Debra Arnold, age 13, of Scarborough, Ontario, Canada, for her question:
WHAT IS HUMIDITY?
Summer is the time almost everyone thinks about humidity. It can affect our comfrot and our health. A person feels uncomfortable and rather sticky when the humidity is high because perspiration on the skin doesn't evaporate easily. But it's just as uncomfortable when there is too little humidity in the air, and overly dry air can also be bad for your health.
Humidity describes the amount of water vapor in the air. The humidity is said to be high when the air contains a large amount of moisture, and when there is just a little moisture in the air, the humidity is low.
The air is called saturated when it holds as much moisture as possible at a certain temperature and pressure. The air is also called at its dew point when this happens.
The warmer the air, the more moisture it is able to hold. At normal atmospheric pressure, for example, an increase in temperature of about 20 degrees Fahrenheit doubles the air's moisture capacity. Therefore, saturated air at 65 degrees Fahrenheit holds twice as much moisture as saturated air at 45 degrees.
The lower air over an ocean is almost always saturated. Far inland, however, and over deserts, the air may be very dry. Usually air is not completely filled with vapor unless it is foggy, raining or snowing.
The amount of water vapor in the air compared to the amount needed for saturation is called the relative humidity. If the air holds only half the amount of vapor that it can hold when saturated, the relative humidity is said to be 50 percent.
A hygrometer is one of the instruments used to determine the amount of moisture in the air so that a humidity reading can be given. A common hygrometer uses two thermometers. One is called a dry bulb thermometer, and it gives the air temperature. The other is wrapped in a piece of wet cloth and is called a wet bulb thermometer. The wet bulb instrument gives a lower reading because evaporation of the water in the cloth cools it. More water evaporates when the humidity is low than when it is high. Therefore, the lower the humidity, the lower the wet bulb thermometer will read.
A special chart has been made for use with a hygrometer so that a determination may be made as to the relative humidity. The equipment is allowed to stand in a breeze away from direct sunlight for at least 15 minutes.
If the dry bulb thermometer reads 70 degrees Fahrenheit and the wet bulb reads 60 degrees, the relative humidity is 55 percent.