1. LIFE CYCLE OF EXTRA-TROPICAL CYCLONE

2. Life Cycle of Mid-latitude Cyclone
Initially there is a boundary or front, separating warm air to the south from cold air to the north
The front is often stationary

3. Life Cycle of Mid-latitude Cyclone
An upper level disturbance embedded in the jet stream moves over the front
A wave forms on the front
The front develops a "kink" where the wave is developing

4. Life Cycle of Mid-latitude Cyclone
Precipitation commences and it is heaviest along the front (dark green)

5. Life Cycle of Mid-latitude Cyclone
As the wave intensifies, both cold and warm fronts become better organized

6. Life Cycle of Mid-latitude Cyclone
The wave becomes a mature low pressure system, while the cold front, moving faster than the warm front, "catches up" with the warm front
As the cold front overtakes the warm front, an occluded front forms

7. Life Cycle of Mid-latitude Cyclone
As the cold front continues advancing on the warm front, the occlusion increases and eventually cuts off the supply of warm moist air, causing the low pressure system to gradually dissipate

8. Winds
Southerly winds east of the low transport warm and moist air northward and this moisture often contributes to the development of precipitation
A warm front marks the leading edge of this warm, moist air mass
Behind the low, northerly winds transport colder and drier air southward, with a cold front marking the leading edge of this colder, drier air mass

9. Satellite Imagery
On satellite images, a mature mid-latitude cyclone is easily identifiable by a comma-shaped cloud
Satellite images show four stages of development of an mid-latitude cyclone

10. Leaf Stage
During the initial development of a mid-latitude cyclone, the cloud pattern appears in the shape of a leaf
This characteristic shape is often observed on the east side of an upper-level trough

11. Leaf Stage
The leaf shaped pattern is caused by the jet stream pushing into the western edge of the cloud system and spreading the cloud down wind

12. Leaf Stage
The highest cloud tops are over the eastern portion of the leaf
Middle-level clouds are over the westward portion
The leaf cloud is a significant region of clouds and precipitation, even if cyclogenesis does not occur
The western edge of the leaf has a well-defined border

13. Open Comma Stage
A comma shaped cloud pattern appears in satellite images when the mid-latitude cyclone has well-developed warm and cold fronts
As the comma cloud pattern develops, pressure at the surface usually falls

14. Open Comma Stage
The back edge of the common cloud pattern is easily identified and represents the position of the cold front and the clouds, often thunderstorms, form because of the frontal lifting
The front edge of the comma cloud pattern is more diffuse as upper-level winds make the clouds to spread out

15. Occluded Stage
As the occlusion forms, the low-pressure circulation separates from the jet stream
This is the mature stage of the storm, when the central pressure in the storm stops falling
The point at which the occluded, cold and warm fronts come together is referred to as the triple point, and it is often seen in satellite imagery where the jet stream cuts across the system

16. Occluded Stage
The cloud pattern is still in the shape of a comma, the clouds spiral around the centre of the storm located in the head of the comma

17. Occluded Stage
The cloud pattern is still in the shape of a comma, the clouds spiral around the centre of the storm located in the head of the comma

18. Dissipating Stage
As the storm system continues to weaken, upper-level winds tend to tear it apart and the system becomes disorganised

19. Dissipating Stage
The cloud system loses its organisational pattern and the comma head gets cut off from the tail

20. Dissipating Stage
The comma head and its associated low pressure may lag behind and continue to rotate
Such cut-off, cold-core lows weaken slowly with time and tend to be very persistent