Condensation problems(PHY)

The crucial point is that dry air can hold a certain amount of water vapour in it. This is similar to a solution that we can dissolve a certain amount of sugar in water until the water is satuated. Air, therefore, can hold water vapour until the air becomes satuated so that no more water vapour can be absorbed by the air. The air becomes "satuated air".

The maximum amount of water vapour that dry air can hold depends very much on temperature. The higher the air temperature, the more water vapour can be held by the air. A very common quantity to describe the degree of satuation of air is "Relative Humidity"(RH), which is expressed in percentage of the maximum mass water vapour to which the air can hold at that temperature. Hence, a 100% RH means the air is satuated. A 0% RH means the air is completely dry.

With the above understanding, the answers to your questions are:
1. The water vapour is "dissolved" in air. That is to say, it is held by the air. The temperature at which the "dissolved" water vapour to be condensed is called the "dew point". In other words, if we cool down the air to its dew point, then the air would become satuated and the water vapour in it begins to come out. That is, dew point is the temperature at which the RH of air reaches 100%.

2. The "dew point" is also temperature dependent. Inside a refrigerator, the dew point may be below the temperature of the refrigerator, hence no water vapour would condense. In other words, the RH inside a refrigerator may not necessarily be 100%.

2010-02-28 00:52:46 補充：
3. As said above, when hot air touches cold air. The hot air would be cooled down by the cold air. When the temperature of the hot air drops to its dew point, the air becomes satuated (i.e. reaching 100% RH), water vapour in it then condenses out.... (contd)

2010-02-28 00:54:37 補充：
(contd).....The mist anf fog formed in spring season in Hong Kong is because of this phenomenon. Cold air from the north meets warm air from the south. The water vapour in the cold air thus form tiny water droplets in suspension in the air, which is the mist and fog we experienced......

2010-02-28 00:54:59 補充：
(contd)..... You may be aware that the RH during such time is almost reaching 100%.

2010-02-28 13:49:56 補充：
Sorry...the word "saturated" has been spelled wrongly in the passage. It should be "saturated" instead of "satuated".