An Austrian climber was thought to be missing on the upper reaches of Denali/NPS file

Days of silence from a 35-year-old Austrian climber high on Denali has prompted an aerial search of the mountain by rangers at Denali National Park and Preserve.

While Matthias Rimml, a professional mountain guide from Tirol, was not considered overdue, the search was launched after a friend hadn't heard from him since April 30. At that time, Rimml indicated he was just below Denali Pass, which is located at 18,200 feet elevation on the West Buttress, a park release said.

Rimml, who set off on his own from the 7,200-elevation Kahiltna Basecamp on April 27, reported being tired, but he was not in distress, the release added. It was unknown whether he intended to climb higher or return to his camp at 14,000 feet.

Rimml came to Denali already acclimatized to altitude due to recent climbs. His plan, the park noted, was to climb alpine style, or travel fast with relatively light gear, and reach the summit in five days.

The Austrian, who was carrying ten days of supplies, was the first registered climber to attempt the 20,310-foot peak this season. He was said to be alone on the upper mountain, with all other teams, including the first National Park Service ranger patrol, camped below 14,000 feet.

A friend of the climber grew concerned about his condition after several days of silence. Rimml had been checking in regularly, the park release said, but that ended on April 30. His friend contacted park authorities on May 3.

On May 4, the NPS helicopter pilot and a mountaineering ranger, already intending to shuttle camp gear to the 14,200-foot basin, flew the route to look for signs of Rimml. Intermittent cloud cover prevented a thorough search of the route, nevertheless they did not see any signs of Rimml. Searchers did observe his tent site at 14,000 feet, however no signs of recent activity were visible. The helicopter was unable to land due to deteriorating weather and wind.

The aerial search continued Thursday, and favorable weather allowed the helicopter to land at the tent site. Rangers confirmed Rimml had not returned to his camp. Clouds on the upper mountain prevented the aerial search team from flying above 17,200 feet.

Temperatures at the upper elevations on Denali have been cold this past week, reaching daytime highs between -25 F and -30 F, as is typical in the early season, the park reported. Wind speeds above 17,200 feet were unknown, but weather stations at 7,200 and 14,200 feet indicated mild to moderate winds up to 30 mph. An estimated 5 inches of new snow have fallen on the upper mountain since Saturday.

Aerial search operations of the upper mountain were to continue as weather conditions allow.