BÀI GIẢNG KHÍ TƯỢNG THỰC HÀNH CHƯƠNG 2

CHAPTER Observation of Atmospheric Pressure CHAPTER Observation of Atmospheric Pressure 2.1 Definition and unit Hecto-pascal (hPa) should be used as the unit of the atmospheric pressure There are some other units for the atmospheric pressure, such as mb, mmHg, inchHg, etc Note the following relations: hPa = 100 Pa = mb mmHg = 1.33322 hPa inchHg = 33.8639 hPa The detailed list of conversion from mmHg to hPa, is shown in the "Guide to Ships' Weather Reports" separately issued by the JMA 2.2 Measuring instruments and their installing conditions The marine aneroid barometer is generally used for the measurement of the atmospheric pressure The marine aneroid barograph is used to record barometric tendencies and amount of change The resonator digital barometer as described later is also adopted to record the atmospheric pressure, barometric tendencies and amount of change In installing these instruments, the following should be reminded: 1) To avoid exposure to the direct sunshine 2) To install instruments at a place with small temperature change 3) To reduce the effect of vibrations and shocks caused by the engine and/ or ocean waves, cushion material such as sponge rubber had better be put on lateral and/ or down sides of the instruments 4) The marine aneroid barometer is usually installed horizontally If you have to put it on a vertical wall, its indicated value should not differ by more than 0.5 hPa from that of normal installation 2.3 Aneroid barometer The aneroid barometer is affected by uncertainties due to variations in the elastic properties of the spring and capsule and due to wear in the mechanical linkages The precision marine aneroid barometer developed with high quality (see Fig 2.1) is recommended for weather observation by ships Before you read the atmospheric pressure, tap the glass face with your finger so that the needle vibrates slightly Then fix one of your eyes right above the needle and read the scale If you find the image of your eye reflected in the glass exactly at the same horizontal position with the needle, your eye is correctly located for observation If your barometer has a mirror CHAPTER Observation of Atmospheric Pressure on the scale plate, you can easily adjust your eye position When the ocean wave is high, your ship suffers large rolling and pitching, resulting in its displacement and vertical acceleration so that the needle of the barometer shows horizontal oscillations In this case, read the maximum and the minimum values of the scale for several reading samples The average value of these data should be recorded as the observed value The observation of the atmospheric pressure should be made exactly on the observation hour and the pressure should be recorded up to the order of 1/10 hPa 2.4 Electronic barometer There are two main types of electronic barometers which are used for weather observations One is a resonator digital barometer and the other is an electric capacitance barometer The former one works on a principle that a frequency of the sensor varies with atmospheric pressure The latter works on a principle that an electric capacitance varies with atmospheric pressure The electronic barometer does not indicate the instantaneous value of the atmospheric pressure but rather the averaged value over a certain short period You can therefore make accurate observations even on board with rolling and pitching You can also avoid observation errors arising from your personal tendency in reading the instrument, because this instrument displays a digitized value and free from parallax 2.5 Correction for instrumental error The indicated value of a barometer is mostly different from the true value to some extent This intrinsic difference is called as "instrumental error" and should be corrected to get the station value (see 2.6) of the atmospheric pressure The barometer should be checked every months by PMOs to maintain the accuracy (see Ch.12) 2.6 Correction to sea level The atmospheric pressure at mean sea level is to be reported and can be derived from the observed value of the instrument installed at a height above mean sea level (referred to as station pressure) This derivation is called "correction to mean sea level" The correction depends not only on the height where the instrument is installed but also on station pressure * and temperature See "Guide to Ships' Weather Reports" for the detailed table to make correction The height of the barometer above mean sea level should be checked when your ship is fully loaded or unloaded If the draught level of your ship varies within + or - m around the averag~ level, you can always apply the height of the barometer above the average draught level to the correction If, for example, the station pressure is 997.4 hPa, the level height of the barometer above mean sea level m and the temperature 13°C, the correction value will be 0.7 hPa according to the table mentioned above And the corrected value is obtained as follows: * Station pressure is defined as the pressure at barometer level CHAPTER Observation of Atmospheric Pressure 997.4 (station pressure) + 0.7 (correction) = 998.1 hPa (pressure at mean sea level) 2.7 Barograph A barograph is used to check observed values of pressure and to record barometric tendencies and amount of change Fig.2.2 shows an aneroid barograph, one of the several available types of barographs The barograph paper which records the continuous record of pressure is called barogram Barograms must be renewed at a certain interval (from day to days, depending on the instrument) The barograph is recommended to be installed because it can show you whether a high, a low, a trough or a ridge is approaching or getting away You could then presume the tendency whether the weather is recovering or getting worse As for where and how to install the barograph, refer to comments noted in 2.2 for the barometer