Maureen Konieczny, age 12, of Staten Island, N.Y., for her question:
HOW IS RAINFALL MEASURED?
An instrument called a rain gauge is used to measure the 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 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 then flows into the tube. The mouth of the funnel has an area 10 times that of 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. When this is the case, the gauge measures the 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. The total amount of rainfall would then equal the amount of these two measurements.
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, however, these gauges do not provide very accurate measurements. Snowfall is difficult to measure accurately because sometimes the flakes are light and fluffy and other times they are heavy and slushy.
Sometimes the snow that falls into a rain gauge is melted and measured as rain. Other times it is weighed.
Because some snow is heavier than other snow, meteorologists must do some calculating. Usually six inches of moist snow will equal about one inch of rain, while a 30 inch layer of dry snow will also equal about one inch of rain.
Meteorologists also use several kinds of instruments called hygrometers to measure the amount of water vapor in the air. One type is called a psychrometer that 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 in the air.