For the second day in a row, residents of the southern United States have awoken to dense fog advisories. A view of the continental United States surface map shows that overall, there is very weak synoptic forcing across the south, which results in calm winds and little mixing or advection. This allows radiative cooling to dominate other processes in the overnight hours, and eventually air temperatures cool until they reach the dew point temperature at which point condensation occurs and fog forms. With little wind to disturb things, the fog persists overnight. During the day, the sun rises and heats the ground, which causes near-surface air temperatures to rise. However, the absolute moisture content of the air is relatively unaffected, so the dew points remain unchanged. The warmer air temperatures mean that the air is no longer saturated, and so the fog dissipates.

Band 13 from the GOES-16 ABI measures the atmospheric window brightness temperature and is often the first stop for observing conditions at night when no visible channels are available. Since fog has effectively the same temperature as the land, it can be challenging to discern where the fog ends and clear air begins using just that channel. This is particularly evident in the pre-dawn hours of 28 January 2025. The Band 13 loop over Alabama and Mississippi from 10:00 – 12:00 UTC (4:00 AM to 6:00 AM Central Time) shows little of note. The clouds moving in from the west indicate some elevated instability due to their convective shape, but otherwise conditions look clear and calm.

However, the Nighttime Microphysics RGB tells a very different story. Here, three different parameters are combined into one image to help users identify various features. In this case, the 12.4–10.4 micron brightness temperature difference (sensitive to optical depth) is assigned to the red channel. The 10.4–3.9 micron brightness temperature difference (sensitive to cloud particle size and cloud phase) is assigned to green, and the 10.4 micron brightness temperature (sensitive to temperature) is assigned to blue. Fog, which is a warm liquid cloud, is going to be a light blue or mint green in this combination. Looping this RGB for the same time as above shows the undeniable presence of fog over the region. Of particular note is the long, narrow Tennessee River valley stretching diagonally from northeastern Alabama into eastern Tennessee. This feature is practically invisible in the traditional IR window, yet the fog that formed in this valley is easily seen in the RGB.

It is important to note that this RGB depends on the 3.9 micron brightness temperature channel. This channel straddles the border between terrestrial emission and solar reflectivity. At night, it represents the infrared emission of the surface, but during the day, it is dominated by reflected radiance from the sun. Because of this, the interpretation of the RGB cannot be used during the day as the relationship between the 3.9 ?m brightness temperature and cloud microphysics is only valid at night.

—————
Free Secure Email – Transcom Sigma
Transcom Hosting
Transcom Premium Domains