Network Pinpoints Lightning Strikes

Few displays of the sheer force of nature are as spectacular and humbling as a bolt of millions of volts of electricity streaking down from the sky in a deafening, hair-raising explosion of thunder. Although lightning strikes the earth 50-100 times every second, its ability to terrify remains justifiably undiminished: in Canada, lightning kills half a dozen people, seriously injures 60-70 others, and ignites some 4 000 forest fires on average each year.

Information on the precise location, strength and timing of lightning is crucial to a wide range of operations that are vulnerable to direct hits or to fluctuations in electric power. By confirming that there is a mature thunderstorm in progress, the presence of lightning assists weather forecasters in issuing warnings to those at risk. For example, the aviation industry relies on such warnings to protect aircraft and ground crew, while forestry and parks services use them to determine areas where lightning-induced fires are most likely to occur, as early detection is critical to saving valuable timber.

A reliance on radar, satellite imagery and ground reports made lightning detection a challenging task, particularly in unpopulated areas, until the Canadian Lightning Detection Network came into being two years ago. Eighty-one sensors across the country, most of them located south of the treeline, form the Canadian half of a North American network that is the largest of its kind in the world, able to process more than 200 000 strikes per hour. The Canadian network not only detects ground strikes, but also is the first national network of its size with the capability to detect cloud-to-cloud lightning.

The cylindrical sensors, which stand over a metre high, determine the strength, polarity and timing of lightning from the electromagnetic pulse it produces. The pulse information is sent via a miniature satellite dish to the network control centre in Tucson, Arizona. There the information from many sensors in the network is integrated to determine the location of the strike, in some areas to within half a kilometre. The lightning location information is then transmitted to Environment Canada's weather centres, where it is mapped on a computer screen in real time. The whole process takes less than 40 seconds from start to finish.

In the northern hemisphere, most lightning occurs between June and August. The circumstances that lead up to a strike occur when hail and supercooled water droplets in a cloud collide, causing them to become electrically charged. The positively charged fragments are carried in the wind to the top of the cloud, while the negatively charged pellets collect in the bottom. Eventually, the two areas build up to such a point that a flash of electric current jumps across the gap.

This usually occurs within the cloud, but a third of the time it jumps from cloud to ground, creating an ionized channel. As it approaches the ground, this channel draws a charge of opposite polarity from a tall object on the ground—often a tree or tall building. When the two meet, the pathway is completed, and a spectacular flash of energy is funneled downward in the form of lightning. As the air in the pathway suddenly superheats to 30 000°C, it produces large sound waves of thunder.

Every strike detected by the network is stored in a data archive, which can be used to produce flash-density maps showing lightning activity in Canada over a certain period of time. Although the network is still too new to identify any long-term trends, scientists are hopeful that these data will help to determine which areas of the country are most or least likely to encounter thunderstorms—and how lightning activity is affected by other weather- and climate-related factors. In the future, more sensors may also be added to locations above the treeline in order to better define lightning activity in more northern remote areas.

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