Chapter 8: Air Masses, Fronts, and Middle-Latitude Cyclones Air masses Fronts Middle-latitude cyclones Air Masses Air mass an extremely 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 Over water? Over dry land? In a cold region? In a warm region? Air mass classification Air mass classification uses a two letter classification scheme First letter: c continental source region m maritime source region Second letter: P polar source region T tropical source region There are four basic air mass types: cp continental polar air mass mp maritime polar air mass ct continental tropical air mass mt maritime tropical air mass There are two additional air mass types for extreme air masses: ca continental Arctic air mass me maritime equatorial air mass Air Masses of North America cp and ca air masses Air mass characterized by very cold and dry conditions
Source region is northern Canada and Alaska What types of weather are associated with cp and ca air masses? Cold, dry air Lake effect snow cp and ca air masses Air mass characterized by very cold and dry conditions Source region is northern Canada and Alaska What types of weather are associated with cp and ca air masses? Cold, dry air Lake effect snow Lake-Effect Snow Cold air goes over warm lake water, picks up moisture, then rises, creates clouds and snow. mp air masses These air masses are characterized by cool, moist, and conditionally unstable conditions. Source region is the N. Pacific and N. Atlantic oceans What types of weather are associated with mp air masses? Rain or snow
mt air masses mt air masses are characterized by warm, moist conditions The source regions for mt air masses are the sub-tropical Pacific and Atlantic oceans and the Gulf of Mexico What types of weather are associated with mt air masses? Figure 8.7 Winter and early spring surface weather patterns that usually prevail during the invasion of cold, moist mp air into the mid-atlantic and New England states. (Green shading area represents light rain and drizzle; pink region is freezing rain and sleet, whit is snow) ct air masses ct air masses are characterized by hot, dry conditions ct air masses form over the deserts of Mexico and the southwestern United States What types of weather are associated with ct air masses? Fronts Front a transition zone between two air masses of different densities There are four basic types of fronts: Cold Warm Stationary Occluded Stationary Front A nearly stationary (not moving) boundary between two air masses A stationary front is drawn on weather maps as alternating triangle and semi-circle symbols on opposite sides of the front What types of weather are associated with stationary fronts? Clear or cloudy skies Light rain possible Cold Front Cold, dry air replaces warm, moist air A cold front is drawn on weather maps as a line with triangle symbols, with the triangles pointing in the direction that the front is moving towards.
How do meteorologists locate fronts on a weather map? Large change in temperature over a short distance Change in moisture content of air Shift in wind direction Pressure and pressure changes Cloud and precipitation patterns Warm Fronts Warm, moist air replaces cold, dry air A warm front is drawn on weather maps as a line with semi-circle symbols, with the semi-circles pointing in the direction that the front is moving towards. What types of weather are associated with warm fronts? Overrunning rising of warm air up over cold air
Occluded Fronts Form when a cold front catches up to a warm front An occluded front is drawn on weather maps as a line with triangle and semi-circle symbols on the same side of the line, with the symbols pointing in the direction that the front is moving towards. Cold-type occluded front (cold occlusion) Air behind the cold front is colder than the air ahead of the warm front Warm -type occluded front (warm occlusion) Air behind the cold front is not as cold as the air ahead of the warm front Ofte n it is difficult to identify occluded fronts from a surface weather map only.
Figure 8.17 The formation of a cold occluded front. The faster moving cold front in (a) catches up to the slower-moving warm front in (b) and forces it to rise off the ground (c). Figure 8.18 The formation of a warm-type occluded front. The faster-moving cold front in (a) overtakes the slower moving war front in (b). The lighter air behind the cold front rises up and over the denser air ahead of the warm front. Diagram (c) shows a surface map of the situation. Middle-latitude Cyclones Middle-latitude cyclones are also called extratropical cyclones or mid-latitude cyclones Polar Front Theory a theory that explains the life cycle of mid-latitude cyclones and their associated fronts Polar Front - the boundary that separates cold polar air from warm subtropical air (our old friend from the three-cell model of global atmospheric circulation) Frontal wave the initial kink that forms on the polar front (b)
Wave cyclone the name given to cyclones that evolve in the manner shown above Open wave fully developed cyclone (c and d) Warm sector warm air mass located between the cold and warm fronts (c, d, and e) Where do mid-latitude cyclones tend to form? Named after the region where they form Nor easters (a favorite of weather fans in the northeastern United States) stroms that fomr along the eastern seaboard of the US and then move northeastward Developing Mid-latitude Cyclones and Anticyclones Cyclogenesis development or strengthening of a mid-latitude cyclone
What causes mid-latitude cyclones and anticyclones to develop? (or how can we change the air pressure at the surface?) Remember, air pressure is just the weight of the overlying air. So, we can change surface pressure by changing the amount of air over a certain location. The secret lies in convergence and divergence Figure 8.23 Convergence, divergence, and vertical motions associated with surface pressure systems. Notice that for the surface storm to intensify, the upper trough of low pressure must be located to the left (or west) of the surface low. Typically we have convergence on the west side of an upper level trough and divergence on the east side of an upper level trough. Convergence (or divergence) can occur due to a change in wind direction and/or a change in wind speed. For a mid-latitude cyclone to intensify (surface pressure decreasing) we need more divergence than convergence over the center of the cyclone. Rising air is found above areas of convergence near the surface or below areas of divergence aloft. Sinking air is found above areas of divergence near the surface or below areas of convergence aloft. Waves in the westerlies Longwave widely spaced troughs in upper level flow (typically spaced 1000s of km apart) Can have 3-6 longwaves in the upper level flow of the Northern hemisphere at one time. Tend to move from west to east fairly slowly. Meteorologists sometimes refer to longwaves as Rossby waves. Shortwave - small disturbance (trough) embedded in upper level flow Tend to move rather quickly from west to east. These short and longwaves are associated with areas of convergence and divergence and as such play an important role in the life cycle of midlatitude cyclones.
The jet stream and mid-latitude cyclones Jet streak core of strongest jet stream winds Areas of upper level convergence and divergence occur near jet streaks. Therefore, jet streaks are also important in the development of mid-latitude cyclones. What seasonal changes occur in the jet stream and how does this affect midlatitude cyclones? Figure 8.24 (a) As the polar jet stream and its area of maximum winds (the jet streak or MAX), swings over a developing mid-latitude cycone, an area of divergence D draws warm surface air upward, and an area of convergence (C) allows cold air to sink. The jet stream removes air above the surface storm, which causes surface pressures to drop and the storm to intensify. (b) When the surface storm moves northeastward and occludes, it no longer has the upper-level support of diverging air, and the surface storm gradually dies out.