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 Support Convergence 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 Fast Trough

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 L 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-Cycle Surface – Initiation Stage

Chapter 10: Middle Latitude Cyclones Mid-latitude Cyclone Life-Cycle Surface – Intensification Stage

Chapter 10: Middle Latitude Cyclones Mid-latitude Cyclone Life-Cycle Surface – Cont. Intensification

Chapter 10: Middle Latitude Cyclones Mid-latitude Cyclone Life-Cycle Surface – 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 L Jet 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

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