"That means clear skies across Hawke's Bay which means maximum radiation, so the temperatures we feel is the earth's radiation going back out again.
"When there's clear skies, there's no trapping of that radiation, it just shoots out and temperatures drop the most under these conditions. It's usually just before sunrise that you get the coldest temperatures."
Keyser said there was no sign of rain on the radar and the frosty conditions would continue until Sunday.
"We're going to be dealing with these temperatures for a wee while yet, but early next week Hawke's Bay will encounter some cloud which will make the temperatures a bit warmer - but there's certainly no rain in sight as of yet."
Keyser said the morning temperatures would move from 0C to at least 5-6C next week.
With the lack of rain, it's been the driest June for Hawke's Bay, along with the rest of the North Island, since 2014.
A total 79.8 millimetres of rain fell in Napier between June 1 to 28, a figure that was "very little", according to Keyser.
"On the 13th of June 12.8mm fell in that one day, but there were only four days of rainfall in Napier for that month."
Last year 113mm fell in Napier during the same time, with 14 days of rain in total.