Discussion 4/10 AOSC200 Sarah Benish

Outline I. Air Masses II. Fronts III. Mid-latitude Cyclonic Storms

I. Air Masses What is it? Extremely large body of air with similar temperature and humidity in any horizontal direction at a given altitude What are their source regions? Where air masses originates; air mass will acquire properties of the source below Either continental (c) or Maritime (m) How do we classify them? Usually in terms of temperature and humidity and source region

cP and cA Air Masses Cold and dry Where does cP air come from? Canada, can head as far south as FL Where does cA air come from? Forms over frozen Arctic bringing bitterly cold temperatures Sometimes called the “Siberian Express” Strong winds and blowing snow can lead to blizzards

mP Air Mass Where do they form? Over oceans at high latitudes, cool to cold and humid In winter, mP from Pacific begin as cP from Siberia These systems run into west coast mountains and dump lots of snow What lifting mechanism is this called? On East coast, mP brings in moist air from Atlantic over land where is meets with cP air What does this lead to? SNOW!

mT Air Mass Where does it form? Forms over Gulf of Mexico, Caribbean Sea, and subtropical western Atlantic Ocean Eastern U.S. strongly affected Stable air mass leads to oppressive heat wave Wintertime precipitation over central and eastern U.S. due to uplift of mT over cold air masses Western U.S. strongly affected by mT air from Pacific

cT Air Mass Where does it form? Form over tropical and subtropical deserts and plateaus Air mass is hot and dry When cT and mT air meet, contrast between systems is called a “dry line.” In summer, large supercell storms often form at the dry line conducive to tornado development

Air Mass Modifications How can air masses be modified? Lifting Heat exchange with the surface Example: Lake Effect Snow

Lake Effect Snow

II. Fronts-Cold

Cold Fronts cont.

Warm Fronts

Warm Fronts cont.

Stationary Fronts Essentially no movement What does it look like? Winds blow along the front but in opposite directions Usually no precipitation (air masses are dry)

Occluded Fronts What is it? Cold front catches up to and overtakes warm front Warm air is forced up over both cold/very cold air masses May have a mix of clouds similar to both warm and cold fronts

Occluded Fronts (cold type) cP air overtaking mP air

Occluded Fronts (warm) mP cool air replacing cold cP air

Occluded cont. What are cold, warm, and occluded fronts often a part of? Mid-latitude cyclone

III. Mid-Latitude Cyclonic Storms Cyclogenesis—development or strengthening of a mid- latitude cyclone Frontal wave—wavelike deformation along a front in the lower levels of the atmosphere. Those that develop into storms are stable waves; those that don’t are unstable waves. Open wave—stage of development of a wave cyclone where a cold and warm front exist, but no occluded front. The center of lowest pressure in the wave is located at the junction of the two fronts.

Cyclone Model (Bjerknes) Birth: begins as a frontal wave along stationary front separating cP air from mT air Young adult: Open wave develops strong cold and warm fronts, precipitation falls along broad area Mature (occluded cyclone): occluded front develops, pressure reaches minimum, winds reach maximum Death: cut-off cyclone develops, pressure rises, clouds and precipitation dissipates

Key Terms Air mass Lake-effect snow cT, mT, cA, mA, cP, mP Front Cyclogenesis Frontal wave Open wave