1 Mid-latitude weather Prof. Jeff Gawrych De Anza College

2 Introduction Here in the mid-latitudes, day-to-day weather changes are closely linked to moving, synoptic- scale disturbances Here in the mid-latitudes, day-to-day weather changes are closely linked to moving, synoptic- scale disturbances These are the high and low pressure systems meteorologists often speak of. These are the high and low pressure systems meteorologists often speak of.  Bay Area is at ~ 37º N  These systems are steered by upper-levels winds (jet streams) Using observations (such as weather balloons), we can track their development and decay Using observations (such as weather balloons), we can track their development and decay  Unfortunately, coverage is sparse. No two systems are exactly alike No two systems are exactly alike

3 Atmospheric Scales of Motion ScaleTime ScaleDistance ScaleExamples Macroscale -PlanetaryWeeks to yearsWesterlies, trade winds -PlanetaryWeeks to yearsWesterlies, trade winds -SynopticDays to weeksCyclones, anticyclones and hurricanes -SynopticDays to weeksCyclones, anticyclones and hurricanes MesoscaleMinutes to daysLand-sea breeze, thunderstorms and tornadoes MicroscaleSeconds to minutesTurbulence, dust devils and gusts 500 – 25,000 miles 50 – 3,000 miles 1 – 50 miles < 1 mile

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6 Upper-level flow Between mb, or 18,000-36,000 ft Between mb, or 18,000-36,000 ft In this portion of the atmosphere, we can most easily see these traveling synoptic events such as: In this portion of the atmosphere, we can most easily see these traveling synoptic events such as:  Cyclones (low pressure systems)  Anticyclones (high pressure systems) Surface features such as mountains, cities, oceans/land can be misleading Surface features such as mountains, cities, oceans/land can be misleading  Ben Franklin noted how rain bands do not always travel in the same direction as the large storm system

7 Cyclone Definition A cyclone is a broadly used term in Meteorology - from small scale phenomena, like tornadoes (~10 km across), to large phenomena like tropical cyclones (~ 500 km across). A cyclone is a broadly used term in Meteorology - from small scale phenomena, like tornadoes (~10 km across), to large phenomena like tropical cyclones (~ 500 km across). Cyclones are defined by air rotating in a cyclonic direction. Typically this corresponds to a low pressure at the surface. Cyclones are defined by air rotating in a cyclonic direction. Typically this corresponds to a low pressure at the surface. anti-cyclonecyclone Northern Hemisphere picture

8 Low and High pressure systems

9 Cylonic flow Anticylonic flow Low and High pressure systems

10 Upper air patterns Aloft, the pressure field normally does not consist of closed high and low pressure contours, it simply flows: Aloft, the pressure field normally does not consist of closed high and low pressure contours, it simply flows:  In a general west to east direction  In a wavelike pattern At upper levels, the pressure field is normally given as the geopotential height. At upper levels, the pressure field is normally given as the geopotential height. The geopotential height: The geopotential height:  is the height of a particular pressure level. Consider a 500mb geopotential map. Consider a 500mb geopotential map. In this map, the height lines (~5000m) are In this map, the height lines (~5000m) are  physically the same as the surface pressure.

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13 Trough Ridge

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15 Surface flow At the surface, friction is more important than at upper levels, At the surface, friction is more important than at upper levels,  thus geostrophic balance is not valid The result, winds not exactly parallel to isobars, rather angled: The result, winds not exactly parallel to isobars, rather angled:  Into low pressure systems  Away from high pressure systems

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17 Friction slows down wind, which decreases coriolis force: Wind now aimed more in direction of PGF.

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22 Why do low-pressure systems cause rain? Aloft: flow is counter clockwise in nh and flow is geostrophic Surface: flow is ~cc, but NOT geostrophic, Instead flow is inward towards low causing convergence Leads to clouds/rain convergence divergence image

23 Why do high-pressure systems cause clear conditions? Aloft: flow is clockwise in nh and geostrophic Surface: flow is ~clockwise but NOT geostrophic Instead flow is outward from high causing divergence Leads to sinking motion (subsidence) and clear skies What happens to air that sinks???

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25 Air Masses A general description of the atmosphere over a certain area. A general description of the atmosphere over a certain area. Tells temperature and moisture profile. Tells temperature and moisture profile. Examples Examples cT: continental tropical (warm and moist) cP: continental polar (cold and dry) mT: marine tropical ( warm and moist) mP: marine polar (cold and moist)

26 Air masses Type of air mass over a region tells what weather may be like. Type of air mass over a region tells what weather may be like. Name of air masses tells you where it originated Name of air masses tells you where it originated Air masses originate in specific areas and can dominate the climate. Air masses originate in specific areas and can dominate the climate.  Remember: climate is what you expect, weather is what you get.

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28 Fronts Separate different air masses of different densities. Cold air is more dense than warm air  cold air sinks, warm air rises. So when a cold front passes, it forces lifting. Separate different air masses of different densities. Cold air is more dense than warm air  cold air sinks, warm air rises. So when a cold front passes, it forces lifting. Cold fronts bring cold air into region Cold fronts bring cold air into region Warm fronts bring warm air into region Warm fronts bring warm air into region

29 Cold Front Warm front

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31 Temperature advection Wind direction can be good indicator of temperature change. Wind direction can be good indicator of temperature change. N. Hem: N. Hem: Wind from the north generally indicates cool air approaching. (cold advection) Wind from the south generally indicated warm air approaching. (warm advection)