The process by which water is transformed from liquid to vapour or gas is known as evaporation. The water vapour suspended in the earth’s atmosphere is the result of evaporation. Water requires a lot of heat to change into water vapour. Approximately 600 calories of energy is required to convert one gram of water into water vapour without any change in the temperature. Heat stored in the water vapour is known as latent heat. Rate of evaporation keeps on changing from place to place and from time to time. Most of the evaporation takes place from the oceans, rivers, lakes, ice-lds, and glaciers which together comprise nearby 75% of the earth’s surface. The surface of the ocean is the greatest source of atmospheric moisture. About 88% of the water which evaporates into the air comes from oceans lying between 60° North and South latitudes. In addition, evaporation also takes place from the moist ground, transpiration from plants and animal respiration.
The amount or rate of evaporation increases with increase in temperature, dryness and movement of the air and decreases with the increase in cloud cover.
Humidity is a general term which refers to the amount of water vapour present in the air. At any specitemperature, the quantity of moisture that can be held by air has a dete limit. This limit is known as saturation point and the air at this point is called saturated air. The capacity of air to absorb water vapour increases with the increase in temperature. For example, at temperature of 10° C, one cubic metre of air can hold 11.4 grams of water vapour. If the temperature of this air is increased to 21° C, it can hold 22.2 grams of water vapour.
Kinds of Humidity
Humidity in the atmosphere is expressed in following three forms:
- Absolute Humidity- The weight of actual amount of water vapour present in a unit volume of air is called the absolute humidity. It is usually expressed as grams per cubic metre of air.
- Specific Humidity- It is the weight of water vapour per unit weight of air. It is expressed as the proportion of the mass of water vapour to the total mass of air. Since it is measured in units of weight (usually grams per kilogram), specihumidity is not aected by changes in pressure or temperature.
- Relative Humidity. It is the ratio of water vapour present in the air at a particular temperature to the amount of water vapour required to saturate the same air at the same temperature. It is always expressed in percentages.
Relative humidity is derived by the following simple formula:
Amount water vapour actually percent in the air at a particcular temperture
(R.H) = ×100
Amount water vapour required to saturate the same air at the same temperature
Relative humidity increases with the increase in water vapour in the air and decreases with the increase in temperature.
The process of change of state from gaseous to liquid or solid is known as condensation. When the moist air is cooled, its capacity to hold water vapour is exceeded by the actual water vapour present in it. The temperature at which saturation occurs (i.e., relative humidity becomes 100%) or water vapour begins to change into water is known as dew point. In free air, condensation results from cooling around very small particles termed as condensation nuclei. Particles of dust, smoke and salt from the ocean are particularly good nuclei because they absorb water. These particles are termed hygroscopic (water-seeking) nuclei.
Condensation takes place under the following four conditions:
- When the temperature of the air falls below dew point but its volume remains constant.
- When the volume of the air is increased without any addition of heat.
- When a joint change in temperature and volume reduces the moisture-holding capacity of the air below its existing moisture content.
- When additional moisture is added to air by evaporation.
Forms of Condensation
Dew, frost, fog, mist, clouds, precipitation etc. are caused due to condensation and they are known as forms of condensation.
Earth’s surface is heated during day time and it cools down at night. Sometimes the cooling is so much that the temperature of the air touching earth’s surface falls below dew point. Water vapour present in the air condenses and is deposited in the form of droplets on cooler surface of solid objects such as stones, grass blades and plant leaves. This is known as dew. Long nights, clear sky, calm air, high relative and humidity provide favourable conditions for dew.
When the dew point is below freezing point, the water vapour will not condense into water droplets but will accumulate on the earth’s surface in the form of small white ice particles. The formation of these ice particles over a large area is called frost or white frost. Frost is formed when the temperature of the air falls rapidly so that the water vapour present in the air is directly turned into solid particles without turning into liquid stage. The conditions for the formation of white frost are the same as those for the formation of dew, except that the air temperature must be at or below freezing point.
Fog and Mist
Fog is a sort of cloud with its base at or near the ground. It is formed when the temperature of an air mass containing large amount of water vapour falls all of a sudden and the condensation takes place around ne dust and smoke particles. The atmosphere appears smoky and visibility is poor (oen less than one kilometre). It is generally formed during winter nights and disappears after sunrise. Normally three types of fog are recognized which are known as radiation fog, advection fog and frontal or precipitation fog. The only dierence between the mist and fog is that mist contains more moisture than the fog. In mist each nuclei contains a thicker layer of moisture.
In urban and industrial centres smoke provides plenty of nuclei which help the formation of fog and mist. Such a condition when fog is mixed with smoke, is described as smog.
It is formed by water particles that have condensed in the atmosphere and the visibility varies between 1 and 2 km.