Mumbai Rains: This was the first pre-monsoon shower in the south Mumbai region.

While a thunderstorm coupled with heavy rain brought much-needed respite from the sweltering heat to residents of Mumbai on Monday evening, air, road and rail traffic in the city took a severe hit.

At least 14 flights were diverted and a secondary runway was put to use at Mumbai's Chhatrapati Shivaji Maharaj International Airport. "Visibility has gone down due to heavy rains, operations have been put on hold. United Airlines Newark to Mumbai flight has been diverted to Delhi," said a spokesperson for the Mumbai Airport, news agency ANI reported. Mumbai is the world's busiest single-runway airport.

Low visibility at the airport also resulted in a minor accident when a Thai Airways plane hit a guard light -- damaging signage and leaving debris -- while vacating the runway.

Although no waterlogging has been reported so far, traffic on the Eastern Express Highway and Western Express Highway slowed down.

Mumbai trains have been running 10-15 minutes late. Rail traffic was hit on the Central Railway's harbour line near Chunabhatti due to technical snag, news agency IANS reported. Train services were also delayed on Western Railway due to a snag in the overhead wire at Mahim station.

This was the first pre-monsoon shower in the south Mumbai region, said an official of the India Meteorological Department.

Monsoon officially hit the coast of Kerala on Saturday, a week later than expected, the weather department said. The ongoing heat wave has sent temperatures soaring across the country. The mercury crossed 48 degrees Celsius in Delhi on Monday, the highest ever in June.