'Error in sensor': What happened in Delhi's Mungeshpur on Wednesday?

The met department quickly corrected the reading and blamed the malfunctioning of the sensors kept at the Automated Weather Station (AWS) for the gaffe

On Wednesday the India Meteorological Department (IMD) said Mungeshpur, on the outskirts of Delhi, recorded an all-time high temperature of 52.9 degree Celsius, which is an abnormal reading.

The reading immediately went viral and became the talk of the town. The Met quickly corrected it and blamed the sensors at the Automated Weather Station (AWS) for the howler.

It has ordered an investigation into it and has stationed a team of senior scientists to ascertain its cause. The error has put a spot of doubt also on Tuesday’s highest reading for the city -- 49.9 degree Celsius -- also in Mungeshpur.

So in what conditions does an AWS malfunction? And how is it different from a thermometer-based temperature reading?

Experts say between the two, thermometer-based reading, which in the case of Delhi is done at the Safdarjung observatory, is a more reliable measurement due to the nature of the device. This is not to say that an AWS does not give correct measurements but it is prone to going wrong if not calibrated properly, placed wrongly, or if the exposure is not correct.

The enclosure where the thermometer is kept, like the one at the Safdarjung observatory, is called a “Stevenson Screen”. This is nothing but a wooden box-shaped enclosure used to protect meteorological instruments against precipitation and direct heat radiation from outside sources, while allowing air to circulate freely around them.

Dry bulb and wet bulb thermometers in the “Stevenson Screen” are placed vertically while the maximum and minimum thermometers are kept in a horizontal position.

The wooden box opens on the north side so that it is not exposed to direct sunlight.

“This helps in measuring temperature accurately,” Mahesh Palawat, vice-president, meteorology and climate change, in private weather forecasting agency Skymet, told Business Standard. He said, on the other hand, an AWS was a sensor-based device and although its installation is also done in accordance with prescribed standards sometimes due to paucity of space the sensors are not accurately calibrated with the actual readings.

Also, if maintenance is not proper and the AWS is placed in an inappropriate place such a rooftop or over a concrete structure it tends to be affected by the nearby topography and overheat.

He said it was natural for an AWS to overheat but then the scientist who had taken the reading should have been careful before making it public.