A Toronto-based physician has developed a free online service for allergy and migraine sufferers, heart patients, diabetics and arthritics that he says will alert them to coming weather patterns that could aggravate their conditions.

The question is, can MediClim.com weather the storm financially.

John Bart, a family physician, and meteorologist Denis Bourque say they have developed an algorithm based on temperature, humidity, barometric pressure, wind speed and direction, and nine other climatic readings to produce what they call a weather health index.

Putting those elements together, they believe that they can predict within a 12-square-mile area whether conditions could cause certain chronic conditions to flare up. At present, the system does not factor in pollution readings.

The belief that weather affects health, even in subtle ways, has been more popularly held among patients than scientists. The number of possible variables makes proving a clear causal connection difficult, but some patients swear that various body aches arise under certain climatic changes.

"It seems to me perfectly logical that as a weather pattern passes over us, everybody responds in some way to it," said Dr. Bart.

He acknowledged the system was not fool-proof, noting that, "We're only as accurate as the weather service and not everybody responds [to climate changes] to the same degree as the next person."

The Mediclim formula is proprietary, but the service is free to anyone who signs up. And Dr. Bart says they now have about 11,000 subscribers in the United States, Canada and the United Kingdom. He said they do not ask for personal information beyond an e-mail address.

Primarily using readings from the U.S. Meteorological Service, the site went live in January; but so far the partners have had trouble getting investors to respond.

Dr. Bart said they approached pharmaceutical companies and other potential advertisers to support the site financially, without success. An agreement with Google has brought in only $900 the past six months.

He figures they'll need to about double their current subscriber list before advertising picks up. Until then, "We're running it from our own pocket," said Dr. Bart. "But it's worth it. There's no question in my mind."

The warnings are not elaborate -- typically, they are simply one-sentence reminders that forecasted weather patterns could mean trouble, accompanied by a suggestion to follow a physician's advice and some tips he gives his own patients.

In Western Pennsylvania, MediClim issued an alert for people with heart disease on May 30 when an afternoon storm was forecast.

Two weeks earlier, on May 17, arthritis sufferers were sent a warning. That day's weather report forecast temperatures dropping from daytime highs in the 70s to overnight lows near freezing.

Dr. Bart estimates that any single subscriber would receive three to four warnings each month.

"I've been accused of being too flippant, reminding someone with asthma to take an inhaler with you," he said. But it may be just enough to ward off an attack that leads to a sick day or worse, he added.

Even when weather isn't extreme in the form of tornadoes or heat waves, Dr. Bart believes that it can have "a destabilizing effect" on some people. "No migraine sufferer will deny the weather has an effect on their migraines."

Rather than prompting a flood of worried calls to doctor's offices, he said, "The idea is that we decrease the number of times the physician and the health system are used" by intervening before the problem worsens.

Ultimately, he hopes to collect enough data to pinpoint not only when weather poses a possible health threat but also how and why that happens.

"It would be really nice to learn how migraine sufferers develop migraines. And if we can find out what the mechanism is, we can interrupt the cycle before it gets going."