Mountains: The Blockers of Clouds (author: Sara Tonks; date: 27 Nov. 2019)

Figure 1: Visible imagery from GOES-16 on 1416 UTC 24 November 2019 (


A curious cloud formation over the southeastern United States appeared on satellite imagery on the morning of 24 November 2019. Visible imagery taken at 1416 UTC on 24 November 2019 showed a well developed extratropical cyclone over the northeast and further south a line of clouds over eastern Tennessee that suddenly began to spread to the southeast once in Georgia (Fig. 1). An infrared cloud-top imagery loop showed low level clouds that initially (As of 0636 UTC 23 November 2019) stretched in a band from the northeast to southwest, but over time the southern part of the band extended southeast while the northern portion remained stationary and did not propagate eastward at all (Fig. 2). The answer to the cause of this cloud development lies in the height of the clouds, shown on IR imagery to be low in altitude.

Figure 2: Cloud-top IR Imagery from GOES-16 from 0636 to 1421 UTC 24 November 2019 (


Extending from the northeast to southeast are the Appalachian Mountains, which start in northern Georgia and end in southern Maine (Fig. 3). These mountains are low in height relative to the Rocky Mountains, but in the case of these clouds, they were just high enough. As the band of low-level clouds propagated eastward, the Appalachian Mountains blocked the majority of that movement due to their height. Some of that moist air is simply trapped by the mountains. Any air that is orographically forced upward over the mountains, upon reaching the other side of the mountain, has lost the majority of its moisture on the windward side and is warming, decreasing the relative humidity even further. Thus, there were no clouds on the leeward side. Further south, past the southern tip of the Appalachian Mountains, the clouds can continue on their march southeastward unperturbed and unbothered by the terrain.

Figure 3: Topographic map of the United States (