If ice water is poured into a glass, a mist will form on the outside of the glass.  This is because the water vapor in the air becomes chilled by contact with the glass and condenses.  Often leaves and grass and sidewalks are so cold that the water vapor in the atmosphere condenses on them, and we say a heavy dew has formed.  If the temperature of the air falls to the freezing point while the dew is forming, the vapor is frozen and frost is seen instead of dew.

The daily evaporation of moisture into the atmosphere keeps the atmosphere more or less full of water vapor; but the atmosphere can hold only a definite amount of vapor at a given temperature, and as soon as it contains the maximum amount for that temperature, further evaporation ceases.  If clothes are hung out on a damp, murky day they do not dry, because the air contains all the moisture it can hold, and the moisture in the clothes has no chance to evaporate.  When the air contains all the moisture it can hold, it is said to be saturated, and if a slight fall in temperature occurs when the air is saturated, condensation immediately begins in the form of rain, snow, or fog.  If, however, the air is not saturated, a fall in temperature may occur without producing precipitation.  The temperature at which air is saturated and condensation begins is called the dew point.

28.  How Chills are Caused.  The discomfort we feel in an overcrowded room is partly due to an excess of moisture in the air, resulting from the breathing and perspiration of many persons.  The air soon becomes saturated with vapor and cannot take away the perspiration from our bodies, and our clothing becomes moist and our skin tender.  When we leave the crowded “tea” or lecture and pass into the colder, drier, outside air, clothes and skin give up their load of moisture through sudden evaporation.  But evaporation requires heat, and this heat is taken from our bodies, and a chill results.

Proper ventilation would eliminate much of the physical danger of social events; fresh, dry air should be constantly admitted to crowded rooms in order to replace the air saturated by the breath and perspiration of the occupants.

29.  Weather Forecasts.  When the air is near the saturation point, the weather is oppressive and is said to be very humid.  For comfort and health, the air should be about two thirds saturated.  The presence of some water vapor in the air is absolutely necessary to animal and plant life.  In desert regions where vapor is scarce the air is so dry that throat trouble accompanied by disagreeable tickling is prevalent; fallen leaves become so dry that they crumble to dust; plants lose their freshness and beauty.

The likelihood of rain or frost is often determined by temperature and humidity.  If the air is near saturation and the temperature is falling, it is safe to predict bad weather, because the fall of temperature will probably cause rapid condensation, and hence rain.  If, however, the air is not near the saturation point, a fall in temperature will not necessarily produce bad weather.