Calcutta could spy its first speck of grey in the April sky on Wednesday evening and the weather office held out hope for more to come.

The atmospheric condition has become conducive to a Nor’wester and the Met office has hinted at the possibility of rain or thundershower, accompanied by squall, in some areas towards Thursday evening or night.

“A well-marked anticyclonic formation that has been hovering over Uttar Pradesh since April 3 has started weakening. This has paved the way for the formation of an anticyclonic circulation over the northern Bay of Bengal,” said D.N. Talapatra, the director of the Regional Meteorological Centre at Alipore.

The formation over the Bay has resulted in “incursion of moisture into the lower troposphere”, a prerequisite for a Nor’wester.

Met officials said that at this time of the year, hot air forms a cyclonic circulation over western Bengal and Jharkhand. Simultaneously, an anticyclonic circulation develops over the Bengal coast, drawing in moisture from the Bay. Thunderstorms occur when the hot and moist air of the two systems collide. Rain is often accompanied by hail.

The presence of moisture in the lower troposphere caused the mercury to dip for the first time in April on Wednesday. The day’s maximum temperature was 37.6 degrees Celsius — 1.3 degrees lower than Tuesday’s maximum but still two degrees above normal. The minimum temperature was 26.8 degrees, three degrees above normal.