It’s a set routine every morning at the Safdarjung Meteorological Office in south Delhi where assistant meteorologist Y N Prasad takes the temperature readings.
The maximum temperature is always recorded in the evening, at 4.30 pm, and the minimum in the morning, always at 6.20 am.
Prasad explains why.
“The maximum and minimum temperatures lies between radiation and re-radiation. When the sun gives out radiation to earth, it’s called solar radiation. When the earth emits heat, it is known as re-radiation.
“When solar radiation is about to end and re-radiation is about to begin, then we get the maximum temperature at 4.30 pm. When solar radiation is about to begin and re-radiation is about to end, we get the minimum temperature at 6.20 am.”
The thermometer is placed in the shade, away from direct sunlight. The box housing it is designed to ensure air gets to the thermometer.
Humidity is calculated by the combination of two thermometers – dry thermometer and wet thermometer, explains Prasad.
“Our centre also provides vital information to Safdarjung airport,” he says.
Visibility information
“Aviation requires weather forecast in loading and unloading the aircraft. Plus, the visibility information is very important.”
“We have a ‘two-type barometer’ that gives QAP – the station-level pressure, which is required for landing of an aircraft – and after getting QAP we derive QNH, the mean sea level pressure, which is essential for take off,” he adds.
“We have a digital barometer also and a self-recording instrument that catches any errors in the readings.”
At Aya Nagar, the Met office floats a balloon which records the temperature and wind direction 16 km above ground.
Prasad gives an equation from his 30-year experience.
“If the humidity is high, the dew difference between yesterday and today is more. Then definitely there will be a change in weather.” He means it will turn worse.
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