Jane Green of Kakanui asks :-

Ever since I was a child, I have been led to believe that 'wild-fire' that occurs on windy nights was different from lightning.What is wild-fire?

Neil Thomson, a physicist at the University of Otago, responded.

The question involves both phenomena and terminology. Wildfires (wild-fires) are generally fires running wild in the countryside or wilderness, burning bush, trees, grass and other vegetation. The commonest agent for starting such fires is lightning.

The lightning itself is caused by the build-up of large amounts of electric charge on clouds from vigorous vertical air movement, usually in storms. When the charge on the cloud becomes large enough, it escapes by forcing a sudden electric current through the air (stripping electrons from the otherwise neutral air molecules). This current flows (along a jagged path) between cloud and ground, emitting light and sound (thunder).

Wildfires can also be ignited by many other sources such as volcanoes or man-made cigarettes but, because lightning is the commonest natural cause, wildfire tends to be associated with lightning. If it's windy, then the wildfire is much more likely to take hold and spread, and so be much more noticeable.

On the other hand, on a summer evening, when the weather near you is warm enough that it's pleasant to be relaxing outside and gazing into the distance, quite possibly towards distant mountains, you may see 'wildfire' far away, not much above the horizon. What you are seeing is actually distant lightning flashes which, in this context, are also called 'wildfire'. The weather far away where the flashes are occurring is stormy and generating lightning which you can see 20-100 km or more away. However, you don't hear the thunder because the sound from such distant lightning is often too weak for your ears to detect after it has traveled so far.

All lightning emits not only light but also radio waves. Professor Richard Dowden of Dunedin invented and implemented the World-Wide Lightning Network (WWLLN) which uses these radio waves to find the locations of lightning continuously, and so track the movement of thunderstorms world-wide (and also help to determine the sources of wilderness fires).