Most of our weather forecasts are based on satellite images, pictures taken from far above the earth. They show the large movements of clouds, wind, and air masses that bring temperature changes, produce precipitation, and create the conditions we experience at ground level.

But the satellite images don't show everything, says Bill Miller, Client Services officer with the Meteorological Service of Canada in Whitehorse.

"The satellites tell you a lot but they don't tell you what's happening under that cloud. That's where you still need people."

And that's where the Severe Weather Watchers come in. For several years, a group of volunteers in British Columbia and the Yukon has been providing valuable information about what's happening underneath the weather systems.

Severe Weather Watchers aren't necessarily specialists or scientists. They include police officers, amateur radio operators, weather enthusiasts, and a variety of other people. Several telephone systems also participate in the network, providing toll-free lines for communicating weather observations.

"You don't have to be a rocket scientist," says Miller. "And with the toll-free numbers, it doesn't even cost anything."

Environment Canada provides Severe Weather Watchers with a quick and easy guide to what they should report and how they can measure it. No equipment is needed, other than a telephone. A rain gauge and a ruler might come in handy, but they aren't necessary.

For example, Severe Weather Watchers are asked to report hail a centimetre or more in diameter -- roughly the size of a marble. Wind gusts of 90 kilometres per hour or more should also be reported. How do you tell if the wind has hit 90 km/h? According to the guide, wind gusts in that range will result in trees being uprooted and damage to buildings.

Some things are always reported, including tornados or funnel clouds and freezing rain. That's because a major concern behind the Severe Weather Watcher program is safety, Miller explains. The kind of conditions that produce tornados, freezing rain, and high winds tend to be very localized and difficult to predict from a distance.

Another inspiration behind the program is the collection of data to improve the accuracy of local predictions.

"We're a data-sparse area," Miller says. "In fact, all of Canada is a data-sparse area."

The lack of data can itself become a safety issue. For example, says Miller, conditions that produce a 20-centimetre snowfall at the observation centre at the Whitehorse airport might produce a 40-centimetre snowfall only a few kilometres away. However, there's no formal observation centre to record that difference.

In order to fill in some of those data gaps, Miller has been recruiting northerners as Severe Weather Watchers. Last fall he drove down the Alaska Highway as far as Fort St. John, looking for volunteers at highway lodges and other isolated locations.

Along the Alaska Highway in the southern Yukon and northern British Columbia, observation centres are rare, he explains. High mountain ranges and deep valley systems make it extremely difficult to translate the large systems recorded by satellite and radar into accurate local forecasts. Miller hopes the information provided by Severe Weather Watchers along the highway will improve the quality of forecasts and the safety of travelers along the highways.

Data is just as sparse on the roads north and west of Whitehorse, he adds, and he's still looking for Severe Weather Watchers in those areas -- or anywhere else where accurate local weather reporting is important.

For more information about the Severe Weather Watcher program, call Bill Miller at (867) 667-8459, email him at William.Miller@ec.gc.ca, or go to Environment Canada's Severe Weather Watcher Resource Site at www.weatheroffice.pyr.ec.gc.ca/severewxwatch/