Heat Science: Hygrometry (part-2)

Consideration of water vapor

When moist air is cooled at a certain temperature it becomes saturate and a further lowering of temperature causes condensation of vapor. The cooling of atmosphere air may be affected for one or more of the following causes: (i) due to radiation of heat or due to contact with cold bodies, (ii) due to the mixing of cold and warm air, (iii) due to adiabatic expansion caused by sudden decrease of pressure.

The first process is responsible for the formation of dew, fog, mist, etc. In  the second process condensation generally takes place because of the fact that when two equal masses of air initially saturated at two different temperatures are mixed, they will acquire a mean temperature and will be supersaturated causing a condensation. If they are not initially saturated there may be condensation or no condensation or even evaporation. The occasional formation and disappearance of certain kinds of clouds may be accounted for in this way. The third process becomes active on a large scale and produces cloud and rain. Ascending current of moist air suffers a decrease of pressure as it ascends and this causes adiabatic expansion of a large mass of air and consequent fall in temperature. If the temperature falls below the dew-point, there is condensation. From simple thermodynamic considerations, it is possible to calculate the lowering of temperature of the moist air due to adiabatic expansion. 

Atmospheric phenomena arising out of condensation

The condensation of water vapor in the air gives rise to the formation of cloud, mist, snow, hail, dew and frost. Brief descriptions of some of these phenomena are given below.

Clouds and mist

The production of mist or cloud is one of the results produced by the condensation of moisture in the air-a cloud simply being mist at high altitude. In order for such small drops of water to be formed some dust particles or electric charge must be present upon which the water vapor may condense. The drops must be extremely small since they do not fall to the earth- the viscous nature of the atmosphere is slight, but nevertheless sufficient to prevent any rapid motion of these particles. There is tendency, however, for these drops to coalesce and when they are sufficiently large, rain is precipitated.

Snow and hail

Snow is probably a consequence of the direct passage of water vapor into the solid state. Hail is most likely due to the freezing which takes place when raindrops pass through strata of air where the prevailing temperature is below 0°C. By cutting a hailstone in halves it has been shown that such a stone may consist of several distinct layers, proving that moisture has condensed upon the original piece of hail several times and after each condensation, freezing has occurred.

Dew and frost

The small drops of water which are seen clinging to stones, grass, leaves, etc. in the early hours of a summer morning and at other times are referred to as dew. These drops have not been produced in the regions remote from the surface of the earth, but in the immediate vicinity of the earth’s surface. Dew is generally noticed when clouds have been absence. The absence of cloud permits heat to be radiated into space and this loss of heat is followed by a local of the temperature. Thus lowering is much more marked when stones, etc. are present, for these are good radiators and if the temperature is lowered below the dew-point the appearance of water-drops on the cooled object is the result.

When there is no wind the layers of the air near to the objects from which heat is being radiated are cooled more rapidly, so that the production of dew is favored. The conditions favorable for the formation of dew were first stated by Wells (1812) in a celebrated “Essay on dew” and are follows:

(i)                 there should be a cloudless sky,

(ii)               there should be no wind,

(iii)             the relative humidity of the atmosphere should be high. In 1886 Aitken extended the above theory. He maintained that there are two types of dew:

(i)                 that which depends on the water vapor present in the atmosphere,

(ii)               that which results from the water given off by the leaves of plants. This is emitted as vapor and under normal conditions passes at once into the unsaturated air. When the air near the leaves is saturated with water vapor, then that which comes from the leaves appears as dew on them.

If the dew-point is below 0°C and the temperature still lower the water is deposited as hoar-frost. When the temperature is below 0°C, but the air is not saturated with moisture, then the prevailing conditions are known as a black-frost.