Study of The Barometer

Barometer

A barometer is an instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air or mercury. From the variation of air pressure, one can forecast short-term changes in the weather.

A barometer is an instrument for measuring the pressure of the, due to the weight of the column of air above it. As the earth atmosphere gets thinner with increasing height, it follows as that as we ascend from sea level and low-lying parts of the earth surface, the weight of the air will decrease. That is the pressure will fall.

Types:

• • Water-based barometers

This concept of ‘’decreasing atmospheric pressure predicts stormy weather’’ is the bases for a primitive weather prediction device called a weather glass or tender glass. it consist of a glass container with a sealed body, half field with water ,a Narrow spout connects to the body below the water level rises above the water level, where it is open to the atmosphere. when the air pressure is lower than it was at the time the body was sealed, the water level in the spout will rise above the water level in the body, when the air pressure is higher than it was the time the body was sealed, the water level in the spout temperature will rise the water level in the spout. A variation of this type of barometer can be easily constructed.

• • Mercury barometers

A mercury barometer has a glass tube with a height of it least 84cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube .mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect. A pressure of one atmosphere is equivalent to about 29.9 inches, or 760mm, of mercury .some barometer give the atmospheric pressure in millibar (one millibar=100 pascal’s , or one hecter pascal ) to convert a reading in inches of mercury to millibars, divide the pressure in inches of mercury by 0.0295.

• • Aneroid barometer

An aneriod barometer uses a small, flexible metal box called an aneriod cell. This aneriod capsule (cell) is made from allow of beryllium and copper. The box is tightly sealed after some of the year is removed, so that small changes in external air pressure cause the cell to expend or contract. This expansion and contraction drives mechanical levers and other devices which are displayed on the face of the anriod barometer. Many models include a manually set needle that is used to mark the correct measurement so a change can be seen. Standard atmospheric pressure the standard atomsphere (symbol;atm) is a unit of pressure equal to 101325 Pa or 1013.25 millibar or hectopascals. It is equivalent to 760mmHg (torr), 29.92inHg 14.696psiThe density of mercury will change with temperature, so a reading must be adjusted for the temperature of the instrument. For this purpose a mercury thermometer is usually mounted on the instrument.

As the air pressure will be decreed at altitude above sea level (and increased below sea level) the actual reading of the instrument will be dependent upon its location. This pressure is than converted to in equivalent sea level pressure for purpose of reporting and for adjusting aircraft altimeter.

A barometer is commonly used for weather prediction, as high air pressure in a region indicates fair weather while low pressure indicates that storms are more likely. When used in combination with wind observation, reasonably accurate short term forecast can be made.