When we think of fog in New Zealand we usually think of big highs and calm weather – however lately around three low pressure systems have helped produce thick fog that have cancelled and delays hundreds of flights – mostly from Auckland, and made for slow commutes across the city.
WeatherWatch.co.nz head weather analyst Philip Duncan says while fog associated with lows isn’t the norm, it does happen when the lows are big enough.
“Some of the lows that have crossed over Auckland lately have been very large – not especially deep and aggressive, but certainly large in size. That basically means the centre is very calm with no winds. Add on top of that the rain that falls before the low and fairly high air pressure then you have a good recipe for fog”.
The air pressure with the current low is around 1008hPa. Most storms drop below 985Pa.
“Normally the centre of lows are where the isobars are packed together – that means windy weather and that blows fog away. But the lows over July especially have been as large as highs – not aggressive, but big and slow with fairly high air pressure too”.
Mr Duncan says a big portion of the windy weather this week was caused by the big high to the east coupled with the low – the low itself hardly capable of producing intense winds with such high air pressure.
As for the heavy rain, that was caused by a funneling effect of air between the large low to the north west and the high to the east. “When we have a low to the west and a high to the east it pulls down sub-tropical air over New Zealand and no matter what time of the year it is that can lead to very high humidity, up to 100%”.
Conversely, a high to the west and a low to the east can produce the opposite wind flow – big, wintry, southerlies.
Mr Duncan says there is a risk of some fog patches early on Wednesday north of Waikato but the risk is reduced as a sou’easter is expected to develop.
WeatherWatch.co.nz says any possible fog will unlikely linger all morning.
on 24/07/2012 11:31pm
I have been living in Auckland for over 10 years now, and I have found that the overall incidence of fog caused by highs (‘normal’ fog) and fog caused by lows is probably about 50/50.
Whilst fog caused by highs tends to be restricted to the colder months (April – October) and usually is a ‘morning’ thing, the fog caused by lows can happen at any time of the year, at any time of the day.
Whilst we know that for fog to form, not only must there be little wind, but the air temperature must drop below the dew point temperature. But what I have noticed is this can also happen after the passage of a ‘warm’ front from a tasman sea low. When this happens the temperature and humidity rises, the wind drops, the rain stops and the skies clear. If all this happens in the afternoon or evening, then the clear skies offer the opportunity for the air temp to drop before the sun rises. Fog can form pretty quickly in this situation.