The frequent change in the wind pattern is behind the seesaw mode of the Celsius in Calcutta, Met officials said.

The Met office recorded a minimum temperature of 19.8 degrees, a notch above normal, in Calcutta on Sunday. Three days ago, it was 23.5 degrees, almost six notches above normal.

From there, the Celsius saw a sharp slide. The minimum temperature fell six degrees to 17.5 on Thursday and 17.3 on Saturday.

By Tuesday, the minimum is slated to rise to 23 degrees, according to the Met forecast.

“The wind pattern has been changing frequently. When the southerly component of the wind flow is becoming stronger in the lower atmosphere, the moisture level is going up and the Celsius is rising. When the northerly component is going up, it is becoming dry and the Celsius is dropping,” said a Met official.

On Wednesday, when the minimum temperature shot up to 23.5 degrees, southwesterly winds from the Bay of Bengal were dominating the lower atmosphere. The anticyclonic flow of winds injected a lot of moisture inland.

That changed on Thursday. The wind pattern altered because the anticyclonic system became weak. The southerly winds lost steam and dry winds from the northern reaches started making rapid inroads, causing the slide in the Celsius.

“The pattern changed again on Sunday. Moisture-rich winds from the Bay are becoming strong again,” the Met official said.

The days have started becoming longer. The strong northerly winds had dragged the maximum temperature down to 27.2 degrees, a couple of notches below normal, on Thursday.

But on Sunday, it was back to over 30 degrees.

“The effect of the minimum temperature is felt for only a few hours at night and early in the morning. The effect of the maximum temperature is felt throughout the day,” said a Met official.

The impact of the rising Celsius was visible on the roads of Calcutta. Umbrellas were out in the afternoon as pedestrians needed a shield against the sun.

The Met office has said fluctuations in the minimum temperature are likely to be arrested now. “The Celsius dropping to 17 degrees is almost ruled out now. For the next few days, it is expected to be between 21 and 23 degrees,” said an official.

But the moisture content is likely to keep rising. The Met office has sniffed chances of light rain in Calcutta and several other parts of south Bengal on Wednesday and Thursday.

If that happens, the overcast conditions will cause fluctuations in the maximum temperature. “The maximum temperature will come down significantly. But as the sky clears, it will rise sharply again,” the Met official said.