Read the given paragraph carefully and answer the following question.

The average rate of temperature decrease upward in the troposphere is about 6°C per km, extending to the tropopause. This vertical gradient of temperature is commonly referred to as normal lapse rate. Certain processes in the lower troposphere create an actual increase in temperature with an increase in altitude, that is, a temperature inversion. Temperature inversions may be produced in five ways (1) When the earth's surface loses more heat by radiation than it gains by any of the energy transfer processes, as on a clear night or at high latitudes in winter, it cools and consequently lowers the temperature of the adjacent layer of air. Inversions due to radiational cooling (radiation inversions) develop best in calm air under clear skies over flat terrain. (2) Cold and dense air from hilltops and slopes tends to flow downslope to collect in valley bottoms, creating an inverted lapse rate in the free air over the valley floor. Air drainage inversions are frequently associated with spring frosts in middle latitudes, and for this reason fruit growers prefer gentle slopes to valley bottoms for orchard sites. (3) When two air masses with different temperature characteristics come together, the colder air, being more dense, tends to push underneath the warmer air and replace it. The boundary zones along which two air masses meet are called fronts, and the inverted lapse rate which results is a frontal inversion. Frontal inversions are not confined to the lower layers of the troposphere. They may form at upper levels wherever cold air underruns warm air or warm air advances above cold air. (4) Advection of warm air over a cold surface creates an inversion in the lower layers of the air mass as the warm air is cooled by conduction. (5) The subsidence inversion forms in an air mass when a large body of air subsides and spreads out above a lower layer. In the process the air heats dynamically more in the upper portion than at its base. Inversions of this type may develop at considerable altitudes.