronts! Fronts! • What are fronts? – A narrow transition zone, or boundary, between differing synoptic scale air masses whose primary discontinuity is density – Fronts highlight baroclinic zones, where temperature advection and atmospheric energy is at work – Fronts are commonly associated with: • • • • Moisture gradient Pressure trough (fronts are areas of lower pressure) Wind shift Various sensible weather phenomena – A front is a convergent boundary of air – 4 main types: • • • • Cold Front Warm Front Stationary Front Occluded Front – In general, fronts are located on the warm side of tight temperature gradients Frontal Symbols / Colors 1. Cold Front • Blue • Triangles point in direction of frontal movement 2. Warm Front • Red • Half circles point in direction of frontal movement 3. Stationary Front • Alternating red & blue • Alternating symbols point in direction of cold air/warm air movement 4. Occluded Front • Purple • Alternating symbols point in direction of frontal movement Cold Front ID Before Passing While Passing After Passing Winds S to SW Gusty; shifting WNW to NW Temperature Warm Sudden drop Steadily dropping Pressure Falling steadily Minimum, then sharp rise Rising steadily Clouds Increasing Ci, Cs and Cb Cb Cu Precipitation Short period of showers Heavy rains, sometimes thunderstorms Showers then clearing Visibility Fair to poor Poor, following by improving Good, except in showers Dew Point High; remains steady Sharp drop lowering Warm Front ID Before Passing While Passing After Passing Winds S to SE Variable S to SW Temperature Cool-cold; slow warming Steady rise Warmer then steady Pressure Usually falling Leveling off Slight rise, followed by fall Clouds Ci, Cs, As, Ns, St and fog Stratus-type Clearing with scattered Sc and sometimes Cb Precipitation Light-to-moderate rain, snow sleet or drizzle Drizzle or none Usually none Visibility poor Poor but improving Fair in haze Dew Point Steady rise Steady Rise, then steady Occluded Front ID Before Passing While Passing After Passing Winds SE to S Variable W to NW Temperature Cold type Warm Type Cold-cool Cold Dropping rising Colder milder Pressure Usually falling Low point Usually rising Clouds Ci, Cs, As, Ns Ns, sometimes Tcu and Cb Ns, As or scattered Cu Precipitation Light, moderate or heavy precip Visibility Poor in precip Poor in precip Improving Dew Point Steady Usually slight drop Slight drop Light, moderate Light-to-moderate or heavy precip followed continuous precip by clearing • In general, look for – – – – Pressure troughing Temperature gradients Moisture gradients Wind shifts • Fronts are generally found on the warm side of significant thickness or temperature gradients • Fronts generally lie within pressure troughs • Fronts generally lie on the edge of regions of neutral advection – Cold fronts reside on the downwind edge of enhanced cold advection patterns – Warm fronts reside on the upwind edge of enhanced warm advection patterns – Occluded fronts reside within thickness “ridge” with cold and warm advection patterns immediately adjacent o the ridge • Stationary fronts are often hard to ID – Warm side of significant thickness gradients – In regions exhibiting no significant thermal advection but still a thermal gradient – Within pressure troughs – In general, there is no upper-level support… parallels geopotential height gradients • Upper levels winds not present to advance the front along
