TEMPERATURE INVERSION

increase of air temperature with altitude. Such an increase represents a reversal of the normal temperature condition of the troposphere (the region of the atmosphere in contact with the Earth's surface), where temperature usually decreases with height. Inversions play an important role in determining cloud forms, precipitation, and visibility. An inversion acts as a lid on the vertical movement of air in the layers below. As a consequence, convection produced by heating the air from below is limited to levels beneath the inversion. Diffusion of dust, smoke, and other air pollutants is likewise limited. In regions where a pronounced inversion is present at a low level, convective clouds cannot grow high enough to result in showers and, at the same time, visibility may be greatly reduced below the inversion, even in the absence of clouds, by the accumulation of dust and smoke particles. Because the air near the base of the inversion is cool, fog is frequently present there. Another important influence of inversions is reflected in the diurnal range in temperature. The principal heating of the air during the day is produced by contact with the ground, the temperature of which is raised by the Sun's radiation to which the air is transparent. The solar radiation is absorbed by the ground, and the heat is communicated to the air by conduction and convection. Since the inversion base represents the upper limit to which heat is carried by convection, only a shallow layer of air will be heated if the inversion is low and large, and the rise in temperature will be great. Inversions are commonly classified into four basic types: (1) ground, (2) turbulence, (3) subsidence, and (4) frontal. A ground inversion develops when the air is cooled by contact with a colder surface until it becomes cooler than the overlying air; this occurs most often on clear nights, when the ground cools off rapidly by radiation. If the temperature of the surface air drops below its dew point, fog may result. Topography greatly affects the magnitude of ground inversions. If the land is rolling or hilly, the cold air formed on the higher land surfaces tends to drain into the hollows, producing a larger and thicker inversion above low ground and little or none above the higher points. A turbulence inversion often forms when quiet air overlies turbulent air. Within the turbulent layer, vertical mixing carries heat downward and cools the upper part of the layer. The unmixed air above is not cooled and eventually is warmer than the air below; an inversion then exists. A subsidence inversion develops when a layer of air descends bodily. The layer is compressed by the resulting increase in atmospheric pressure, so that its upper portion descends farther than does its lower part. As a consequence, the heat of compression will be greater for the upper part than for the lower, and the lapse rate (q.v.) of temperature will be reduced. If the air mass sinks low enough, its upper part becomes hotter than its lower part and a temperature inversion is produced. Subsidence inversions are common over the northern continents in winter and over the subtropical oceans; these regions have generally subsiding air because they are dominated by large high-pressure centres. A frontal inversion occurs when a cold air mass undercuts a warm air mass and lifts it aloft; the front between the two air masses then has warm air above it and cold air below. This kind of inversion has considerable slope, whereas other inversions are nearly horizontal. In addition, humidity may be high, and clouds may be present immediately above it.