● In the big picture, most of the weather action is where the temperature contrast is strongest between the equator and the pole. And..... 250mb surface lines: surface height colors: 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 surface lines: surface height colors: 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 surface lines: surface height colors: 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 surface lines: surface height colors: 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 surface lines: surface height colors: 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 surface lines: surface height colors: 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 surface lines: surface height colors: wind speed zone of north- south temperature contrast ● Not coincidentally, this is also where the jet stream is strongest! 250mb surface lines: surface height colors: wind speed zone of north- south temperature contrast jet stream 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 Why is most of the action in this region of strong north-south temperature contrast? Well...... ● 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! Mid-latitude cyclones are the big low pressure centers on the weather map: ● 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 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 .