In summer, extreme humidity – high and low – can send us round the bend. So what is it, why does it make us feel so bad, and how can we beat it?
"Humidity," explains Dr Mike Revell, NIWA's Principal Scientist, Meteorology, "refers simply to water vapour suspended in the atmosphere.
"Sometimes it's high – like on foggy mornings or those 'sticky' summer days familiar to people living in the north of the North Island – and sometimes it's low, like when the nor'wester blows in Canterbury.
"Humidity is always there; there's always some water vapour suspended in the air."
It's the combination of extreme humidity and high temperatures that can really play havoc with our wellbeing, says Dr Revell. That's why summer is the time to beware of humidity's more unpleasant effects.
"The higher the air's temperature, the more water vapour it is capable of holding. So on a hot summer day when the humidity is high – or in the tropics at any time of the year – there's a lot of water vapour out there interfering with the way our body functions.
"And a hot day with very low humidity feels different – but can be just as debilitating."
The term 'relative humidity' is the actual amount of water vapour present in the air divided by the maximum amount the air could hold at its current temperature, usually expressed as a percentage.
"So when relative humidity gets up into the 80s and 90s, you know the air is nearly as full as it can be with moisture. On the other hand, it can sometimes fall as low as 20 or 30 per cent," explains Dr Revell.
Natural air conditioning
To understand how extreme humidity and high temperatures affect us, we need a quick explanation of how our body's natural cooling system works:
When our body temperature rises, we sweat. The water on our skin then evaporates into the air and, "if you recall your year 11 science," explains Dr Revell, "you'll know the process of evaporation draws energy from the air, reducing its temperature.
"That's how our body attempts to keep us cool. Evaporating sweat creates a thin layer of cooled air close to our skin.
"But when humidity is high and the air is already laden with moisture, our sweat can't evaporate as quickly and the cooling effect is reduced."
The result? That feeling like we're covered in a warm, damp blanket.
When extremely high temperatures and humidity strike, the result can be dramatic – and even dangerous.
"As our body works harder and harder to cool us down," explains Dr Revell, "precious water is diverted from our brain and internal organs, slowing them down and making them work harder. Our blood thickens, making it harder for our heart to pump it around our body."
Hyperthermia, or 'heatstroke', is the ultimate result of dehydration and an overheated body. Symptoms are unpleasant and potentially life-threatening – particularly among the elderly or unwell.
And as if that's not enough, high temperatures and humidity cause irritants like mould spores and dust mites to proliferate – creating misery for sufferers of allergies and asthma.
Beating the blues
When conditions turn hot and muggy, there are some basic precautions you can take to lessen the danger.
"The ultimate response is to stay indoors in air-conditioned comfort," says Dr Revell. "But of course that's not always possible or desirable.
"So the key is to reduce activity, pace yourself, and drink plenty of water to replenish your body's cooling system."
In New Zealand, prolonged periods of extremely high temperatures and humidity are mercifully rare. For most, bad hair is the most alarming effect we're likely to experience.
'The frizz' happens when abundant moisture in the air begins to break down temporary hydrogen bonds in our hair. The process allows more permanent sulphur bonds to prevail.
So in fact, high humidity allows our hair's natural tendencies to show through: if it's naturally inclined to curl or wave, high humidity will bring out the frizz; if it's inclined to be straight, a limp coiffure will greet us in the mirror on a muggy morning.
"Interestingly, human hair is so sensitive to humidity changes that it was once used as the active component in meteorological instruments called hygrometers, which graph changes in humidity over time," says Dr Revell.
The dry horrors
In the east of both islands, and in inland Canterbury and Otago, a combination of high temperatures and low humidity is a much more common occurrence.
"Here the effect is quite different," says Dr Revell. "Sweat evaporates off our skin much faster because the air has plenty of capacity to take up more moisture. As a result, the cooling effect is more pronounced. In fact, it's possible you'll feel quite chilly, even though the temperature may be in the twenties."
But dehydration is just as big a danger, he adds. "Cantabrians know to drink plenty of water when the nor'wester blows and the humidity drops, because moisture is being sucked up at such a great rate."
Dry lips, a dry throat and irritated skin and eyes are common problems on Canterbury's low-humidity days. What's more, dried-out mucus membranes in our nose and throat can ease the passage of bacteria and irritants into our system – prolonging the misery for some.
Static electricity build-up is another side-effect of very dry air. "Take care when reaching for the light switch or car door handle on low-humidity days," says Dr Revell. "A short sharp zap could be in store."
Of course it's not all gloom and doom. Warm, humid weather encourages luxuriant growth in the garden, while low humidity is great for getting the washing dry and enjoying a good night's sleep.
"We just need to understand humidity's effects and take some basic precautions," says Dr Revell. "Then we can get on with enjoying summer."
