Freezing dense fog developed across the far eastern Colorado plains and surrounding states during the early morning hours of 6 Feb 2018. By mid morning, the dense fog began to lift and retreat east, revealing hoar frost had been deposited on surfaces. The hoar frost was first apparent to NWS PUB forecasters in the GOES-16 Day Cloud Phase Distinction RGB, where dark green colors appeared along the outside edge of the retreating low clouds (Fig 1).

This dark green color comes from a relatively high reflectance (compared to bare surface, but not as high as with fresh snow) in the VIS, combined with warm temperatures in the IR, and low reflectance in the snow ice band. All of these details indicate a blanket of ice crystals on the surface. Analyzing the snow ice single-band imagery, it further appears that there was indeed a layer of ice left behind by the retreating fog, as ice crystals have a very low reflectance (Fig 2). In the channel 2 VIS, the frost layer is subtly apparent as slightly higher reflectance compared to the bare surface, but slightly lower reflectance compared to the low clouds (Fig 3). In all examples, the frost is seen quickly retreating/melting as the low cloud shield also retreats, exposing the frost to the sun.

Finally, webcams in the area confirm the thick layer of hoar frost (Fig 4 and 5).

Bill Line, NWS