- Slides: 26
Air Masses and Fronts – I
• • Air Masses A large (thousands of kms) body of air with more or less uniform properties in any horizontal direction at any given latitude ♦ Temperature ♦ Humidity Next time: fronts – the boundary between two different air masses
• • Origin and formation of air masses Source Region: the location where the air mass formed its properties ♦ Large, flat areas where the air stays stagnant long enough to take on the characteristics of the underlying surface. ♦ Ideal source regions areas of high pressure Types of sources. ♦ Tropical: warm ♦ Polar: cold ♦ Continental: dry ♦ Water (maritime): humid
• • • Classification of air masses c. P ♦ ♦ c. A ♦ ♦ continental polar cold, dry, stable continental arctic very cold, very dry, stable m. P ♦ maritime polar ♦ cool, moist, unstable m. T ♦ maritime tropical ♦ warm, moist, usually unstable c. T ♦ continental tropical ♦ hot, dry, unstable surface air
• • • Stability of air masses Which is lighter? ♦ Dry air ♦ Moist air Dry air: ♦ Nitrogen ♦ Oxygen ♦ Argon Moist air: all this plus ♦ H 2 O
• • The weight of humid air Mean molecular mass of dry air: gas m % mass of the gas mixture ----------------------N 2 28 78% 28 x 78/100+32 x 21/100=28. 6 O 2 32 21% Mean molecular mass of wet air. gas m % mass of the gas mixture ----------------------Dry air 28. 6 90% 28. 6 x 90/100+18 x 10/100=27. 5 H 2 O 18 10% Moist air is lighter and less dense than dry air at the same temperature. Moist air rises more readily. Evaporation enhances convection in the atmosphere.
Air mass source regions and their paths
Continental Polar (c. P) and Continental Arctic (c. A)-winter time • • • Forming conditions: ♦ Above the northwest part of Canada and Alaska ♦ Long clear winter nights result in radiational cooling. Properties: ♦ Very cold temperatures; ♦ Little evaporation -> low water content (low specific humidity and dew point); ♦ Temperature inversion near the ground -> stable atmosphere; ♦ The cold subsiding air results in broad high P centers (anticyclones). Weather in the winter: ♦ Bitterly cold, dry weather, clear sky; ♦ It is slightly warmer on the west and the east coast because of compressional heating of the (slightly) sinking air. ♦ The stagnant, stable ground layer can create poor visibility and air quality -> pollutants are trapped near the ground
c. P, c. A
Effect of c. P air masses on the weather • Upper level wind flow and surface position of anticyclones during two cold spells in 1989 and 1990. Numbers represent minimum temperatures.
• • • Lake-Effect Snow Description: increased snow showers on the downwind side of large unfrozen lakes. It is a local phenomena. Conditions: ♦ The water is warmer than the ground (the first half of the winter). ♦ Cold dry air is moving over the lake. What happens? – The cold dry air is warmed up at the surface and picks up moisture. The boundary layer becomes unstable. Warm and moist air rises and forms cold cumulous clouds. There is a continuous supply of moisture from the lake. On the downwind side the air is additionally cooled and/or lifted up by topography -> precipitation.
Lake Effect Clouds – satellite image
Snow Effect on the Great Lakes
• • • c. P air masses-summer time Forming conditions: ♦ Above the northwest part of Canada and Alaska ♦ The melting ice adds moisture to the air. Properties: ♦ Cool temperatures (the sun is never directly overhead); ♦ Increased water content and humidity; ♦ No temperature inversion near the ground, surface instability Weather in the summer: ♦ Cooler and comfortable temperatures, the added moisture and atmospheric instability result in fair-weather clouds;
• • • c. P air mass moving over the ocean Additional moisture is added to the air. The warm ocean surface heats up the air -> instability, cloud formation The c. P air mass loses its characteristics and is given a new designation
• • • Maritime Polar (m. P) Air Masses Forming conditions: ♦ It may start over Asia and the frozen polar pacific regions as c. P. ♦ As it moves over the Pacific (around Aleutian low) the boundary layer warms up and accumulates significant amount of moisture. Properties: ♦ Cold (freezing) and dry air aloft (similar to c. P) ♦ Milder temperatures at the ground 40 -50 F (no T inversion) ♦ It is conditionally unstable Weather in the winter: ♦ This air enters the US from the NW coast; is forced over the mountains ♦ Rain on the upwind side ♦ Snow high in the mountains
mp da ld Pacific aircold but not VERY cold Co ow sn air , in Ra m. P
Effect of m. P air masses on the weather
• • • Maritime Tropical (m. T) Air Masses Forming conditions: ♦ Over the subtropical east Pacific Ocean-spends a long time over the warm ocean water. Properties: ♦ In winter it is very warm and moist ♦ Increased water content and humidity ♦ The atmosphere is unstable (m. T is the most unstable of all air masses) Weather in the winter: ♦ As it moves over the colder continent it produces heavy rain. ♦ Flooding and mudslides in California in the winter.
y. R eav r. H nte Wi og, f r e t Win uds o l c w o l ain m. T
“The Pineapple Express” –IR image
Continental Tropical (c. T) Air Masses • ♦ • • Forming conditions: In the summer over northern Mexico and SW US. Properties: ♦ Very dry and hot; ♦ The atmosphere is unstable; Weather: ♦ Dry and hot. ♦ Practically no precipitation. ♦ Results in heat waves and droughts across the great planes.
HOT & DRY c. T
Effect of c. T air masses on the weather
• • Motion of Air Masses The prevailing upper level winds determine the motion of the air masses. The direction of the tropospheric jet steams impacts the change in weather The Jet stream on Sunday ♦ The zonal component results in the E-W motion of the weather patterns ♦ The meridional component drags tropical air masses to the North and Polar air masses to the South.