Chris Cowan says forecasters use the 570 line as a shorthand for the presence of hot, dry weather.

Chris Cowan says forecasters use the 570 line as a shorthand for the presence of hot, dry weather.

Watching weather on the 570 line

For forecasters, it’s a sure sign that hot weather is here

People mark the start of summer in different ways. For some, it’s the last day of school; for others, the first jump in a lake.

For Chris Cowan, it’s the 570 line on a weather map.

“The 570 line is a measure of the height of a certain air pressure level,” says Cowan, pointing to a map on the wall of the weather office at the Southeast Fire Centre in Castlegar. The map is marked with lines and circles, indicating different weather patterns, high and low pressure areas.

“Up here is winter, down here is summer,” says Cowan, pointing to the 570 line. “And in here, where we are, it’s spring.”

The weather forecasters at the Southeast Fire Centre watch things like temperatures and air pressure to try to predict how hot it’s going to get, where rain may fall and where lightning might strike. Over the years, one line — the 570 line — has come to be a shorthand for just what’s in store for the days and weeks ahead.

When the pressure of the atmosphere is at 500 millibars at 5700 metres altitude, that’s the “570” line — and it only gets that way when it’s hot, and sunny. Since that’s the weather firefighters worry about, that’s the line they watch closely as it moves higher up in latitude.

“It’s a rough and ready average temperature indicator, and it’s common currency with the fire people,” says Cowan. “It’s an easy concept to get across … they know if the 570 line goes to Northern B.C., we’re deep into summer.”

And that’s just where the line is now, and the southern part of the province has been dry and hot this week. Temperatures are expected to hit 30C by Friday.

“It’s not particularly unusual to see the 570 line here now,”says Cowan. “This time of year, it bounces around.

“What’s significant is that it’s sticking around for a while, and that gives things more of a chance to warm up.”

And with the hot, dry weather comes low humidity. Great for helping the snow disappear, but with that comes the risk of fires.

“The humidity is down below 20 per cent in some areas of the Southern Interior,” says Cowan. “The air mass is very dry, but there’s no green-up yet.

“So there’s a hazard for grass fires, but not like major forest fires yet,” he says. “It’s still damp in the forest, but there’s a hazard in grassy areas.”

And while it’s always difficult to say what the long-term forecast is, Cowan says that indications are the 570 may retreat south, at least for a few days. Other systems are supposed to bring rain into the region.

“Enjoy the dry week, because the weekend’s probably not going to be as nice,” says Cowan.

So the 570 line may not be here for long … but no doubt will be back again soon, likely to stay until the fall.