MetService has issued a heat alert for Auckland — driven by the warm nights ahead with “no relief in sight” for those sweltering in the City of Sails.

Explaining the alert, MetService told 1News it has been triggered by the warm nights, which are currently forecast between 20C or 21C until Monday.

Auckland’s single day threshold for a medium heat alert is 28C, and 29C for a high alert. But if the median temperatures across two days reach over 22C or 23C respectively, a heat alert is also triggered.

The forecast maximum temperature is currently sitting at 27C in Auckland, but with the minimum temperatures at night sitting between 20C and 21C, the median temperature is set for 23C or 24C, triggering the heat alert.

MetService general Meteorologist Alain Baillie said, unfortunately for Aucklanders, the warm and northeasterly flow will persist until Sunday, when a low crosses the upper North Island.

“This brings rain and a change in the winds but doesn’t remove the humid airmass over the region, so the muggy nights continue through the forecast period.

“Humidity, or more correctly the high dewpoint, is the chief driver of the current run of warm nights. The dewpoint is the temperature to which air needs to cool before water vapour condenses to liquid water.

“Effectively, the temperature cannot fall below the dewpoint, so if the dewpoint is consistently in the high teens or low 20’s, nighttime temperatures are going to be stuck above that level. The dewpoint generally drops a bit at night as dew forms on the ground, removing some moisture from the air, but that effect is currently limited by the northeasterly flow continually replenishing the moisture.

“Cloud also plays a role by locking in heat, keeping the temperature well above the dewpoint.”

To get past the technical talk, Bailie said the basic message is the heat alerts have been caused by warm nights, which are going to continue “for a while yet”.

During heat alerts, MetService recommended people keep hydrated, seek shade, check in with vulnerable people and look after pets and livestock.