Page 29 - Niên giám

Average air temperature in year is the average of average air
temperature of days in the year.

• Air temperature is measured by normal thermometer, maximum
thermometer (mercury), minimum thermometer (alcohol liquid) and thermo
graph (sensor is a bi-metal plate) exposed to the air in a meteor bust at
altitude 2m away from the ground, sheltered from direct solar radiation.

• Daily average air temperature is calculated using the simple
arithmetic mean from the results of 4 main observations in the day at
1 a.m., 7 a.m., 13 p.m., 19 p.m. or from the results of 24 observations at
the time of 1 a.m., 2 a.m., 3 a.m.,... 24 p.m. of the thermometer.

Number of sunshine hours in months is the sum of sunshine hours
of the days in the month. Numbers of sunshine hours are hours with direct
2
2
solar radiation equal or exceed 0.1 kw/m (≥ 0.2 cal/cm min.). Sunshine
duration is measured by heliograph.
Total number of sunshine hours in the year is the total number of

sunshine hours of the days in the year.
Rainfall in months is the total rainfall of the days in the month.
Rainfall is the thickness measured in millimeters (mm) of the floating

water layer made by rain on a flat surface at a site, which measured by
rain-gauge/ pluviometer.

Total rainfall in year is the total rainfall of the days in the year.
Average humidity in months is the average of average relative

humidity of the days in the month.

• Relative humidity is the ratio between the vapor in the air and
saturation vapor (maximum) at the same temperature. It is indicated in
percentage (%). Humidity is measured by hygrometer and hygrograph.

• Daily average relative humidity is calculated by the simple
arithmetic mean from the results of 4 main observations in the day at:
1 a.m., 7 a.m., 13 p.m., 19 p.m. or from the results of 24 observations at
the time of 1 a.m., 2 a.m., 3 a.m.,... 24 p.m. of the hygrograph.

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