



HOW MOISTURE GETS INTO THE AIR 



31 



a fresh supply is always in contact with the clothes, 

 they will dry rapidly. The sun's rays warm the air 

 and, as you will learn later in this unit, this heat from 

 the sun not only aids evaporation but also is the cause 

 of winds. 



Clothes will not dry on a damp day because the air 

 around the clothes is nearly filled with moisture and 

 unable to take up much more. However, a wind blow- 

 ing fresh air to them, even on a damp day, will greatly 

 aid the evaporation of the water. 



What is the importance of relative humidity and 

 how is it measured? The amount of water vapor in the 

 air varies from day to day. All have experienced those 

 sultry days in midsummer when the clouds hang low 

 and one perspires easily. We commonly speak of such 

 days as "humid." We have also experienced the other 

 extreme of a cold, dry day in win- 

 ter. On a humid day the air-sponge 

 is nearly full ; on a crisp, dry day in 

 winter it contains much less mois- 

 ture. 



As you have already learned, 

 warm air can hold more water va- 

 por than cold air, and further, at 

 any given temperature it can hold 

 only a certain amount. When this 

 amount has been reached, the air is 

 said to be saturated or filled and 

 can hold no more unless the tem- 

 perature rises. When the air at any 

 given temperature is completely 

 saturated it is said to be 100 per 

 cent filled. If it contains only half 

 as much as it can hold at that temperature it is 50 

 per cent filled. 



The amount of water vapor actually present at any 

 temperature compared with the amount which the air 

 can hold at that same temperature is called relative 

 humidity. This is expressed in percentage as explained 

 above. When the amount present is half that which the 



air can hold, the relative 

 humidity is 50 per cent. 



The relative humidity is 

 usually measured with one 

 of the devices shown in Fig- 

 ures 50, 51, and 52. These 

 are slight variations of the 

 wet- and dry-bulb method 

 which you used in your ex- 

 periment. The evaporation 

 from the wick of the wet 

 bulb is controlled by the 

 moisture or dryness of the 

 air about it. The heat 

 needed for this evaporation 



Tycos 



FIG. SO 

 HYGROMETER 



Tycos 



FIG. 51. HYGRODEIK 



is taken from the thermometer, which becomes cooler 

 and as a result reads lower. The difference of tempera- 

 ture between the wet and dry bulbs measures the rate 

 of the evaporation. Tables like the one on page 30 have 

 been prepared to give the relative humidity from the 

 readings of the wet- and dry-bulb thermometers. Fig- 

 ure 50 shows a common form of the wet- and dry- 

 bulb hygrometer which is fastened to the wall. The 

 hygrodeik (Fig. 51) is made with tables printed on 

 it so that the relative humidity may be obtained di- 

 rectly. The psychrometer (Fig. 52) is composed of 

 the wet- and dry-bulb thermometers so constructed 

 that they may be rapidly spun around. 

 This gives a more accurate reading than 

 either of the other instruments. 



When the relative humidity is too low 

 in our homes and buildings, the moisture 

 from the skin and from the linings of 

 the nose and throat tends to dry out. This 

 seems to create a condition favorable for 

 germs of various types and may lead to 

 colds and other diseases. When the rela- 

 tive humidity is between 50 and 65 per 

 cent we have the most healthful condi- 

 tion. 



Buildings in which the relative humid- 

 ity is kept between these points are eas- 

 ier to heat because a room which is dry 

 causes the evaporation of body moisture 

 and a consequent cooling. A room at 68 F. and 60 

 per cent relative humidity is more comfortable than 

 one at 72 F. and 35 per cent relative humidity. When 

 humidity is low, furniture and woodwork dry out 

 and crack. 



Several different types of home humidifiers are on 

 the market for homes heated by steam or hot water. 

 Usually these are some type of water pan which is 

 placed on the radiator so that the water in the pan 

 will be warm enough to evaporate rapidly. For hot 

 air heating systems, water pans are placed in the fur- 

 nace jacket, and as the warm air passes through it 

 may take up some of the water. Large buildings such 

 as theaters, hotels, and hospitals usually have humidi- 

 fying systems built in with the ventilating system. 



How does the electric refrigerator work? One of 

 the experiments taught you that heat aids evaporation. 

 It is a familiar fact that a kettle of water must be 

 placed over a fire to boil. Liquids must have heat to 

 evaporate into gas or vapor, and when they absorb 

 this heat, energy is taken from the surroundings, such 

 as the fire or whatever they are in contact with. If 

 a drop of a liquid which easily evaporates is placed 

 on the hand, the hand will feel cold. When one comes 

 from a bath and stands over a register or near a heater 

 he may feel very cold, because the warm air currents 



