Joanne Wilson, age 13, of Asbury Park, N.J., for her question:
HOW IS RAINFALL MEASURED?
An instrument called a rain gauge measures rain that falls in a certain location or region during a certain period of time.
An extremely simple rain gauge is used by the National Weather Service. It is shaped like a narrow cylinder and has a removable cover. Inside the cylinder is a long, narrow tube where the rainfall is actually measured.
At the top of the rain gauge's tube is a funnel. The rain falls into the funnel and flows into the tube. Because the funnel has an area 10 times that of the tube, an inch of rain that falls into the funnel would fill 10 inches of the tube. The rain in the tube is measured by a special ruler. With this ruler, a depth of 10 inches gives a reading of one inch of rainfall.
Some rain gauges use the metric system to measure rainfall in millimeters.
In case the rainfall is so heavy that the water in the tube overflows, this extra rain goes into a space between the outside of the cylinder and the tube. After the rain in the tube is measured, it is poured out and the extra rain is placed in the tube and measured.
A rain gauge is usually placed on the level ground quite a distance from buildings and trees to insure accuracy.
Some rain gauges can be used to measure snowfall. Unfortunately, these do not provide very accurate results. Snowfall is difficult to measure because sometimes the flakes are light and fluffy and other times they are heavy and slushy.
Sometimes snow that falls into a rain gauge is melted and measured as rain. Other times it is weighed.
Meteorologists must run special calculations on snowfall to gauge the amount of moisture. Usually six inches of moist snow will equal about one inch of rain, while a 30 inch layer of dry snow will also equal one inch.
Meteorologists also use several kinds of instruments called hygrometers to measure the amount of water vapor in the air. One type, called an apsychrometer, is made up of two thermometers set on a support so they can be whirled in the air.
One thermometer, called a wet bulb, is covered with a wet cloth. When the two thermometers are whirled in the air for a few minutes, water evaporates from the wet cloth and cools the wet bulb thermometer.
The amount of evaporation depends on the amount of moisture in the air. The difference between the temperatures of the wet bulb and dry bulb thermometers measures the moisture.