Cold Fronts and Air Masses (author: Sara Tonks)

Figure 1: True-Color RGB Visible imagery from GOES-16 (East) from 1726 UTC to 1931 UTC 31 October 2019 (


On 31 October 2019, a cold front moved through the eastern United States in association with an extratropical cyclone located over the Great Lakes region. Early in the afternoon (1726 UTC), a squall line feature formed due to the passage of the front and aided by daytime heating, which is visible in Fig. 1. The most interesting features of the visible satellite imagery, however, are the clearly distinct air masses associated with the front. When cold fronts move into a region, they bring cold, dry air. This air is more dense than the moist warm air from the tropics ahead of the front, and as such the cold air moves below the warm, forcing the warm air up (this is what causes the precipitation, and the added boost of instability from daytime heating can lead to severe weather) (Fig. 2).


Figure 2: Conceptual model of a cold front (


Extratropical cyclones in the Northern Hemisphere lead to cold air moving from the northwest, and in states such as Georgia, prior warm, moist air is from the Gulf of Mexico. This leads to the difference in wind directions in front of and behind the front – behind experiences northwesterly wind, and ahead experiences south-southwesterly. This is visible on the true-color visible satellite imagery for this event, as the cumulonimbus clouds ahead of the cold front move north-northeast, and behind the front, lower level clouds within the cold air mass are moving southeast (Fig. 1).


Figure 3: Composite Radar Reflectivity from 1725 UTC to 1935 UTC 31 October 2019 (


Radar imagery from the same time period also shows the different types of precipitation associated with the frontal passage (Fig. 3). The squall line ahead of the front represents convective precipitation, notable on radar imagery as tight regions of intense precipitation. Closer to and behind the front, within the cold air mass, is weak stratiform precipitation from the low level clouds that remain.