"It creates the foehn effect, where the wind warms and dries the air as it makes its way over the ranges. It can also get quite warm when there is a westerly wind, seen [yesterday], but not as warm as a northwesterly because there is less land for the wind to travel across."
Yesterday Napier peaked at just over 27C, while Hastings also hit 27C about 2pm. Splash Planet manager Peran Hutchings said the hot weather was great for business and tourists.
"It's been the same sort of busy weekend, the last few weekends have been quite busy."
Splash Planet closes for the summer on March 30 and is open Saturdays and Sundays.
Mr O'Connor said for a northwesterly to reach Napier and Hastings the wind needed to be "quite strong" to push its way over the hills.
Temperatures are forecast to be cooler today as a southwesterly brings a high of only 22C in Napier and Hastings, while overnight lows plummet to 11C.