By BRENDAN MONTAGUE, Daily Mail

Last updated at 19:24 13 July 2006

When a rainbow formed in the sky people stopped and stared at the natural wonder.

But then lightning sparked across the evening panorama as two of nature's most spectacular phenomenon created an unusual alliance.

See more incredible storms and lightning pictures here

The clash of weather was seen above the affluent city of Fort Smith, in the Southern state Arkansas.

One onlooker said: "It was awe inspiring. The lightning made a huge rumbling sound and when you looked up there was also this incredible rainbow forming on the horizon."

The intracloud lightning, known as an anvil crawler, is the most common form of lightning, with the electrical charge contained within a single cumulonimbus cloud.

Lightning often occurs during heavy storms while rainbows are generally formed after the rain has stopped, making an appearance of both simultaneously relatively rare.

The actual electric charge in a flash of lightning comes from particles from the sun sent out in the solar wind which gather in the outer atmospheric layers before creating a strike.

Scientists are still divided by what actually causes lightning, with one theory suggesting falling droplets of ice and rain become electrically polarised as they fall through the natural electric field in the Earth's atmosphere.

This would explain why lightning often accompanies storms and heavy rain. The same droplets also cause the rainbow, when light from the sun is refracted by the water to cause a spectrum.