Mountaineering experts have criticised irresponsible walkers who are ill-equipped for challenging conditions on the Cumbrian fells.
Arctic survival specialist Craig Palmer described some of the sights he had seen as "crazy".
In some cases, whole families were setting off in inadequate clothing.
The former marine commando makes a daily ascent of Helvellyn on alternate weeks to gather information for Lake District National Park's Weatherline.
On Saturday, a strong wind chill factor led to temperatures of -16C (3.2F) on Helvellyn's 3,114ft (950m) summit.
'Very dangerous'
Mr Palmer, an outdoor guide and instructor, who is qualified in Arctic survival, said: "We've seen some crazy sights over the Christmas holidays.
"One walker was wearing wellies, jeans, t-shirt and a fashion jacket on Swirral Edge, a challenging route at the best of times.
"A lot of family groups are setting out in sunshine on the valley floor and find themselves freezing cold as they head up the hill.
"There is virtually no snow so people are getting on to the tops relatively easily.
"However, without warm clothes and the right gear they are putting themselves at great risk.
He added: "It's fantastic being out on the hills in this bright, clear weather.
"But people have to realise that bitterly cold conditions on the tops can be very dangerous if they are not wearing full winter gear."
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