Heat Science: Hygrometry

Water vapor in the atmosphere

The amount of water vapor present in the atmosphere may be considered to be exceedingly small in comparison with amount of oxygen, nitrogen, etc. present it. Yet the importance of the presence of water vapor in the atmosphere can not be ignored because it plays a very important part in influencing the weather condition at a given place and at a particular time. The source of supply of this water vapor to the atmosphere lies in the rivers, oceans, lakes, snow-mountains, damp soils, etc.

Hygrometry is that part of physics which deals with the measurement of the amount of water vapor present in a given volume of air. Condensation of this aqueous vapor present in the atmosphere is responsible for various phenomena like formation cloudy fogs, rains, snows, hails, etc.

Dew-point

The capacity of the air to hold water vapor is limited and controlled by temperature only. If the air in any place and time contains the maximum amount of vapor that it can hold it is said to be saturated, otherwise it is unsaturated.

Water vapor usually remains in an unsaturated state near the earth. When temperature falls, unsaturated vapor gradually approaches saturation and finally at a particular temperature, it reaches saturation. If the temperature falls further below, saturated vapor condenses on cold surface in the form of minute droplets of water, known as dew.

Dew-point is the temperature at which water vapor actually present in the air just reaches saturation. We may, therefore, define dew-point as that temperature at which the pressure of water vapor actually present in the atmosphere is equal to the saturation pressure of water vapor corresponding of dew-point. Knowledge of dew-point enables us to find out the pressure of water vapor actually present in the atmosphere. For example, dew-point today is 20°C at a particular place at the time of observation. From Regnault’s chart, it is found that the saturation pressure of water-vapor at 20°C is 17 mm. Hence today at the particular place, the pressure of water vapor actually present in the atmosphere is 17 mm.

Sensation of dampness or dryness

The feeling of dampness or dryness depends upon the factor relative humidity. For example, two rooms having same temperature but of different relative humidity will cause a difference in the sensation of dampness and dryness. The room having greater relative humidity will give a sensation of dampness more than that in the other room. The dampness or dryness of the air is judged by the rate at which evaporation goes on and this depends upon how far the air is from the saturation state. i.e., how much more vapor it can take up and does not depend only on the actual amount of water vapor in the air. A low relative humidity will thus encourage evaporation and therefore, the air will appear dry and high value of it will discourage evaporation and the air will appear damp. This is why wet clothes are found to dry up more quickly if relative humidity of the atmosphere is low.

If the water vapor in the air is very close to saturation we can not perspire freely as the air now contains nearly as much moisture as it can possibly have and thus we feel uncomfortable. Conversely, we can perspire freely if the water vapor is not close to the saturation value and we are correspondingly comfortable.

We can now explain the reason why a hot day at Cox’s Bazaar being a place on the sea-cost usually has got sufficient amount of water vapor in the atmosphere while Dhaka has a lesser amount.

Importance of determining relative humidity

The hygrometric state of the air or its relative humidity is of great service in weather-forecasting. Dew-point predicts minimum temperature at night. Information regarding relative humidity is necessary for health and comfort can be produced artificially. The humidity of the air in refrigerators storing meat must not exceed a definite value, otherwise the meat deteriorates; artificial seasoning of timber takes place under pre-arranged value of air humidity. Tobacco deteriorates unless the air in the store-room has a specified humidity.

Humidity control

In many industrial and laboratory operations, it is necessary or desirable to keep the humidity between certain limits. Addition of moisture, when required, is usually accomplished by using sprays of water or jets of steam. Removal or moisture from incoming air is achieved either by cooling the air by passing it over pipes of brine fed from a refrigerator or by using absorbing agents more particularly silica gel, which can be freed from its moisture subsequently by strong heating. Often the most convenient method of keeping the relative humidity at or near a given figure is by first reducing the moisture content to a safe level and then controlling the addition of moisture subsequently from a spray by the us of a air hygrometer controller in which the lever operated by the hank of hair opens or closes a valve. If, however, the temperature of the incoming air is to be controlled in any case, it is sufficient first to cool the air to a predetermine temperature, at which moisture is added if necessary to saturate it and then to drew it off and warm it to its final temperature. The first operation fixes the dew-point of the air and its final state is, therefore, determined.