Air Masses and Fronts Air Mass Properties • Air masses take on the properties of the underlying surface • Air masses are classified according to their location of origin • Geographical Characteristics – Tropical, Polar, Arctic • Surface Properties – maritime, continental • Source region characteristics most prevalent if air

Air Mass Classifications

• cP - continental Polar – Cold, dry, stable – Extremely cold cP air mass may be designated cA (continental Arctic) • mP - maritime Polar – Cool, moist, unstable • mT - maritime Tropical – Warm, moist, usually unstable • cT - continental Tropical – Hot, dry – Stable air aloft, unstable surface air Fronts Typical Cold Front Structure • Cold air replaces warm; leading edge is steep in fast-moving is the boundary between air masses; normally A Front - front shown below due to friction at the ground refers to where this interface intersects the – Strong vertical motion and unstable air forms cumuliform clouds ground (in all cases except stationary fronts, the – Upper level winds blow ice crystals downwind creating cirrus and symbols are placed pointing to the direction of cirrostratus movement of the interface (front) Warm Front Cold Front Stationary Front Occluded Front

Typical Warm Front Structure

• In an advancing warm front, warm air rides up over colder air at the surface; slope is not usually very steep • Lifting of the warm air produces clouds and precipitation well in advance of boundary • At different points along the warm/cold air interface, the precipitation will experience different temperature histories as it falls to the ground

Midlatitude Cyclones Cyclone Development Nascent stage of begins with a Cyclone Development stationary front

Mature stage of Cyclone Mature Wave Cyclone Development

Adulthood The Partially Occluded Stage begins Partially occluded wave cyclone when the cold front starts to overrun the • Cold-occluded front warm front – Approach brings weather sequence like a warm front – Frontal passage brings weather more like a cold front

• Warm-occluded fronts also possible Cold-occluded front

The final decay stage of the cyclone. The warm air is isolated aloft with What maintains the surface low? cold air beneath. Imagine a surface low forming below an upper level low. Cyclone development: Need upper level divergence to maintain Imagine a short wave trough passes overhead (looking North): surface low. This occurs Where will surface low develop? downstream of the upper level trough.

Low DIV High

east

(looking North): Near the surface, where will we have cold and warm advection? Will this amplify or weaken the upper level low? How about the upper level divergence? Will a more intense upper level low strengthen or weaken the surface low?

Low DIV High

warm •Upper level shortwave passes. cool •Upper level divergence -> sfc low. Low •Cold advection throughout lower troposphere. •Cold advection intensifies upper low. •Leads to more upper level divergence. •Intensifies sfc low.

ADVECTION IS KEY. • What is the vertical structure of a developing storm? • Where is the largest upper/lower level divergence/convergence occurring? • Why aren’t the ‘lows’ vertically stacked? • What is required for a storm to develop? • Where is rising motion occurring? • Precipitation? • What is the ultimate source of energy for a midlatitude storm? • Why does a storm “die”? • During what time of year would you expect more midlatitude cyclones? • Why doesn’t baroclinic instability occur in the tropics?