At least 13 people have been killed by lightning in eastern India, as storms raged across the region.

Those killed were farm workers in West Bengal state, who were in the fields when they were struck by lightning. Twenty more people were injured.

The severe weather was brought by what is known as a kalboishakhi, an immense storm which forms ahead of the monsoon rains.

Dry, cold air which originates from Iran and Turkenistan drifts east and is forced to the south of the Himalayas.

This air is a stark contrast to that which is across the rest of India, and the two air masses interact with explosive results.

Huge storms form bringing destructive winds, torrential rain and plenty of thunder and lightning. These generally move southeast, often drifting across the border into Bangladesh.

A number of these storms have been seen over the past few days, including one which blasted New Delhi on May 30. As the rain started, the temperatures dropped 15C in just an hour and a half.

Winds of up to 55kph were recorded, as lightning lit up the sky.

More severe weather is expected in northeast India over the next few days. This will bring more heavy rain to the region, ahead of the more persistent monsoon rains.

This year the summer monsoon is lagging about a week behind schedule, and has yet to make it onto the coast of Sri Lanka.