Outline

1. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (1 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Further Reading: Chapter 08 of the text book Outline - air masses - air masses of the world and of N. America - fronts: warm, cold and occluded

2. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (2 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Previously, We discussed the general circulation of the atmosphere and oceans We looked at surface winds and pressure patterns as well as upper-air winds We also discussed surface circulations and thermohaline circulations of the deep ocean An overarching theme for all of this discussion was the role these circulations play in transporting energy from low latitudes to high latitudes Introduction • Today, • We begin our discussion of phenomena that affect temperature, rainfall, etc. on a day to day basis, i.e. weather • We will define characteristics of air masses involved in producing weather • We will also look at the characteristics of fronts • Throughout, we will have to keep in mind all of the principles we’ve learned before • Atmospheric humidity • Instability • Dynamic forces

3. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (3 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Air Masses Large bodies of air with (more or less) uniform properties (temperature and moisture) Air masses can be very large: ~ 1000’s of km across Properties reflect properties of the source region - hot and moist air masses develop over tropical oceans - cold and dry air masses develop over continental polar locations Described bases upon Latitude (defines temperature) Surface type (defines moisture content) Air Masses • Weather Systems: Recurring or common circulation patterns and their associated weather • - Example: traveling low pressure centers (cyclones) where inspiraling air that brings warm • moist air in contact with cold dry air to results in clouds and precipitation • - 1 km (Tornado) to 100s km (anti-cyclones) • - may last from hours to weeks • - weather systems are associated with movements of air masses

4. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (4 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Classification of Air Masses Name Symbol Source Region Arctic/Antarctic A/AA Arctic/Antractic Ocean Polar P 50-60N Tropical T 20-35N Equatorial E <10N Maritime m Oceans Continental c Continents • Ex.: cA - Continental Arctic mP - maritime Polar

5. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (5 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Air Masses of the World • The maritime tropical airmass (mT) and maritime • equatorial air mass (mE) originate over warm oceans • - high specific humidity • - warm (20+C) • - capable of producing heavy precip • The continental tropical airmass (cT) originates • over the subtropical deserts of the continents • - substantial water vapor • - low relative humidity • Polar air masses (mP, cP) originate in the subarctic • latitude zone • - mP has low water vapor (moderate precip) • - cP originates over N. America and Eurasia • (low humidity and very cold) • The continental arctic (cA) and antarctic (cAA) • air masses are extremely cold (-45C) and have little or • no water vapor

6. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (6 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University cP: cold, stable air originating from high pressures in N. Canada mP: cool, moist air, usually unstable originating from Aleutian low; region of cyclone formation; heavy precip in coastal ranges mT: warm moist unstable air originating from the Gulf of Mexico; good for producing thunderstorms cT: hot dry air from Mexico; impacts southern AZ and CA Air Masses of North America mP cP mP cT mT mT mT

7. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (7 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Fronts • Implicit in all of our discussion is that air masses can move and still retain their identity • May result in the collision of two air masses with very different properties • The zone of contact is called a front • A front is an interface between air masses with different temperatures and moisture content • This figure shows the polar front of which you learned in the past • Here the front is between cold air to the north and warm air to the south

8. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (8 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Warm Front • Warm air is less dense than cool air • The warm air rises over the cool air • As it does so, it goes through “adiabatic cooling” which allows moisture to condense into clouds and rain • Typically there is gradual lifting leading to relatively mild precipitation

9. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (9 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Cold Front • Warm air is forced to flow over the cooler air • As it does so, it goes through “adiabatic cooling” which allows moisture to condense into clouds and rain • For these fronts, the boundary tends to be sharp with strong instability, leading to heavy precipitation

10. Myneni Lecture 21: Air Masses and Fronts Mar-09-07 (10 of 10) Natural Environments: The Atmosphere GE 101 – Spring 2007 Boston University Occluded Front • Similar to cold front in that warmer air is forced to rise over cold air • Cold fronts move along the ground faster than warm fronts • Thus, a cold front can overtake a warm front • The warm air mass may be completely lifted off the ground • For these fronts you may find hail and sleet -> as rain from the warm air falls through the cold air below it, it freezes and forms hail