Summer forecast: Hot weather to only get hotter

It's been a warm start to summer - and it's only going to get hotter.

Temperatures will be rising across the country over the next week - particularly in the South Island, which has been facing heat in the upper 20s and low 30s, NIWA forecaster Chris Brandolino told The AM Show on Monday.

"We're gonna see more of that pretty much the entire week."

Mr Brandolino also explained how the sun's intensity is measured and emphasised the need to get sun-smart.

"UV index is one way to measure [it] and its harmful effects - when it gets to about three, that's when it's advised that you wear sunscreen.

"That happens as early as late August-early September for the upper North Island, but this time of the year it gets to extreme levels greater than 11 for pretty much the entire country - you can get skin damage very quickly."

Mr Brandolino also explained why NIWA and other weather forecasters no longer talk about 'burn time' - how long it takes to get sunburned.

"We've gone away from burn time because that can be a bit misleading if you're fair skin or have darker complexion."

The sun reaches its maximum intensity between 10am and 4pm, but you can get sunburnt regardless of cloud cover. 

"You don't associate overcast with getting sunburnt, but the sun's that powerful." 

"Chances of a white Christmas?" asked host Duncan Garner.

"A sandy white Christmas? 100 percent," replied Mr Brandolino.