Researchers in Japan turned to detailed logs kept by farm families and government officials hundreds of years ago, looking for mentions of thunder and lightning events. Original copy of the Diary of Hirosaki Clan Government Office, held at the Hirosaki City Library, is shown
Diary entries dating back to the 1700s could help scientists understand the link between lightning activity on Earth, and the rotational cycle of the sun.

Researchers in Japan have turned to detailed logs kept by farm families and government officials hundreds of years ago, looking for mentions of thunder and lightning events.

The study shows this activity lined up with the time it takes sunspots to make a complete rotation, suggesting the cycle plays a 'very important role,' in daily weather.

thunder lightning
'It is well known that long-term - centennial to millennial-scale - variations of solar activity influences terrestrial climate,' said Hiroko Miyahara, first author on the paper and an associate professor of Humanities and Sciences/Museum Careers.

'However, it is not well established whether the sun influences the daily or monthly weather.'

A team, including researchers from numerous universities in Japan and the National Institute of Polar Research, examined old Japanese diaries that were kept continuously for over 150 years.

This began with the 'Diary of the Ishikawa Family' - a farm family in Hachioji (now western Tokyo).

They also examined the Diary of Hirosaki Clan Government Office, which was kept by civil servants from Hirosaki (Aomori Prefecture).

The team focused on events between May and September, when Japan is less affected by the cold Siberian air mass, and found a distinct pattern.

Every 24-31 days, the researchers found lightning and thunder activity peaked.

According to the team, this is the same window for a sunspot rotation, and was seen to be especially strong in years with a high number of sunspots.

'The cyclic behaviour of the sun is playing a very important role in the changes of weather in Japan,' Miyahara said.

The researchers now plan to investigate this influence in greater detail to see how it may affect weather in Japan today and in the future.

'Our ultimate goal is to include the influence of solar activity in the weather forecast,' Miyahara said.

'It would improve the accuracy of the forecast, and it may even enable a longer-term weather forecast.'

Lack of sunspot activity in the sun is due to a continuing period of inactivity in the star's magnetic field.

As the sun moves through its 11-year cycle, it experiences active and quiet periods known as the solar maximum and solar minimum.

As solar minimum approaches, certain types of activity - such as sunspots and solar flares - will drop, but it's also expected to increase long-lived phenomena.

This includes coronal holes, where fast moving solar winds are created when the star's magnetic field opens up into space. This happens more regularly as the sun's magnetic field becomes less active.

coronal hole sun

Strong solar winds emanating from three massive 'holes' on the surface of the sun have begun to bombard Earth, scientists say. Fast moving solar winds are created where the star's magnetic field opens up into space, pictured here as vast black regions
Charged particles make their way out into the solar system through these gaps and hit the atmosphere of our planet.

This can lead to a number of complications, including magnetic storms which can result in power grid fluctuations, impact on satellite operations and can affect migratory animals.

An increase in solar winds can also alter the chemistry of Earth's upper atmosphere, which may trigger more lightning and aid in cloud formation.

It can also affect air travel, as more radiation is able to penetrate planes. This means passengers on long-haul flights may receive doses of radiation similar to dental X-rays during a single trip, and puts flight crews in additional danger.

The effects of solar minimum may also include Earth's upper atmosphere cooling and shrinking slightly, thanks to less heat reaching the planet. This can allow space junk to accumulate in low Earth orbit.