What is an acre-foot?

One acre-foot is equal to 43,560 gallons (164,875 liters) of water, which is what it would take to bury an acre of land in a foot of water. The term is usually used to measure rainfall runoff, reservoir capacity, and irrigation.

How is sea water salinity measured?

The amount of salts in sea water is important because it affects ocean currents, which, in turn, affect the world's climate. Sea water contains a variety of dissolved elements, including chlorine, sodium, magnesium, calcium, sulfur, and potassium. In the past, measurements of salinity were taken simply by going out onto the ocean, filling a bucket with sea water, and testing the salt levels by measuring electrical conductivity (the more salts, the quicker electricity flows through the water because there are more ions present). There are also techniques to measure chlorine or other dissolved elements.

More recently, sophisticated equipment has become available for measuring ocean salts remotely. Low-frequency radiometers mounted on C-130 aircraft can scan the ocean during flights, covering over 38 square miles (100 square kilometers) every hour. The European Space Agency plans to launch its Soil Moisture and Ocean Salinity (SMOS) satellite in 2009 to take readings from space using a two-dimensional interferometric radiometer, a new technology that captures images based on microwave radiation emitted at a frequency of 1.4 gigahertz (GHz).

How is wind speed measured?

Wind speed is measured with a device called an anemometer, which was an invention of English physicist Robert Hooke (1635-1703). The most commonly used type is the rotating cup anemometer, which uses three or four small cups that spin around a central pole. Modern anemometers of this sort work using electricity and magnets. As the cups spin, a reed switch within the central pole detects each time a magnet in a cup swings by. This sends out an electronic pulse that has been calibrated to calculate wind speed. The data is then transmitted to a weather station.

What are some other types of anemometers?

Besides the rotating cup anemometer, there is the sonic anemometer, swinging-plate (or pressure-plate) anemometer, pressure-tube anemometer, bridled (or windmill) anemometer, and the

An early anemometer designed by John Thomas Romney Robinson in 1846. (photo by Sean Linehan, NOS, NGS, courtesy NOAA)

An early anemometer designed by John Thomas Romney Robinson in 1846. (photo by Sean Linehan, NOS, NGS, courtesy NOAA)

aerovane. Weather stations often use sonic anemometers, which calculate both wind speed and direction. Four ultrasound transducers are set up in a circle, evenly spaced apart, in two pairs placed across from each other. A transducer will send out an ultrasonic signal to the one directly across from it. Winds blowing across this path will cause the signal to travel faster, slower, or change direction, thus indicating wind conditions. Pressure-plate and pressure-tube anemometers work by the fact that wind blowing against a plate or through a tube will exert a measurable pressure. Aerovanes and windmill anemometers can measure both speed and direction. As the blades on these devices spin, it is possible to calculate wind speed, and both will turn into the oncoming wind, which indicates direction.

How is wind direction usually measured?

A wind vane is the common instrument used to discover wind direction. Wind vanes look like windmills mounted on a pole that allows them to rotate toward the direction of the oncoming wind. Historically, wind vanes have often come in decorative models, often with a rooster or some other farm animal mounted on the top. Of course, there are many other ways to discover wind direction, ranging from the primitive (analyzing the direction smoke is blowing or how balloons are moving) to the more sophisticated, such as Doppler sodar (sound radar) and lidar (light radar). Gyroscopes and GPS devices mounted in airplanes can calculate air speed by comparing the indicated speed to the actual distance covered (i.e., the amount of thrust from the airplane's jets or propellers may be slowed or sped up, depending on whether winds are blowing with or against the plane).

What standard unit of measurement is used to indicate wind speed?

In most forecasts in the United States, wind speed is described in miles per hour. (Outside this country, it would be expressed in kilometers per hour; most other scientists also prefer to use the metric system). However, the Federal Aviation Administration, National Weather Service, and other groups that work with air and ocean travel, will use knots (one knot equals 1.15 miles per hour, or 1.85 kilometers per hour). Internationally, wind is also commonly measured in meters per hour. For vertical wind speeds, meteorologists use microbars per second, which indicates pressure change with altitude over time, or centimeters per second.

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