New Zealand’s summer has been defined by repeated bursts of intense rain, as subtropical systems have swept down over the upper North Island and beyond.

Floods, slips and storm damage – most recently in Christchurch – have dominated the headlines. But many of the season’s events have come with another feature we rarely talk about: the sheer amount of moisture in the air before the rain even starts.

Even by Auckland’s standards, recent humidity readings have been notable.

On February 2, the Whangaparaoa automatic weather station on the city’s North Shore recorded a temperature of 24°C with 98% relative humidity. That implied the dew point – a direct measure of humidity – also came close to 24°C, making for conditions not merely muggy, but oppressive.

A fortnight earlier, on January 21–22, humidity in Tauranga reached similarly sticky levels, with dew point measuring 20C to 24C. Over that period, a record-breaking 274mm of rain fell within 24 hours, triggering separate landslides that claimed eight lives.

While humidity is often treated as just an uncomfortable part of summer in the upper North Island, it can, like extreme rainfall, tell us something important about the warming state of our atmosphere.

It also presents a real risk to human health that can be overlooked when we........

© The Conversation