Chapter 10: Middle Latitude Cyclones
Chapter 10: Mid-latitude Cyclones
Most active weather is associated with a cyclone (i.e. low pressure system)
Here we will examine how these systems evolve:
Life cycle Bring the ingredients together for active
weather Central U.S. focus.
Chapter 10: Middle Latitude Cyclones
Hydrostatic Equilibrium
To create low pressure, the mass in the air column must be reduced.
At the surface low pressure systems have convergence … a build up of air in the column
To compensate the surface low must have support .... upper level support
Chapter 10: Middle Latitude Cyclones
Upper Level SupportConvergence at the surface must be compensated by divergence aloft
Divergence aloft = upper level support
For the system to intensify divergence aloft must be greater than surface convergence
Chapter 10: Middle Latitude Cyclones
Intensifying Mid-latitude Cyclone
L
Jet Stream
Surface Low
Slow
FastTrough
Chapter 10: Middle Latitude Cyclones
Intensifying Cyclones Characteristics Surface low is between trough and ridge
Westward tilt with height of system Jet forms on boundary between warm and
cold air and surface low is on that boundary The circulation around the surface low
introduces: Warm air advection (i.e., warm front) out
ahead of the low Cold air advection (i.e., cold front)
dragging behind the low
Chapter 10: Middle Latitude Cyclones
Intensifying Mid-latitude Cyclone
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Warm Front
Cold Front
We anticipate these locations for the fronts.
The exact locations depend upon the situation
Warm Sector
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclones
The pattern we have examined so far is point of most rapid intensification
How do we get to that point? How does the system dissipate
What is the life cycle of a low pressure system?
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclone Life-CycleSurface – Initiation Stage
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclone Life-CycleSurface – Intensification Stage
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclone Life-CycleSurface – Cont. Intensification
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclone Life-CycleSurface – Occlusion Stage
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclone Life-Cycle The surface system propagates (moves)
during its life cycle The surface interacts with the upper levels
Upper levels support the surface Near surface circulations redistribute the
warm/cold air which causes the jet to adjust.
Now that we have looked at the surface, lets consider both surface and upper levels
Chapter 10: Middle Latitude Cyclones
Intensifying Mid-latitude Cyclone
L
Jet Stream
Surface Low
Cold Air Advection
Warm Air Advection
Dashed lines represent locations at a later time
Chapter 10: Middle Latitude Cyclones
Intensifying Mid-latitude Cyclone
LJet Stream
Surface Low
Cold Air Advection
Warm Air Advection
L
Dashed lines represent locations at a later time
Chapter 10: Middle Latitude Cyclones
Intensifying Mid-latitude Cyclone
L
Jet Stream
Surface Low
Cold Air Advection
Warm Air Advection
Dashed lines represent locations at a later time
Chapter 10: Middle Latitude Cyclones
Chapter 10: Middle Latitude Cyclones
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclones These systems move mainly west to east
Some move NW → SW, some SW → NE The track is often tied to the jet stream
pattern Often a location will first experience the
warm front and then the cold front Some have strong intensification others
don't Some systems go through the life-cycle
quickly (3-5 days), or not (7-10 days)
Chapter 10: Middle Latitude Cyclones
Mid-latitude Cyclone Weather
Chapter 10: Middle Latitude Cyclones
High Pressure Systems
Form in the middle of air masses not on the boundaries
Have no systematic life-cycle Work with rather than against gravity so no
need for upper level support Though upper level convergence can
generate high pressure
Chapter 10: Middle Latitude Cyclones
Main Points Dropping pressure requires a loss of mass
in the column Systems only deepen if they have upper-
level support Systems have self-development/life cycle
Surface affects upper-levels Upper levels affect surface
Developing systems tilt westward with height
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