Visibility — Page 274, Lesson 258

Let’s pick up where we left off with advection fog. I’ve already covered the basic definition — warm, moist air moving over a cold surface — so now I want to walk you through the specific conditions that must be in place for advection fog to form, when and where it typically occurs, how it disperses, and then we’ll look at a special type called steaming fog, or Arctic Smoke. First, the conditions necessary for advection fog. There are three key requirements. Number one: winds up to 15 knots to move the air. Over sea areas, the wind may be stronger and still produce advection fog, but over land, 15 knots is the typical upper limit. Number two: a high relative humidity, or RH, so that relatively little cooling is required to bring the air to saturation and then condensation. In other words, the air is already nearly full of moisture, so only a small temperature drop is needed to form fog. Number three: a cold surface whose temperature is lower than the dew point of the moving air. The dew point, or DP, is the temperature to which air must be cooled to become saturated. If the surface is colder than that dew point, condensation is guaranteed. Now, times of occurrence and location. Over land areas, advection fog happens mainly in winter and early spring. Over sea areas, it occurs in late spring and early summer, but it can happen at any time of the year when tropical maritime air moves over sea areas whose temperature is below the dew point of that air. A specific example: advection fog occurs particularly when a southwesterly wind brings tropical maritime air to the UK. Let’s talk about dispersal — how advection fog clears. There are two main ways. First, by a change of air mass, meaning a wind change that brings in different air. Second, by a wind speed greater than 15 knots, which will lift the fog to form stratus cloud. So instead of fog at the surface, you get a low cloud layer. I want to highlight some special areas. Nearly all sea fogs are caused by advection. Two classic examples are the extensive and persistent sea fogs in the region of the Grand Banks of Newfoundland, and around the Kamchatka Peninsula in the North Pacific. In both cases, warm air from the south moves over a cold sea current flowing down from the north. That temperature contrast is perfect for advection fog. Now, a different type: steaming fog, also called Arctic Smoke. This occurs over sea in polar regions — for example, the fjords of Greenland, Iceland, and sea areas of high latitudes. It is caused by cold air from a land mass moving over a warmer sea. The small amount of evaporation from the sea is enough to cause saturation and condensation, but the air itself must be very stable. This fog can be persistent and up to 500 feet thick, and it may drift inland. It will be dispersed by an increase in wind speed or a change of direction. Usually, steaming fog is only significant in Arctic regions, but the ‘steam’ may be seen at any latitude when cold air moves over a wet surface. So to summarise: advection fog needs wind up to 15 knots, high relative humidity, and a surface colder than the dew point. It clears with a wind change or stronger winds that lift it into stratus. And steaming fog is a polar phenomenon where cold air moves over warmer water, producing a shallow, stable fog layer.