The state got two reasons to cheer despite the soaring mercury level on Tuesday.

While the India Meteorological Department's (IMD) headquarters issued a forecast of an above average monsoon rainfall this year, weathermen at Patna Meteorological Centre sounded hopeful for thundershowers in the entire northern and some parts of south Bihar this weekend.

Almost the entire state has been experiencing heat waves for the past few days with the mercury breaching the 40-degree mark almost everyday (Tuesday being an exception). In meteorological terms, a heat wave is declared at a place where the maximum temperature exceeds 40 degrees Celsius and is at least 4.5 degrees Celsius above normal.

The weather office, however, said the maximum temperature could drop below 40 degrees Celsius or even below 35 degrees Celsius when clouds start hovering from Friday evening.

The thundershowers have been attributed to Nor'wester - sudden thunderstorm that act as summer coolers. Patna meteorological centre director Ashish Sen told The Telegraph: "A western disturbance passing over Nepal has led to the formation of an induced cyclonic circulation. Moisture feeding from the Bay of Bengal will cause this cyclonic circulation to lead to thundershowers. Skies are expected to turn cloudy from Friday evening itself and rain might occur in an intermittent manner over the next two days."

A cyclonic circulation forms around a low-pressure area and attracts moisture towards itself.

The Met chief claimed the north Bihar and some parts of south Bihar would receive maximum rainfall because of the upcoming Nor'wester. "Places like Patna and Gaya are unlikely to receive much rainfall but the moisture feeding and change in wind direction from dry and hot westerly to moist and cool easterly winds will bring down the mercury level this weekend," said Sen.

Met experts however claimed the rains would provide temporary relief as hot and dry westerly winds blowing at an abnormally high speed of up to 35kmph is leading to rapid evaporation of surface water at a rate of 10mm per day.

Abdus Sattar, the assistant professor of meteorology at Rajendra Agriculture University, Pusa, Samastipur, said: "At present, the average rate of surface water evaporation in Bihar facing heat wave conditions, is not less than 10mm a day. This implies that rainfall of 10mm in a day can get completely evaporated the next day when it is dry and hot."

Evaporation normally leads to good showers but it's not happening in Patna because of rising pollution level, unchecked urbanisation and cutting of trees among others.

"Despite high evaporation this year, there are no frequent summer thundershowers. The Celsius surge isn't unnatural but the continuous 40-degree plus reading amidst such dry conditions is," said Sattar. "The average level of relative humidity is only around 12 to 14 per cent, which is very low. Besides, the wind speed of up to 35 kmph at this time of the year is quite abnormal."

Explaining the reason behind the high-speed westerly winds, Sen said: "The continuous flow of westerly winds in Bihar and neighbouring states, including Bengal, Odisha and Jharkhand over the past few days has led to the formation of a localised atmospheric vacuum, something like a low-pressure area. This low-pressure pushes the wind speed under high temperature especially between 11am and 5pm, when the day is hottest."