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A closer look at forecasts for SLD

Updated: Nov 8, 2021

For those pilots that utilize the airframe icing analysis and forecasts found on aviationweather.gov, it's important to take a closer look at the guidance for supercooled LARGE drop (SLD) icing. The Current and Forecast Icing Products (CIP and FIP) provide three distinct forecasts. This includes an analysis and forecast for (1) icing probability, (2) icing severity and (3) supercooled LARGE drop (SLD) icing. First, let's define SLD.


What is SLD?


Clouds are made up of tiny droplets that are suspended in the air. Most of those droplets are 15 to 40 microns in size. For those who are micron-challenged, 1000 microns is equal to 1 millimeter. The naked eye can see objects as small as 40 microns. As a point of reference, the diameter of a human hair is approximately 100 microns, and a red blood cell is 8 microns. When the droplets grow in size to 50 microns or greater, they are considered a "large" drop (and drop the "let" at the end of droplet).


For icing certification purposes the drop sizes are actually based on a medium volumetric diameter (MVD). That is, when the median size of the volume of drops exceeds 50 or more microns, this becomes a large drop environment. That means when a drop becomes barely perceptible to the naked eye, it's considered a large drop for certification purposes. If the temperature is below 0°C this is called a supercooled LARGE drop environment. When the MVD is less than 50 microns, it's considered a small drop environment. Certified ice protection systems (IPS) are only certified into small drop environments.


Simply put, SLD is a "large drop" icing environment. If you hear terms like freezing rain (FZRA), freezing drizzle (FZDZ) or thunderstorms during your weather briefing, you are also likely dealing with the risk of SLD. Despite what some pilots will tell you, SLD does not stand for supercooled liquid drops; they are truly missing the point. Yes, they are liquid, but SLD stands for supercooled LARGE drops.