Moisture in the air

There is always some moisture in the air-even the seemingly driest air has moisture in it. You have only to put some ice in a tumbler on a dry to find this out. How does moisture get into the air? Let us find out.

Evaporation of Water

Take some water in a beaker and heat it. It soon boils and begins chang- ing into steam. The steam vanishes into the air. Put equal amounts of water in several watch- glasses. Cover one and place it in the sun. Keep one in the room, one in the open where it windy. Put an equal amount of water in a saucer and place it out with the others. Leave them for some time. Then note what has happened.

There has been a loss of water in all the glasses except in the closed one. The greatest loss occurs when the water has more surface ex-posed in warm and windily place.

This shows that water is con- tinously being changed into vapour at all temperatures. The rate of change into vapour increases when:

  1. the temperature increases,
  2. it is windy, and
  3. More surfaces are exposed to air.

The process by which water changes into vapour or gas is called


Water is continually evaporating from the oceans, lakes, rivers, and other water bodies, into the atmosphere.

Condensation of water vapour

When you put ice in a tumbler, its sides get cooled. Air touching these sides is also cooled and drops its moistures as droplets of water sticking on the outer sides of the tumbler.

Thus, change of water vapour from a gas into liquid droplets is known as Condensation. This takes place when the air is sufficiently cooled. When the temperature of the moist air goes down, the air become incapable of condenses into tiny particles of water. The condensation of water-vapour can be in from of the clouds, fog, dew, hail, rain or snow.

Warm air can hold more moisture water vapour is cooled; it slowly loses its capacity to hold moisture in it. At a certain temperature, Cooling marks unsaturated air saturated. Further cooling result in the air dropping its moisture abouve the limit of saturation at that temperature. On the other hand, heating will make it unsaturated i.e., it can now take in more water vapour.

Dew Point

The temperature at which air just begins to shed moisture is called its dew point. Dew point is variable, depending upon the amount of water vapour held in the air.

How does wind get cooled? You know when winds are forced to rise by intervening mountain or when they blow from a lower to higher latitude, they bring rain, snow, etc. Now tell what will happen if a moist wind descends downhill or blows from a higher latitude to a lower one?

The amount of water vapour in the air at any place does not always remain the same. The air may hold more water vapour on a warm day than on a cold one. Again, air abouve warm places near the seas will have more water vapour in it than air abouve cold dry land areas. Thus the air amount of water vapour or humidity of the air changes from day to day and from place to place.







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