It's hot and muggy, but last year it was worse

Prepare for two more hot, muggy days this weekend - but give thanks that the humidity of the past few days is still lower than it was a year ago.

The first week of school is usually the hottest in our calendar. The warmest day recorded in our history was February 7, 1973, when temperatures averaged 39.2C in the North Island and 42.4C in the South Island.

Yesterday's temperatures reached only 27C in Auckland and 30C in Hamilton, and humidity was three points below the usual summer peak.

"Normally the hottest weeks are the last week of January and the first week of February," said MetService weather ambassador Bob McDavitt.

"Once we get past Waitangi Day we can expect that, because the days are getting shorter and the longest day is now six weeks ago, we'll have gone over the top and start to go down."

Vineyard workers went home early in parts of Marlborough yesterday where a Havelock resident recorded 42C shortly after midday, and Molesworth Station was closed because of the extreme fire danger. The Warehouse spokeswoman Cynthia Church said a lot of stores had sold out of fans.

Mr McDavitt said the heat spike was caused by a "blocking" high-pressure system that had sat for the past few days southeast of the country. "They are not unusual and can hang around for some time," he said. "This particular high's grip over New Zealand will probably be released on Monday or Tuesday as a low-pressure trough goes through, so there will be a drop in humidity at that time."

Humidity, or the amount of water vapour in the air, is measured in "dewpoints" - the temperature at which water vapour condenses into dew or frost. This point of complete saturation of water in the air is usually reached at night at this time of year, making the dewpoint the same as the minimum night-time temperature. Mr McDavitt said summer dewpoints usually peaked around 23C, but were only 19 to 20C this week. "Once the dewpoint gets over 20, people start to notice the humidity and have problems coping with it," he said. "At a dewpoint of 21, 50 per cent of the population are feeling it. At 22, 90 per cent are feeling it. Just drink more water if you're feeling the humidity."

The cold front due on Monday or Tuesday is the same one that caused storms in Australia this week, but the MetService expects it to be much weaker by the time it hits New Zealand, and forecasts another spell of hot weather later next week.