Mid-latitude weather
Prof. Jeff Gawrych
De Anza College
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Introduction
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Here in the mid-latitudes, day-to-day weather
changes are closely linked to moving, synopticscale disturbances
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
 Unfortunately, coverage is sparse.
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No two systems are exactly alike
Atmospheric Scales of Motion
Scale Time Scale Distance Scale
Examples
Macroscale
-Planetary
Weeks to years
-Synoptic
Days to weeks
50 – 3,000 miles
Cyclones, anticyclones
and hurricanes
Mesoscale
Minutes to days
1 – 50 miles
Land-sea breeze,
thunderstorms and
tornadoes
Microscale
Seconds to minutes
500 – 25,000 miles
< 1 mile
Westerlies,
trade winds
Turbulence, dust
devils and gusts
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Upper-level flow
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Between 500 - 250 mb, or 18,000-36,000 ft
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
 Ben Franklin noted how rain bands do not
always travel in the same direction as the
large storm system
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Low and High pressure systems
Cylonic flow
Anticylonic flow
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Upper air patterns
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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.
The geopotential height:
 is the height of a particular pressure level.
Consider a 500mb geopotential map.
In this map, the height lines (~5000m) are
 physically the same as the surface pressure.
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Ridge
Trough
Trough
Trough
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Surface flow
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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:
Into low pressure systems
 Away from high pressure systems
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Friction slows down wind, which decreases coriolis force:
Wind now aimed more in direction of PGF.
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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
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convergence divergence image
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Why do high-pressure systems
cause clear conditions?
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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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Air Masses
A general description of the atmosphere over a
certain area.
 Tells temperature and moisture profile.
 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)
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Air masses
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Type of air mass over a region tells what weather
may be like.
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Name of air masses tells you where it originated
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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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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.
 Cold fronts bring cold air into region
 Warm fronts bring warm air into region
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Cold Front
Warm front
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Temperature advection
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Wind direction can be good indicator of temperature change.
N. Hem:
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Wind from the north generally indicates
cool air approaching. (cold advection)
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Wind from the south generally indicated
warm air approaching. (warm advection)
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