According to the Norwegian cyclone model, the situation where a cold front catches up with a portion of a warm front above the cold frontal surface. This conceptualization of cold type occlusion development is not often observed in nature; however, it is undisputed that in strong cyclones the low center often retreats toward the cold air separating itself from the cold and warm fronts. A trough in sea level pressure is found between the cyclone center and the wave on the front, and this trough is the occluded front. Regardless of the formation processes, characteristics of a cold type occlusion are 1) a warm temperature or thickness ridge along the occluded front; 2) a trough in the sea level pressure field along the occluded front; 3) relatively colder air behind the front; and 4) an increase in lower-tropospheric static stability behind the front.