Post on 03-Jul-2020
● In the big picture, most of the weather action is where the temperature contrast is strongest between the equator and the pole. And.....
250mb surfacelines: surface heightcolors: wind speed
....this north-south temperature contrast means that upper-level pressure surfaces slope strongly downward (i.e., the height contours are closely packed)
250mb surfacelines: surface heightcolors: wind speed
zone of north-south temperature
contrast
250mb surfacelines: surface heightcolors: wind speed
....this north-south temperature contrast means that upper-level pressure surfaces slope strongly downward (i.e., the height contours are closely packed)
250mb surfacelines: surface heightcolors: wind speed
zone of north-south temperature
contrast
....this north-south temperature contrast means that upper-level pressure surfaces slope strongly downward (i.e., the height contours are closely packed)
250mb surfacelines: surface heightcolors: wind speed
zone of north-south temperature
contrast
....this north-south temperature contrast means that upper-level pressure surfaces slope strongly downward (i.e., the height contours are closely packed)
250mb surfacelines: surface heightcolors: wind speed
zone of north-south temperature
contrast
....this north-south temperature contrast means that upper-level pressure surfaces slope strongly downward (i.e., the height contours are closely packed)
250mb surfacelines: surface heightcolors: wind speed
zone of north-south temperature
contrast
....this north-south temperature contrast means that upper-level pressure surfaces slope strongly downward (i.e., the height contours are closely packed)
● Not coincidentally, this is also where the jet stream is strongest!
jet stream
250mb surfacelines: surface heightcolors: wind speed
zone of north-south temperature
contrast
Why is most of the action in this region of strong north-south temperature contrast? Well......
Why is most of the action in this region of strong north-south temperature contrast? Well......
● The north-south temperature contrast is fundamentally unstable, in that the colder, denser air wants to slide southward under the warmer air
● But because of the earth's rotation, this adjustment happens in a rather complicated way
● The end result of this process: mid-latitude cyclones● Mid-latitude cyclones are nature's way of sliding colder, denser air
southward under the warmer, less dense air!
Why is most of the action in this region of strong north-south temperature contrast? Well......
● They account for much what we call weather, including rain, snow, high winds, etc.
● Cyclones develop and die on a time scale of several days to a week or so (synoptic scale)
● Key features of cyclones are the cold and warm fronts extending outward from the low center
Mid-latitude cyclones are the big low pressure centers on the weather map:
The development of cyclones is rather complicated, but it's described to a first approximation by the Norwegian cyclone model (aka polar front theory), first developed in the 1920s and then later refined......
The Norwegian Cyclone Model
Stationary front: It all starts with a zone of temperature gradient (called a stationary front or baroclinic zone) along the polar front region.
surface view
3D view
The Norwegian Cyclone Model
Frontal wave: A low pressure center forms along the zone of temperature contrast, usually due to upper-level divergence. Warm and cold fronts start to organize and move around the low.
surface view
3D view
The Norwegian Cyclone Model
Open wave: The low strengthens and the warm and cold fronts rotate counter-clockwise around the low center. Colder air begins to slide underneath warmer air at the fronts, causing lifting and clouds.
surface view
3D view
The Norwegian Cyclone Model
Mature: The system is at its strongest, with its lowest pressure and strongest winds. The cold front catches up to the warm front, forming an occluded frontal zone. Cold air slides further under warm air.
surface view
3D view
The Norwegian Cyclone Model
Occlusion: The cold front catches the warm front along most of its length, cutting off the warm air from the low center. The system dissipates as the warm sector is lifted completely off the ground.
surface view
3D view
The Norwegian Cyclone Model