The temperature rose to a balmy 55 degrees on the 7,100-foot rim of Crater Lake early this week, yet surface ice was forming on the west side of the lake.
What kind of trick is Mother Nature playing?
None at all, says Marsha McCabe, chief of interpretation and cultural resources at Crater Lake National Park.
"We've been having a temperature inversion up here — it has been 55 degrees during the day at the rim," she said Tuesday afternoon of a weather pattern that sometimes occurs at the high lake during the winter months.
During an inversion, cold air can be trapped during the day at lower elevations while the heights have balmier temperatures.
Yet the day temperature at the lake surface has remained much colder than at the rim, McCabe said.
Moreover, the temperature at the rim has been plunging some 30 degrees at night, she noted.
But it is very unlikely the lake surface will freeze over this winter, she said, noting the last time that happened was in 1949, when it remained frozen for nearly three months.
Some 95 percent of the surface froze in 1985, according to park records.
The lake surface averages 6,173 feet above sea level.
Yet Diamond Lake, roughly 1,000 feet lower at 5,183 feet elevation, routinely freezes over each winter, as do other lakes in the High Cascades.
The difference, McCabe explained, is that Crater Lake's surface is resistant to freezing because of its large heat reservoir in the immense volume of water contained in the caldera. Other lakes such as Diamond are shallow, she said. But skim ice does occasionally form on the surface of Crater Lake during the winter, she said.
At 1,943 feet deep, the lake is the deepest in the United States and the ninth deepest lake in the world. In fact, the bottom of the lake is about the same elevation level as Fort Klamath, McCabe observed.
Typical winter temperatures at the park range from a high of about 35 degrees to an overnight low of around 19 degrees, with temperatures dropping much lower on occasion, officials said.
While the lake rarely freezes over, the park receives an average of 533 inches — more than 44 feet — of snow each year, receiving snow from the fall through late spring.
There are 29 inches of snow on the ground at the park headquarters so far this winter. Come spring, there often is 10 to 15 feet left on the ground, according to park officials.
The most snow ever recorded on the ground at park headquarters was 211/2; feet, measured on April 3, 1983.
Since the National Park Service began keeping weather information at park headquarters in 1926, it measured a record 879 inches — some 73 feet — in 1932-33.
And the park established a state record for snowfall in a calendar year with 903 inches — nearly 75 feet — in 1950.
Meanwhile, the weather forecast is calling for more snow at Oregon's only national park later this week.
Reach reporter Paul Fattig at 541-776-4496 or email him at firstname.lastname@example.org.