Which descriptor denotes shallow fog in METAR reports?

The descriptor for shallow fog in METAR reports is indeed represented by MI, which stands for "shallow." In aeronautical meteorology, when visibility is reduced due to fog, it can be classified into different types. Shallow fog implies that the visibility is significantly reduced but not to the extent that typical fog would.

In METAR reports, shallow fog specifically describes a layer of fog that is usually less than 2 meters deep, causing limited visibility at lower altitudes but with potentially clearer conditions above. This distinction is important for pilots and air traffic controllers, as it helps them understand how the weather may affect operations at the airport.

Fog is denoted by the code FG, but it does not specify whether it is shallow or not. The codes PR and SH refer to different weather phenomena, specifically for partial obstructions to visibility and showers, respectively. Thus, MI is the correct indication for shallow fog in METAR reports.