Japanese scientists have fired cloud seeding equipment to help top up reservoirs serving the 35 million people of greater Tokyo, officials said Friday, amid a sweltering summer dry spell.

Months of below normal rainfall and soaring temperatures have left supplies around 60 percent of the average for the time of year, sparking calls to economise on water in the heaving Japanese capital.

Using a piece of equipment nearly half a century old, the Bureau of Waterworks sent a plume of silver iodide up through a chimney over an area outside of Tokyo, an official told AFP.

Around 17.5 millimetres (two thirds of an inch) of rain was recorded over the following two hours, the Asahi Shimbun reported.

"It's difficult to judge whether the machine was direct cause of the rain, but we'd like to think that the run was effective," the official said.

A bakingly hot summer has seen the mercury go as high as a record 41 degrees Celsius (106 Fahrenheit) in Japan this month.

Around 47,000 people have been taken to hospital nationwide suffering from heatstroke or exhaustion, the Fire and Disaster Management Agency said. More than 100 people have died because of the heat in the capital alone, Tokyo government figures show.

Seeding is commonly carried out by planes spraying substances directly into the air. The idea is that the silver iodide provides something for water molecules to bind to. When the droplets become large enough, they fall as rain.