Chapter 8: Air Masses, Fronts Mid-Latitude Cyclones Air Masses  large body of air whose properties of temperature and humidity are fairly similar in any horizontal direction at any given latitude Source Regions  Regions where air masses originate Air Mass Classification/Characteristics  Table 8.1

Air Masses of North America Continental Polar (cP) Continental Arctic (cA)  Air masses originate over ice and snow covered regions  Clear night allow for strong radiational cooling Maritime Polar (mP) Maritime Arctic (mA)  Source region is over the ocean of higher latitudes  North Pacific and north Atlantic  Responsible for west coast low clouds and fog  This air mass also contributes to rainfall that occurs during the winter months

Air Masses of North America Maritime Tropical (mT)  May be associated with warm and unstable air  This air mass may combine with maritime polar air to give us more rain from a storm  Pineapple Express type storms  Atmospheric Rivers: stretches of maritime tropical air from the west Pacific Continental Tropical (cT)  May bring hot and dry weather to the mountains and deserts  May be associated with dust devils, wind storms in the deserts

Fronts The leading edge of an air mass may serve as a frontal boundary. A frontal boundary can either be a continental polar air mass moving south or maybe a maritime tropical airmass moving to the north The airmass can either be moist or dry, depending on its source region

Fronts cont… Front: is the transition zone between two airmasses of different densities. Also the humidity can differ with the airmasses Cold air is often heavier and drier than moist, less dense air Warm air is often lighter and more moist than heavy, more dense air Ocean air masses are more moist than land air masses, which are typically drier.

Fronts Stationary Front: has essentially no movement Cold Front: Cold, dry, stable air is replacing warm, moist, unstable tropical air Warm Front: air from the south (Gulf of Mexico) moves northward and moves over colder air to the north. Occluded Front: a cold front catches up to a warm front. See Table 8.3 page 220 Also see Figure 8.20

Dry from the North

Warm Unstable Air

Middle Latitude Cyclones Low pressure system forms where cold air is moving south and warm air is moving north Fronts represent ‘battle grounds’ where air masses meet. Middle latitude cyclones represent this ‘battle ground’. Middle latitude cyclones are associated with rain, wind and snow. Middle latitude cyclones move with the upper level winds at 500mb.