Visibility — Page 274, Lesson 260

I want to walk you through the remaining fog types that affect aviation visibility. We've covered radiation fog, hill fog, advection fog, and steaming fog — now let's look at frontal fog, freezing fog, and ice fog. Frontal fog occurs at a warm front or an occlusion. The main cause is precipitation lowering the cloud base all the way down to the ground. So the rain or drizzle from the warm front saturates the cold air below, and the cloud base simply descends until it touches the surface — that's frontal fog. There are subsidiary causes as well. First, evaporation of standing water on the ground adds more moisture to the already saturated air. Second, mixing of saturated air with non-saturated air below can also contribute. So even if the precipitation alone isn't enough, these extra processes can push the air to saturation and produce fog. Frontal fog can form along a belt up to 200 nautical miles wide, and that belt then travels with the front itself. The fog can be increased by orographic lifting — that means when the air is forced upward by rising terrain, it cools further and thickens the fog. The fog will be dispersed by the passing of the front, so once the warm front moves through, the fog clears. shows the conditions for frontal fog. Now let's move to freezing fog. This one is tied to the Bergeron theory. The Bergeron theory tells us that at temperatures below 0°C, the air becomes saturated for the formation of ice before it becomes saturated for the formation of water. In other words, the air reaches its saturation point relative to ice at a higher temperature than it does relative to liquid water. So water vapour will go directly to the solid state at these temperatures — that's deposition, skipping the liquid phase entirely. However, here's the key practical problem: the rarity of freezing nuclei in the atmosphere. Freezing nuclei are particles that trigger ice formation. Because they're scarce, when the dew point is below 0°C, condensation will still take place first — producing supercooled water droplets. These droplets are liquid even though their temperature is below freezing. Those supercooled droplets will then freeze on contact with a solid object, giving hoar frost or rime ice. So freezing fog is essentially a fog of supercooled water droplets that freeze instantly when they hit your aircraft, runway, or any surface. Freezing fog will also occur in another scenario: when the dew point is above 0°C, forming ordinary fog, but the air then cools to a temperature less than 0°C. So you get a pre-existing fog that subsequently freezes. There's an important note here: if water vapour is in contact with a solid object at temperatures below 0°C, then it will immediately form ice — hoar frost — missing out the liquid state entirely. That's direct deposition onto the surface, not through a droplet phase. Finally, ice fog. Ice fog forms in extremely low temperatures, usually below -40°C. It happens when warm moist air is introduced into cold saturated air. The typical source of that warm moist air in such extreme cold is combustion — for example, from car engines, aircraft engines, or other combustion sources. The water vapour from the exhaust instantly freezes into tiny ice crystals, creating a fog that can severely reduce visibility, especially around airports and taxiways in polar or high-altitude winter operations. So to summarise: frontal fog is precipitation-driven at warm fronts, freezing fog involves supercooled droplets that freeze on contact, and ice fog is a direct ice-crystal fog from combustion in extreme cold. Each has different formation mechanisms and different implications for your flight operations.