Green or yellow skies can be a sign of an incoming tornado. Here's what we know

A multi-vortex tornado appears near Patricia, Texas. (John Finney Photography)

Ontario residents facing tornado alerts this week reported seeing skies in unusual colours as they anticipated the potentially destructive funnel clouds.

Some experts believe green or yellow clouds are signs of a potentially destructive tornado.

Tornados can be tough to predict, but an extremely dark sky that appears to turn yellow or green is among the warning signs, according to Public Safety Canada.

Officials including research meteorologists and Canadian public safety experts say they could signal that a severe thunderstorm is approaching, bringing with it a chance of heavy rain, large hail, strong winds, lightning, thunder and even funnel clouds.

Tornados often happen later in the day as the sun approaches the horizon, according to oft-cited research by meteorologist Scott Bachmeier from the University of Wisconsin-Madison.

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A thundercloud has water droplets that make the light appear mainly blue, which are illuminated with red light from the sunset and make the sky appear green, he explained, though typically the combination creates purple based on colour theory.

For the sky to appear green, the thunderstorm clouds filling the sky need to be "very deep" in height, he wrote. He notes thunderstorm clouds can often be identified by their height, as they are the tallest type of clouds.

While green clouds may signal a risk of tornado, they don't always mean there will be one, Bachmeier has said in research published in 2008 on the University of Wisconsin-Madison website.

Other clues a tornado may be coming include dark, brownish or yellow clouds, caused by a thunderstorm's massive size and blockage of sunlight, according to the U.S. Federal Emergency Management Agency (FEMA).

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What else to watch for

A tornado appears as a funnel-shaped cloud, or as an "approaching cloud of debris," especially at ground level, FEMA says.

Witnesses may even hear a loud roar like a freight train, or a rumbling or whistling sound, according to FEMA and Public Safety Canada. There may be silence during or soon after a thunderstorm before a tornado appears, or you may even see debris falling from the sky.

FEMA warns those who see what they think is a tornado or notices any of the warning signs to "take protective action immediately."