UNIT 5—LESSON 2
OVERVIEW Identify the
characteristics of lithometeors (haze, smoke, dust, sand, and dust
devils). OUTLINE Haze Smoke Dust Sand Dust devils LITHOMETEORS Lithometeors comprise a class of atmospheric
Lithometeors comprise a class of atmosphericphenomena of which dry haze and smoke are the most common examples. In contrast to hydro-meteors, which consist largely of water, lithometeors are composed of solid dust or sand particles, or the ashy products of combustion.
Learning Objective: Identify thecharacteristics of lithometeors (haze, smoke, dust, sand, and dust devils).
Haze is composed of suspended dust or salt particles which are so small that they cannot be individually felt or seen by the unaided eye. They reduce visibility and lend a characteristic opales-cent appearance to the air. Haze resembles a uniform veil over the landscape that subdues all colors. This veil has a bluish tinge when viewed against a dark background and a dirty yellow or orange tinge hwen viewed against a bright background.
Differences in air temperature may cause a shimmering veil over the landscape called optical haze.
Smoke is fine ash particles suspended in theatmosphere. When smoke is present, the disk of the Sun at sunrise and sunset appears red, and during the daytime has a reddish tinge. Smoke at a distance, such as from forest fires, usually has a light grayish or bluish color and is evenly distributed in the upper air.
Dust is finely divided solid matter uniformlydistributed in the air. It imparts a tannish or grayish hue to distant objects. The Sun’s disk is pale and colorless or has a yellow tinge during the day.
Blowing dust consists of dust raised by thewind to moderate heights above the ground and restricting horizontal visibility to less than 7 miles. When visibility is reduced to less than five-eighths of a mile but not less than five-sixteenths of a mile, it is classified as a dust storm and, if less than five-sixteenths of a mile, as a severe dust storm.
Fine particles of sand picked up from the sur-faceby the wind and blown about in clouds or sheets constitute a troublesome lithometeor in some regions.
Blowing sand consists of sand raised bythe wind to moderate heights above the ground which reduces horizontal visibility to less than 7 miles. When the visibility is reduced to less than five-eighths of a mile but not less than five-sixteenths of a mile, it is classed as a sandstorm and, if less than five-sixteenths of a mile, as a severe sandstorm.
Dust devils, or whirling, dust-laden air, are caused by intense solar radiation which sets up a steep lapse rate near the ground. They are best developed on calm, hot, clear afternoons and in desert regions. As the intense surface heating sets up a steep lapse rate, a small circulation is formed when the surrounding air rushes into fill the area of the rising warm air. This warm ascending air carries dust, sand, leaves, and other small material to a height of a few hundred feet.