T/G Layout 1

figure 24.

Divergence associated with jet streak circulations can be compared to a chimney. If the
chimney flue is open, smoke escapes and more air is drawn in to feed the fire. If the
flue is closed, smoke backs up into the fireplace and no new air feeds the fire. As a fire
needs an open flue to grow, a deepening low pressure system must contain a deep
region of divergence (figure 24).

figure 25.

Mature Extratropical Cyclone

cold

1. At surface:

• cold front with strong
  temperature gradient


• low pressure center

2. In mid-troposphere:

• amplified wave


• cloud system in
  comma form

3. At tropopause:

• jet stream with embedded
  jet streak


• cloud free zones


• north of comma cloud and
  behind cold front shows position
  of jet steam

L

warm

jet stream

jet streak

Divergence aloft: Chimney example