South Wales Fire Service have seen their calls increase tenfold since the start of April, as they battle against a rising number of grass fires.
Crews have had 3000 calls, many over grass fires over the last six days, said the service's Mike Flanagan.
He appealed directly to youngsters who he said were largely responsible for are setting the fires not to do so.
"We would say 'Look it's just not cool to light a fire on the mountains, it really isn't'," Mr Flanagan said.
On Thursday night fire crews in south Wales attended 85 emergency calls dealing with grass fires alone, he said.
One was on a mountain at Nantyglo in Blaenau Gwent while in north Wales crews battled through the night on Wednesday to tackle blazes across 700 square metres of National Trust land in Conwy.
"A fire on a mountain can be as difficult for us to deal with as a medium sized commercial premises or a school or a large shop, " Mr Flanagan explained, adding one firefighter had collapsed with heat exhaustion on Friday night.
" It really does beggar belief that they are setting fire on their own doorstep"
"There's no doubt in my mind that every time the schools are on holiday - and it's almost seasonal, we almost know when we're going to get busy.
"It's young people who are going up the mountain sometimes on mountain bikes, sometimes on motor bikes deliberately setting a string of fires that we then have to deal with."
He said fighting the fires was depleting resources and killing a lot of wildlife and greenery in the area.
Mr Flanagan said the fire service would be co-operating with police who would be using helicopters this weekend to try to identify people in hotspots responsible for setting the grass fires.
"I don't think sometimes our young people think about the consequences of what they're doing but the knock-on effect to the fire service and to the community is significant, both financially and in terms of resources, "he said.
Nowhere to go
"There's a significant risk to people who go up the mountain and set fires.
"The winds can change and they can find themselves caught with nowhere to go."
Mr Flanagan appealed to parents and older brothers and sisters to discourage younger people from lighting these fires.
"The community belongs to all of us. It really does beggar belief that they are setting fire on their own doorstep," he added
His appeal follows one by Forestry Commission Wales warning people of the dangers of starting grass fires over the Easter holiday.
The commission said recent dry weather had "heightened" the risks.
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