CARIBOU – Clearing skies and dry-air wind caused a surprising drop in late-night temperatures Saturday as residents of northern and eastern Maine pulled on coats and heavy jackets.

The temperature dropped to 41 degrees in Houlton shortly before midnight, Victor Nouhan of the National Weather Service in Caribou said Sunday. That tied the record for lowest temperature set two years ago.

The cool air didn’t stick around long, Nouhan said. Low clouds moved in early Sunday and ended the cooling trend.

Temperatures are expected to be closer to normal this week, creeping up into the 80s by Friday. Skies are expected to be partly cloudy through midweek as the humidity continues to rise.

Sunday’s sunny skies brought some relief from last week’s six-day stretch of dense fog, which blanketed the state’s coast. Forecasters were reluctant to predict whether it would return.

All told, the National Weather Service recorded 18 fog days in the first three weeks of the month. Ten of those days featured dense fog, with visibility limited to less than a quarter of a mile. July’s average is five days of dense fog.

There are a number of factors, but one is colder water after a harsh winter. “Because of cold ocean water, we receive more fog and more dense fog at times,” said Tom Hawley, a meteorologist with the National Weather Service in Gray.

Tugboats were at a near standstill last week. Brian Fournier, president of Portland Tugboat LLC, said the low-visibility weather stranded four cargo vessels inside and outside the harbor over the past five days. Delays in entering and leaving the harbor have reached 24 hours.

“It’s a nuisance,” Fournier said. “The fog drains you, especially working in the fog at night – that makes your job that much more difficult.”

The inclement weather continues to affect tourism.

Larry Legere, a shore-side operations agent with Casco Bay Lines, said July ferry business is down 16,000 riders from last year because of the foggy and generally overcast summer.

“I’ve been in this business 32 years,” Legere said. “[The fog] is as thick as summer people talk about back in the ’60s and ’70s.”

The Associated Press contributed to this report.