"There are actually six equations that describe the atmosphere," says Michael Purves, meteorologist with Environment Canada's Weather Centre in Whitehorse. However, that doesn't mean that predicting the weather is about to become an exact science.

The equations are extremely complicated, Purves says. "In order to solve them exactly, you would have to know the state of every molecule in the atmosphere at any given time."

Since that level of data collection is impossible, weather forecasting remains a delicate balancing act, incorporating computer models, scientific measurements, and human observation and experience.

The computer models used by Environment Canada use a grid system based on measurements every 30 kilometres. Meteorologists can't actually sample the atmosphere at each point on the grid, Purves says, so the model predicts conditions at each point based on available information.

Some of the information comes from the many satellites circling the globe, photographing atmospheric conditions. Most of our pictures of the Yukon come from satellites sitting over the equator, says Purves. However, the satellite view of the Yukon is limited by distance and the curvature of the earth.

"They look at us sort of edge-on, and the resolution is poor at that angle," says Purves. In addition, because of the time limitations, the satellites often skip photographing latitudes north of 65 degrees, which cuts off a significant portion of the Yukon.

Another source of information is the weather balloon program. Twice a day, at midnight and noon Greenwich time, weather balloons designed to sample the upper air are released from locations all around the world, including from the Whitehorse Weather Centre. The program has been going on for more than 50 years, says Ken Roth, the Weather Centre's electronics technician.

Each balloon carries a lightweight package containing sensors to measure temperature, dew point, and atmospheric pressure. Some balloons are tracked by satellite navigation systems, providing a record of upper level winds as well.

"There's a battery and a radio transmitter in the package, and that transmits the information back," says Roth. "The average ascent will last about two hours and attain a height of 30 kilometres. Then the balloon breaks and falls back to earth."

Information from weather balloons, satellite photographs, automated weather stations on the ground and from human observers is transmitted to Montreal several times a day, and entered into the computer that provides a model of what the atmosphere over Canada is doing.

"For the most part we use the computer models to show us how things are changing, the motion of storms, the motion of air," says Michael Purves. Detailed local forecasts rely on the skill of the meteorologist in balancing the large systems shown on the model against local information and knowledge of local weather patterns.

The Yukon community of Burwash is an example of the limitations of the computer model, Purves says. The 30-kilometre grid used in the computer model ignores the finer details of terrain, he explains.

"The computer model thinks of Burwash as on the southwest side of the St. Elias Mountains, near the top, instead of in the valley on the northeast side. So the forecast for Burwash is almost always cloudy and rainy if we use the computer model with no human input."

In putting together a local forecast for Burwash and other parts of the territory, Yukon meteorologists go well beyond the computer models and the satellite photographs.

They also consider reports called in from sites as far south as Dease Lake and Fort Nelson. Some reports come from automated weather stations, some from human observers, and the information is often contradictory. The meteorologists weigh all that information against their knowledge of the terrain and local weather patterns, and their understanding of how the computer models work, in attempting to come up with an accurate forecast.

"We look at all these different things," says Purves. "It makes it very tricky to do. It's an extremely complicated business."

For recorded weather information, call 668-6061. For more information about weather forecasting, contact the Yukon Weather Centre in Whitehorse.