Tonight’s low in Tauranga is 17C. Tomorrow, Tauranga’s high is…17C.

The humid sub-tropical flow is moving down across the upper North Island with thick cloud, limiting how warm the days get but significantly increasing the overnight lows.

A bit like how closed curtains can stop the sun from warming your home – but also at night can keep that daytime warmth trapped inside.

Tauranga’s low of 17 tonight and high of 17 tomorrow is a great example of this. Whangamata, further north in the Coromandel Peninsula, also has a low of 17 tonight and high of 17 tomorrow.

Auckland has about a 2 degree difference from overnight tonight to Thursday’s maximum temperature, as does Thames.

Whangarei has a 3 degree difference.

Further down NZ and in Napier tonight’s low is 16, tomorrow’s high is 19 – a 3 degree difference. New Plymouth on the other coastline has a low of 16 tonight and a high of 18 on Thursday, only 2 degrees separation.

Wellington has a 3 degree difference from night to day – although Wellington often has a smaller margin between daytime highs and overnight lows due to windier weather overall keeping temperatures fairly steady – not too hot, not too cold.

Only 3 degrees separates Nelson and Greymouth’s overnight low tonight with Thursday’s high (15C & 18C).

But the eastern South Island has more ups and downs. Sunny nor’westers in Christchurch means the low gets down to 12 and tomorrow’s high is 24, maybe even a few degrees higher than that in some suburbs. At least 12 degrees difference between night and day.

In fact by Thursday night Dunedin also has just 3 degrees between overnight low and daytime high – but this is due to a cooler change that comes through. Thursday’s low is +7C and Friday’s high is only 10C at this stage.

By Philip Duncan, head forecaster WeatherWatch.co.nz / RuralWeather.co.nz