Retired IT engineer Gerry Coyle, 65, who is originally from Glasgow, has not been seen since he made his way to the top of Mount Snowdon on Thursday, last week.
The keen amateur photographer took the narrow-gauge railway line that runs up the mountain to the summit, and sent a text message to his wife to say he had arrived.
However, it is thought that the father-of-two decided to walk back down the tourist path. He was last seen by staff at a cafe on top of the mountain at 1pm.
Mr Coyle made the trip on a dry day, but hopes for his safe return are now fading as the weather has deteriorated rapidly since he went missing.
Emergency services were alerted on Saturday and North Wales Police and the Llanberis Mountain Rescue Team, supported by the RAF Mountain Rescue and the Search and Rescue Dog Association Wales, have been searching the area.
His sister-in-law Josie Coyle said Mr Coyle was used to hillwalking but was not an expert mountaineer.
His wife Pat, their two sons and his two brothers have gathered at a hotel near the mountain to wait for news while the search is carried out.
Mrs Coyle said: "The family are facing a horrible waiting game. The weather has been really bad since Gerry went missing, with wind and rain on the mountain.
"It is looking very bad. It will be a week today since he has last been seen. Gerry is very organised and not one of those people who would just head off on his own. The last we heard he was on his way back down and there has been nothing since. It is like a nightmare."
Mr Coyle, who now lives in Rugby, Warwickshire, had taken a camera and tripod with him on his trip.
He had been staying at the Padarn Lake Hotel in Llanberis and left at 11am to walk to the summit of Snowdon.
Sergeant Rob Rands, of North Wales Police, said: "Mr Coyle left the hotel stating his intentions to walk to the summit and indeed, upon speaking with staff there, it appears he made it.
"However, he has not been seen since and with the passing of time, and the deteriorating weather conditions, we are increasingly concerned about his safety. I am appealing to anyone who has been walking the mountains over the last few days that if you have seen Mr Coyle to make immediate contact with the police."
At 3560ft, Mount Snowdon is the highest peak in Wales and the tallest mountain in the British Isles outside of Scotland.
The visitor centre at the summit, which cost £8.4 million, was constructed in 2009 on the site of a dilapidated structure once described by Prince Charles as the "highest slum in Wales".
Despite having a reputation as one of the safer mountains to climb in the UK, there have been a number of fatalities on Mount Snowdon in recent years.
In February 2009 four people, including experienced climbers Christopher and James McCallion, from Bristol, died in the space of eight days when the peak was hit by bad weather.
Temperatures at the summit this summer fell to 5C (41F), and so far the mountain rescue team has been called out on 183 occasions, making it one of its busiest years.
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