The current weather pattern is feeding subtropical air onto the North Island.
Pohutukawa trees are revelling in the warmth and high humidity. This one, near Auckland Harbour Bridge, is already near full bloom and it will not be long before its colleagues follow, all around the coastal North Island.
On the weather map shown here, for 1pm on Wednesday in the middle of the first week of December, some typical signs of summer can be seen: The incoming high pressure system is, for a while, at near 45°S. The low pressure system in the Tasman Sea has been transporting some warm air from the subtropics on to northern New Zealand, raising our humidity. (Next week, the high is expected to return to its typical El Nino summer latitude of 35°S, so let’s enjoy some subtropical warmth when we can.)
The relative humidity has been “flat-lining” at 100% in many places all around the country this week. The bottom part of the graph shown here gives the relative humidity at Whangarei, Auckland, Wellington, Christchurch and Dunedin for comparative purposes. Out of all these locations, Christchurch was the place hugging 100% the most, so did it feel the most humid?
The answer is NO.
Relative humidity measures how close the air is to saturation (see here for more) but not necessarily how muggy it feels to people.
In cool air, saturation is reached at relatively low amounts of water vapour – when the air temperature is below around 8°C your breath
can often contain sufficient moisture to produce saturated air or misty cloud – at standard pressure that’s 6 g of water vapour per kg of air,
and this has been the amount of water vapour in Christchurch at times this week. Places such as Auckland or Whangarei have this week been having around 12 g of water vapour per kg of air – twice the ‘humidity’ of Christchurch, even when the relative humidity is 100%!
A better index for measuring mugginess is the dew point … the temperature at which, if cooled, water vapour from the air will condense on to a surface such as grass. This condensation requires 100% relative humidity and occurs when the air temperature equals the dew point.
The air feels muggy when we get hot and sticky – and that’s when 1) the air is warm enough to make us perspire, and 2) the air contains sufficient water vapour to interfere with the evaporation of our perspiration and not allow us to cool down much from that. So mugginess depends on both heat and humidity.
In the graph shown above the dew point is plotted on the upper table. Notice how Whangarei and Auckland have both been on top this week. They have often been having nearly the same dew point and so have been nearly equally muggy. However, only Auckland has, occasionally at night, been flat-lining at 100% relative humidity. Even though both places are equally muggy, Whangarei has slightly warmer air than Auckland and thus slightly lower relative humidity.
The dew point temperature is a great mugginess parameter, for it combines heat and humidity into one number. Wikipedia has more information about the human perception of mugginess depending on the dew point, ranging from extreme discomfort above around 24°C, to noticeably humid from 16°C-18°C, to noticeably dry below 10°C
I hope this has posting has helped to clear up any misconceptions between relative humidity and perception of mugginess. However, leave me a comment if you have any queries.
Jon Tunster of Wellington comments:
Here’s a photo of the same tree at Oriental Bay, Wellington taken on 6 Dec 2008, and a year later on 7 Dec 2009.
Oriental Parade Pohutukawa 7 Dec 2009
Flowering is considerably delayed this year, maybe due to the cold October?
Bob McDavitt answers:
The name for the study of plant (and animal) life cycle events in relation to the changing seasons is Phenology.
These photos dramatically illustrate the difference between the start of a La Nina summer (Dec 2008) and an El Nino summer (Dec 2009). The chilly air and soil in October is indeed a likely source of the delayed flowering of the pohutukawa.