The clouds can only form in temperatures lower than minus 80 degrees Celsius (minus 112 Fahrenheit). Meteorologist Renae Baker who photographed the clouds, said a weather balloon sent up about 12 miles above the Earth's surface measured temperatures as low as minus 87 Centigrade (minus 124.6 F).

"They reveal extreme conditions in the atmosphere, and promote chemical changes that lead to destruction of vital stratospheric ozone," Baker said.

Australian scientist Andrew Klekociuk said said temperatures in the stratosphere, between 5 and 31 miles above Earth, would be expected to drop as global warming increases. Data collected over the past 25 years had reflected this.

"Over that time there has been a small decrease in temperature and that change is actually occurring faster than the warming at the surface of the Earth," he said.

The delicate cloud colours are created at sunset when fading light passes through tiny water-ice crystals blown along on strong jets of stratospheric air.

"It frees up what we call the militia - agency employees whose regular job may be as a biologist or realty specialist but who are trained in fire duty and can now be called up to help," said Randy Eardley, a U.S. Bureau of Land Management spokesman at the federal firefighting center in Boise.

More than 24,000 firefighters were working on fires across the West on Monday, including 58 large fires of 500 acres or more.

The biggest active fire in the country was in northern Nevada, where nearly 300 square miles of grass and sagebrush had burned. It was 10 percent contained Monday, and fire bosses had no estimate when it would be surrounded.

No homes were in immediate danger, though one outbuilding had been destroyed, said Jamie Thompson of the U.S. Bureau of Land Management in Winnemucca, Nev.

More people were told to evacuate Monday from areas south of Chadron, Neb., on the fourth day of fires that have scorched nearly 80 square miles.

About 45 to 50 people were affected by the latest evacuation orders, and hundreds of others evacuated over the weekend were kept from their homes.

Four rural houses have been destroyed and several more damaged since lightning sparked the fires last week.

In Montana, more firefighters, equipment and aircraft arrived Monday as crews fought to corral a fire that blew up rapidly in Glacier National Park over the weekend, fanned by strong winds and blistering heat.

Firefighters got some relief Monday with calmer winds and lower temperatures, but officials said the fire - estimated at 34 square miles - still posed a threat to the gateway community of St. Mary.

The blaze came within a mile of the town over the weekend. The National Park Service on Sunday evacuated its administrative site there, as well as several area campgrounds.

Most of the park remained open to visitors, officials said.

Residents of a subdivision in central Oregon were allowed to return late Monday as crews tamed a fire there, though evacuation orders remained in effect for another 500 residents of two subdivisions near the tourist town of Sisters.

The subdivisions appear to be protected from the 14-square mile fire, which is 30 percent contained, said Scott Brayton, a fire spokesman.

An evacuation order was also lifted for several dozen residents near Weaverville in northern California after a wildfire that destroyed one home calmed down.

In Idaho, a 5-square-mile fire in the mountains fed on bug-killed evergreen stands as it neared a cluster of vacation homes and a mining museum.

More than 70,600 timber and range fires have burned on federal land so far this year, higher than the 10-year average of 50,984, according to the National Interagency Fire Center. Because of unusually large early-season range fires in Texas and Oklahoma, the acreage burned so far in 2006 is 5.5 million, compared with a 10-year average of 3 million acres for the same period.

Normal precipitation for the month of July is 70.6 millimetres.

Dykes said the period of June-July was also the driest on record, with a total of 40 millimetres of rain. That broke the previous record, set in 1886, by more than seven millimetres.

Hottest days on record

July wasn't just dry: it was hot. Sixteen of the month's 31 days recorded temperatures above 30 C. Normally, there would be four days that hot.

"The daytime high - the average temperature in Winnipeg in the afternoons of July - was 29.8 C, and that is a new record," said Dave Phillips, a senior climatologist at Environment Canada.

"Never has it been so warm in the afternoons over the month of July than this past month. You have to go back to 1974 to see 28.9 C, almost a full degree cooler than what you've had."

Phillips notes Winnipeggers have experienced a series of record-breaking summers in the past few years.

"Two years ago, the big buzz, of course, was about how cool it was. There was no summer," he said. "And then last year, you just couldn't dry out! Now it's the dry and the heat. You just can't get any normal kind of weather there."

August is also expected to start off dry. Despite some cloud cover expected Tuesday, there is only a 30 per cent chance of hit-and-miss showers across southern Manitoba. Temperatures, however, are expected a bit lower, in the mid-20s.

The precipitation situation looks more promising later in the week, as a warm front is expected to bring showers to the region on Saturday.

Fall harvest already underway

The heat and aridity of the past two months have forced crops to mature much earlier than usual, so the fall harvest is already well underway in much of the province.

Swathing of barley is complete in some areas, and farmers are also cutting some oats and spring wheat, with some combining even underway, according to the latest provincial crop report.

Yields are expected to come in below normal, with many crops displaying signs of heat and drought stress.

Livestock farmers have noticed the water levels in their dugouts dropping fast, but there is still enough for cattle to drink in most areas.

With dry pastures and disappointing hay cuts, some farmers are cutting cereal crops and baling them for the winter feeding period.