Visibility — Page 274, Lesson 261

I want to walk you through the visibility reducers that go beyond just fog and mist. Let's start with a rare but interesting phenomenon: when condensation and freezing happen almost simultaneously, you get a build-up of ice crystals in the atmosphere, which reduces visibility. This is a rare occurrence, and you'll only really encounter it in northern North America and northern Eurasia during the winter months. Now, let's look at the main visibility reducers. Apart from very small water droplets, visibility can be reduced by solid particles or by precipitation. First up is smoke. Smoke consists of solid particles produced by combustion. The conditions will be worse under stable conditions — that means subsiding air, where the air is sinking and not mixing vertically. Smoke can cause widespread reductions in visibility; a classic example is forest fires in Indonesia. The actual reduction in visibility depends on three things: the rate of production of the smoke, the rate of dispersal by wind, and the distance from the smoke source. There's an important knock-on effect here: those smoke particles provide ample hygroscopic nuclei — that means particles that attract water vapour — for vapour to condense onto. This increases the severity of radiation fog, because you have more condensation nuclei available. Next is dust. A dust particle is defined as being less than 0.08 millimetres in diameter. Because dust is so light, it can be carried high into the atmosphere. The surface wind speed is likely to exceed 15 knots, and as the wind speed increases, so does the height to which the dust will rise. There's a figure in your materials showing a dust storm rising to 11,000 feet, 75 miles south-southeast of Damascus. Dust storms mainly occur during daylight hours due to the diurnal variation of wind — that's the daily cycle where wind speeds tend to increase during the day. But here's the tricky part: because the dust particles are very small, they may stay in suspension, and visibility might not improve for a day or so. Examples of such dust storms are the Khamsin and the Haboobs, which we'll cover in more detail later. Now let's talk about sand. Sand consists of particles between 0.08 and 0.3 millimetres in diameter. Because sand particles are heavier, the wind speed needs to be 20 knots or more to lift them. And because of their greater weight, sand particles will only be carried a few feet above the surface. Again, this is more of a daylight event due to the diurnal variation of wind. For both dust storms and sandstorms, the key figure you need to remember is this: visibility is likely to be reduced to less than 1000 metres. That's a significant reduction for aviation operations. Finally, we have precipitation as a visibility reducer. The reductions in visibility caused by precipitation have already been covered in Chapter 13, Cloud Formation and Precipitation. To recap briefly: different types of precipitation — rain, snow, drizzle — each affect visibility in their own way, but the detailed mechanisms are in that earlier chapter.