WEATHER AND OBSTRUCTIONS TO VISION

Q33. Where can shipboard observers obtain visibility range information at sea? Q34. When is sector visibility reported? Q35. When is differing level visibility reported? WEATHER AND OBSTRUCTIONS TO VISION LEARNING OBJECTIVES: Identify six types of lithometeors. Compare the condensa- tion/sublimation and precipitation forms of hydrometeors. Explain wind-blown forms of hydrometeors. Describe two types of electrometeors. Define and list four types of photometeors. The occurrence of weather and the presence of obstructions to visibility are directly related to sky condition and the visibility. Observing the type of weather occurring and the presence of any obstructions to visibility is usually the third task undertaken in an observation. A pilot may use this information to determine the impact of the conditions at a station on the aircraft being flown. In this discussion, we use the term weather to refer to any particles suspended in or precipitating from the atmosphere, or to the process that causes these particles to precipitate from the atmosphere. Observable weather elements may be broken down into four groups: lithometeors, hydrometeors, electrometeors, and photometeors. LITHOMETEORS A lithometeor is any dry particle suspended in or falling from the atmosphere. The particles are usually formed on earth’s surface and then are carried aloft by either wind or thermal currents. Haze, smoke, dust, dust-devils, ash, and sand are all lithometeors. Haze Haze is composed of suspended dust, plant pollen, or salt particles that are so small that they cannot be seen by the unaided eye. It is opalescent, reducing visibility. Haze typically produces a bluish tinge when viewed against a dark background. It produces a dirty yellow or orange tinge when viewed against a brighter background because of the scattering of light. When haze is present and the sun is well above the horizon, its light may have a silvery tinge. Haze particles are hygroscopic—they attract moisture. Because they attract moisture, they are good condensation nuclei. When conditions are favorable, haze may attract sufficient moisture to thicken into fog as the sun sets and the temperature drops. Smoke Smoke is composed of fine ash particles and other by-products of combustion. When concentrated at its source, smoke may appear white to bluish-black, or yellow to brown, depending on its composition and the amount of water vapor present. After it is dispersed in the atmosphere, smoke is distinguished from haze by its characteristic reddish tinge, especially near the horizon at sunrise and sunset. Dust Dust is composed of fine solid matter uniformly distributed in the air. It typically imparts a tan or gray hue to distant objects. The sun’s disk may appear pale and colorless, or may have a yellow tinge when viewed through dust. Although dust and haze appear similar, when the visibility is less than 7 miles, dust may be differentiated from haze or fog by the low relative humidity associated with dusty conditions. In certain areas of the world, suspended dust may reduce visibility to less than a mile. Normally, the lower visibility associated with dust is limited to blowing dust—dust picked up and carried by the wind. The term dust storm usually refers to blowing dust reducing visibility to 5/16 to 5/8 of a mile, while the term heavy or severe dust storm is reserved for use with blowing dust that restricts visibility to less than 5/16 mile. Dust/Sand Whirl Dust/sand whirls or dust devils, are rotating columns of dust or sand-laden air, caused by intense solar radiation. They are best developed on calm, hot, clear afternoons and in desert regions. Warm, ascending air in a dust devil may carry leaves and other small debris to a height of a few feet or a few hundred feet. Ash The phenomenon called ash in a surface meteorological observation usually refers to the heavier volcanic ash particles falling from a volcanic cloud. It may also be used to identify heavier solid particles precipitating and falling from an industrial smoke 1-30