Barometer, a device for measuring atmospheric pressure (the pressure resulting from the weight of the air above a given point). Because a change in atmospheric pressure is a sign of changing weather, this instrument is one of the chief tools of the weather forecaster. A falling" barometer, indicating reduced pressure, shows that a low-pressure mass of air is approaching. Such low-pressure systems generally bring storms. A rising" barometer generally means an approaching high-pressure air mass and fairer weather.
Another use of a barometer is as an altimeter, a device that measures heights. As altitude increases, atmospheric pressure decreases because there is less air to press down. A barometer used as an altimeter has a scale marked in feet or meters above sea level.
Two forms of barometers in common use are the mercury barometer and the aneroid barometer.
The mercury barometer consists of a hollow glass tube, about three feet (0.9 m) long, closed at one end and filled with mercury. The open end is placed in an open vessel of mercury called a cistern. The mercury column in the tube will drop until the weight (pressure) of the air pushing down on the mercury in the cistern is equal to the weight of the mercury remaining in the tube. The higher the atmospheric pressure, the higher the mercury will stand in the tube. Similarly, the lower the atmospheric pressure, the lower the mercury will stand.
A change in the height of the mercury, or barometric column, therefore, indicates a corresponding change in the weight of the atmosphere. These changes may be measured by means of markings on the tube. The average height of the barometric column at sea level is 29.92 inches (760 mm). This height indicates a pressure of one atmosphere, or about 14.7 pounds per square inch (101,300 pascals). (On weather maps, barometric pressure is usually expressed in millibars; one atmosphere is equal to about 1,013 millibars.)
The mercury barometer was invented in 1643 by Evangelista Torricelli, an Italian physicist. It is used mainly in laboratories and weather stations.
A more rugged and easily portable instrument is the aneroid, or dry, barometer. While not as accurate as the mercury type, the aneroid barometer will show much smaller changes in atmospheric pressure.
The aneroid barometer contains a cylindrical container with a flexible top. Air is partially removed from inside the container, and the top, or diaphragm, flexes with each change of atmospheric pressure. Delicate levers transmit these movements to a pointer that moves over a graduated scale.
Aneroid barometers with altitude scales are commonly used for aircraft altimeters. In determining altitude, adjustments must be made to account for changes in air pressure due to the weather. Barometers sold for home use are generally of the aneroid type.
3)Rain Gauge
Rain Gauge, an instrument used to measure precipitation. Rain is measured directly; snow, hail, and sleet must first be melted. Approximately 10 inches of snow equal one inch of rain, but this figure varies with the compactness of the snow.
A standard rain gauge consists of a funnel leading to a measuring tube. The area of the circular top of the funnel is 10 times that of the tube's cross section, so that one inch of water falling on the funnel will rise to 10 inches in the tube. The tube thus multiplies the height of the fallen water by 10, making it possible to measure small amounts of precipitation.
A tipping bucket rain gauge contains a small bucket mounted on a pivot. The bucket is divided into two compartments. When one compartment receives 1100 inch (or, for metric gauges, 0.2 mm) of rain or meltwater, the bucket tips, dumping the water. The other compartment then receives the precipitation. Each time the bucket tips, an electric circuit is closed and a mark is recorded on moving paper. The marks show how much precipitation has fallen and the rate of fall.
A weighing-type rain gauge weighs the precipitation as it falls into a container resting on a scale and records the weight, usually on revolving graph paper.
4)Hygrometer
Hygrometer, a device for determining the humidity of the atmosphere. It can indicate relative humidity (the percentage of moisture in the air), absolute humidity (the amount of moisture), or both. Some crude hygrometers will simply tell whether the air is dry or damp. An important use of hygrometers is in weather forecasting. Hygrometers are also used for monitoring the humidity in laboratories, storage areas, and manufacturing plants where specific levels of humidity must be maintained. Some hygrometers form part of devices called humidistats, which are used to control humidifiers or dehumidifiers for regulating the humidity of the air.
There are several types of hygrometers. One common type contains animal fibers, usually human hair, that stretch when the relative humidity increases and contract when it decreases. The fibers are usually attached to levers that turn the pointer on a dial.
The psychrometer, sometimes called the wet-and-dry bulb hygrometer, consists of two thermometers mounted side by side. One of the thermometers—the dry-bulb thermometer—measures the temperature of the air. The bulb of the other thermometer is covered with a sleeve of muslin or candle-wick. The cloth is wetted and the water is then made to evaporate by blowing air over the cloth with a fan or by whirling the psychrometer in the air. As the water evaporates, the bulb is cooled. In general, the drier the air, the greater is the drop in temperature. The relative humidity is determined by comparing the readings of the two thermometers with published psychrometric tables or charts.
Electrical hygrometers typically measure the electrical resistance of a substance, such as lithium chloride, whose resistance to an electric current varies with the humidity. In the dew-point hygrometer, a smooth, shiny surface is cooled until water vapor in the air begins to condense on it. The humidity is determined by comparing the temperature at which the condensation occurs (the dew point) with the temperature of the air. Chemical hygrometers use a substance such as phosphorus pentoxide to absorb moisture from a given volume of air. The difference in the weight of the substance before and after its exposure to the air indicates how much moisture is present.
5)Thermometer
Thermometer, an instrument for measuring temperature. The measurement is always indirect—that is, it is a measurement not of temperature itself but of the effect of temperature on some physical property of a material used in the thermometer. In response to changes in temperature, for example, solids, liquids, and gases expand and contract; the pressure of a gas that is kept at a constant volume changes; and the electrical resistance of a metal or alloy increases and decreases.a)Fahrenheit,
used in everyday temperature measurement in the United States. On this scale, the freezing point of water is 32 and its boiling point 212. (The zero point was assigned to the temperature of a mixture of ice and salt, which in the early 1800's was the lowest temperature attainable in the laboratory.)
b)Rankine,
used primarily by American engineers. Like the Fahrenheit scale, the Rankine scale has a difference of 180 degrees between the freezing point (491.7 R.) and boiling point (671.7 R.) of water, but the zero point is absolute zero (–459.7 F.), the lowest temperature theoretically possible.
c)Celsius, or Centigrade,
used generally throughout the world and in scientific work. On the Celsius scale, the freezing point of water is O and its boiling point 100. (The common name centigrade derives from the 100-degree difference between the freezing and boiling points of water.)
d)Kelvin,
used primarily by physicists working with very low temperatures. The unit of temperature on the Kelvin scale is called the kelvin. It is equal to a Celsius degree. The zero point of the Kelvin scale, like that of the Rankine scale, is absolute zero (–273.15 C).
e)Raumur,
used to a limited extent in parts of Europe. On the Raumur scale, the freezing point of water is O and its boiling point 80.
| Fahrenheit to Celsius: | C 59 (F. - 32) | ||
| Celsius to Fahrenheit: | F. (95C.) + 32 | ||
| Fahrenheit to Rankine: | R.F. + 459.7 | ||
| Rankine to Fahrenheit: | F. R. - 459.7 | ||
| Celsius to Raumur: | R. 45C. | ||
| Raumur to Celsius: | C. 54R. | ||
| Celsius to Kelvin: | KC. + 273.15 | ||
| Kelvin to Celsius: | C. K - 273.15 | ||
