Page 28 - Niên giám 2021

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 solar
2
2
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.
Average humidity in year is the average of average relative humidity of
all days in the year.

The water level is elevation of the water surface at the observation place
in relation to the sea surface, measured in centimeters (cm). A system of piles,
rulers and self-recording machines are used to monitor the water level.

Water flow is the amount of water flowing through a cross section of the
river in a unit of time, measured in m³/s. Monthly average flow is the average
value of flow of the days in the month. Water flow is measured by flowmeters,
drifting buoys or ADCP machines.

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