Three children from a care home are missing on moorland after they became separated from a group on a field trip.
The boys, aged nine, 12 and 13, were last seen at 4pm on Wednesday during a visit to the Coldwell Reservoir between Widdop, near Nelson and Hebden Bridge.
They were with a party of two adults and five boys from The Nook children's home in Knott's Lane, Colne.
The boys did not have waterproofs and would have been in "horrendous" conditions if on the moor overnight.
Five children originally went missing after a visit to the Coldwell Activity Centre, but two returned to the group.
"If they are not in shelter they are going to be very, very cold when we find them," a spokesman said.
The area was cold and windy on Wednesday night, with heavy showers and poor visibility between 9pm and 3am on Thursday.
A Lancashire Police spokesman said the children are in care at to The Nook school, but none of them are residents there.
They are believed to be from addresses in Macclesfield and Chester, Cheshire, and Bury, Greater Manchester.
"We're going to the local areas to see if they've made their way back there," a spokesman said.
The search was called off at 4am on Thursday but began again just after dawn, with 60 members of three mountain rescue teams, two police helicopters, officers from Lancashire and West Yorkshire Police and dog search teams.
The grandfather of one of the boys told the BBC he was extremely concerned for their safety.
"I just hope he's gone to Burnley or something and he's gone to somebody's house," he said.
"I don't think they realise the consequences."
The nine-year-old is described as 4ft 5in with dark hair, wearing green combat trousers, a beige t-shirt and white and orange trainers.
The 13-year-old is 5ft 2in also with brown hair and was dressed all in blue, including a baseball cap.
The 12-year-old has blond hair and was wearing black tracksuit bottoms and a dark t-shirt with striped sleeves.
A spokeswoman from The Nook, an independent school for boys aged between eight and 16, refused to comment.