Forecasters Guide - (IMD), Pune

DEFINITIONS
Pressure:
The pressure of the atmosphere at any point is the w eight of the air column w hich
stands vertically above unit area w ith the point as its centre. For meteorological purposes,
atmospheric pressure is usually measured by means of a mercury barometer w here the
height of the mercury column represents the atmospheric pressure. The C.G.S. of pressure
is the dyne per square centimetre but this is inconveniently small. The unit employed for
reporting pressure for meteorological purposes, is, therefore, the hPa w hich is defined as
equal to 1000 dynes/ Sq. Cm.
Temperature:
The temperature of a body is the condition w hich determines its ability to
communicate heat to other bodies or to receive heat from them. Heat energy appeals to the
sense of touch and the degree of hotness or coldness of a body is know n as its temperature.
In a system of tw o bodies, that w hich loses heat to the other is said to be at a higher
temperature and is the hotter one. Meteorologists are interested in the temperature of the
air, of the soil and of w ater bodies. Temperature is measured by means of a thermometer.
Thermometers are graduated in different scales. The scale accepted for use in the India
Meteorological Department is the Celsius scale giving temperature in degrees Celsius.
Maximum Temperature:
The highest temperature reached, since the time of the last setting of the maximum
thermometer. The maximum thermometer is routinely set after the morning (0830 hrs. IST)
observation.
Minimum Temperature:
The low est temperature reached, since the time of last setting of the minimum
thermometer. The minimum thermometer is routinely set after the morning (0830 hrs. IST)
and afternoon (1730 hrs. IST) observations.
Relative Humidity:
Relative Humidity is the ratio of the actual mixing ratio of a sample of air at a given
temperature and pressure to the saturation mixing ratio of the air at the same temperature
and pressure.
It is usually multiplied by 100 and expressed in percent.
Dew Point Temperature:
It is defined as the temperature to w hich moist air must be cooled, during a process
in w hich pressure and mixing ratio remain constant, in order that it shall become just
saturated w ith respect to water.
Note that it is an isobaric process in w hich the pressure remains constant. No w ater vapour
is added or subtracted from the sample of air and so its mixing ratio remains constant.
How ever, the air becomes saturated at the dew point (temperature), and so the value of
mixing ratio becomes equal to the saturation mixing ratio at this temperature.
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