1. A Presentation on Instrument use For Measurment of Atmospheric Pressure Presented By KHILESH KUMAR SAHU M.Sc. (Ag.) Previous Dept. of Agro meteorology

2. CONTENTS Introduction Uses of Barometer Atmospheric pressure and its Units Types of Barometer

3. Introduction :- atmospheric pressure is important parameter in weather forecasting. Instrument which is used for measuring atmospheric pressure is called barometer. Barometer can be used for following purposes For measuring atmospheric pressure. For determination of altitude of a station above mean sea level(MSL) Approximate forecasting of weather.

4. Atmospheric pressure Air has a weight and it exerts pressure on all sides. The pressure exerted by the air column of atmosphere on a unit area on earth surface is known as atmospheric pressure. Units of atmospheric pressure: The basic unit pressure in international system (SI) is Newton (N). The unit used to measure the atmospheric pressure for meteorological purpose is hecta Pascal (hPa) or millibar (msb)

5. Barometer Mercury filled 760 mm = 1 atm Water filled 10400 mm = 1 atm The barometer measures air pressure Water column (34.0 ft. high or 10.4 m) Atmospheric pressure Mercury column (30.0 in. high or 76 cm )

6. Types of barometers Mercurial barometers: There are two types of mercurial barometers. i) Fortin’s barometer ii) Kew pattern barometer i) Fortin’s Barometer: this is simple and accurate instrument for measuring atmospheric pressure. Fig.- Fortin’s Barometer

7. Kew Pattern Barometer In observatories controlled by India meteorological department (IMD) kew pattern barometer is used for measuring atmospheric pressure. This is also similar to fortin’s barometer were the cistern vessel is fixed and has no adjusting screw. The division are made unequal in order to allow rise or fall of mercury column in the cistern. in this barometer initial adjustment of cistern is not required. Fig.- Kew Pattern Barometer

8. Aneroid barometer: This is simple portable and without mercury barometer. It consists of partially evacuated German –silver circular box corrugated at its top and bottom. This is called sylphon cell. It is very flexible and very sensitive to atmospheric pressure. When the pressure increases the box is compressed and when the pressure decreases, the box expands. Every aneroid barometer before its use must be set and standardised with the help of standard mercury barometer. Fig.- Aneroid barometer

9. Altimeter The pressure decreases with height. The rate of decrease in pressure with height can be used for measuring altitude of a station above msl. When the scale of altitude is given on the aneroid barometer, it is called altimeter. It is used by pilots.

10. References- N.Manikandan, Dr. A.S.R.A.S. Sastri, Ramagya Singh, J.L. Chaudhary 2013. Practical manual on agricultural meteorology College OF Agriculture Raipur. Ghadekar S.R. 2002. Practical meteorology: Data Acquisition Techniques, Instruments and Methods. Agromet Publ.