Chapter 12, Part 2 Fronts Fronts • A transition zone between two air masses of different densities is called a front. • There is usually a temperature difference across a front as well. • The humidities may differ as well. Types of Fronts Stationary front Occluded front Warm front Cold front 1 Stationary Fronts • Do not move. • On the map, semicircles point to warmer air and triangles point to cooler air. • Here, air blows parallel to the fronts. Cold Fronts • Cold air is replacing warm air. • The triangles indicate the direction the front is moving. Determining Location of Front 1. Sharp temperature changes over short distance 2. Changes in the air’s moisture content (dew pt.) 3. Shifts in wind direction 4. Pressure and pressure changes 5. Clouds and precipitation patterns 2 Cold Front Example Isobar kink at front Lower T, Dew Point (low pressure – trough) Wind northwesterly Pressure rising Higher T, Dew Point Pressure lowering Wind southwesterly Typical Airflow at a Cold Front • Cold dense air forces warm air upwards. • Warm moist air rises and condenses into clouds, producing rain showers at the front. • Leading edge of front is steep (1:50 for a 25 knot front). Regenerated Fronts • Frontolysis – temperature contrast lessens and front weakens • Frontogenisis – temperature contrast increases and front strengthens 3 Typical Weather at a Cold Front Before During After Wind S or SW Shifting W or NW Temperature Warm Dropping Cooler Pressure Falling Minimum Rising Precipitation Short Heavy Clearing showers showers Visibility Fair to poor Poor Good Dew point High Sharp drop Lowering Warm Fronts • Warm air is replaces cold air. • The semicircles indicate the direction the front is moving. • Front moves slowly (10 knots), about half the speed of an average cold front. Warm Front Example Lower T, Dew Point Pressure rising Wind southeasterly Higher T Dew Point Pressure lowering , Wind southwesterly Isobar kink at front (low pressure – trough) 4 Typical Airflow at a Warm Front • Warm air rises over cold air, • creating clouds and rain showers ahead of the front. • Leading edge of front has a gentle slope (1:300). • There is a temperature inversion (frontal inversion) just ahead of the front. Typical Weather at a Warm Front Before During After Wind S or SE Shifting S or SW Temperature Cool Rising Warmer Pressure Falling Minimum Rising Precipitation Light to Drizzle Usually moderate none Visibility Poor Poor, improving Fair Dew point Steady rise Steady Rise Occluded Front • When a cold front catches up to and overtakes a warm front, the boundary is called a occluded front (or occlusion). • On a weather map, there are alternating cold-front triangles and warm-front semicircles pointing in the same direction. 5 Cold-Occluded Front • The cold air of the cold-front lifts both the warm and cold air ahead of it. Warm-Occluded Front • The cold air of the cold front rises over the colder air ahead of it. Typical Weather - Occluded Front Before During After Wind E, SE, or S Variable W or NW Temp. (Cold) Cold or cool Dropping Colder Temp. (Warm) Cold Rising Milder Pressure Falling Low point Rising Precipitation Yes Yes Clearing Visibility Poor Poor Improving Dew point Steady rise Slight drop Slight drop 6 Summary • When two air masses meet, a front is created. • There are four kinds of fronts: – Stationary – not moving – Cold – cold air replaces warm air – Warm – warm air replaces cold air – Occluded – cold front catches up with warm front 7.