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Warmer week ahead

Despite a cool change currently moving up the South Island most of New Zealand is in for a much warmer week than last week predicts WeatherWatch.co.nz.

Head weather analyst Philip Duncan says conditions look warm with today’s southerly very short lived.  “The southerly is a very weak one and while today in the south and east of the South Island will be affected by a cooler southerly it will be nothing like last week”.

10 days ago a cold southerly came through bringing snow to the mountains and frosts as far north as Central Plateau.  The chill in the air remained for a number of days before easing gradually as last week ended.  Today’s southerly isn’t strong with winds dying out as quickly as this afternoon.

“By Wednesday warmer winds move back into the South Island while the North Island remains warm, even hot, with highs in the low to mid 20s”.

Overnight lows have also slowly crept back up to average levels following last weeks cold snap.  They are expected to remain where they are for most centres for the rest of the week.

Another front will move in to the south west of New Zealand on Wednesday and Thursday with conditions predicted to be settled almost everywhere by the weekend – although rain may affect the upper North Island on Sunday as a low from the northern Tasman Sea approaches.

– WeatherWatch.co.nz
– Image – File, Taranaki on a sunny day / Greg Taylor

Comments

Guest on 14/03/2011 9:49pm

I understand that low pressure is caused by rotation and a thinner layer of atmosphere at the low’s centre. So why isn’t this centrifugal affect seen in high pressure systems, afterall the atmosphere is spinning too but in the opposite direction?

WW Forecast Team on 15/03/2011 1:21am

Hi there,

Thanks for the question.  We thought such a scientific question should be answered by a scientist, so we asked friend of the site Dr James Renwick from NIWA for an answer – hope this helps!

– WeatherWatch.co.nz

Dr Renwick:  It sounds to me like the questioner is saying that the atmosphere is "thinner" because there’s rotation. And if so, wouldn’t that happen with any rotation, including anticlockwise around a high?
 
The answer is that it’s not the rotation that gives rise to the low pressure, it’s the other way around. Air moves in towards lower pressure, and as it does so, it begins to move clockwise (in the Southern Hemisphere) because of the Earth’s rotation. Air near the centre of the low pressure is rising, helping to create a region of high pressure several kilometres above the surface low pressure centre.
 
In an anticyclone (a high), air’s descending and spreading out from the high pressure centre. Again because of the Earth’s rotation, the air moves in an anticlockwise direction as it spreads out.
 
Understandable I hope.  Your correspondent could also check out
 
 
– one paragraph of which reads:
 
Perhaps the most commonly encountered rotating reference frame is the Earth. Because the Earth completes only one rotation per day, the Coriolis force is quite small, and its effects generally become noticeable only for motions occurring over large distances and long periods of time, such as large-scale movement of air in the atmosphere or water in the ocean. Such motions are constrained by the 2-dimensional surface of the earth, so only the horizontal component of the Coriolis force is generally important. This force causes moving objects on the surface of the Earth to appear to veer to the right in the northern hemisphere, and to the left in the southern. Rather than flowing directly from areas of high pressure to low pressure, as they would on a non-rotating planet, winds and currents tend to flow to the right of this direction north of the equator, and to the left of this direction south of the equator. This effect is responsible for the rotation of large cyclones (see Coriolis effects in meteorology).

James Renwick, NIWA

Guest on 15/03/2011 1:48am

Thanks for the excellent explanation, I really appreciate it!
I guess one answered question often prompts another !!
The motion of air is out of a High, so how is it that high pressure systems can have such a long life, given that there would be a limited amount of air available to flow out?

WW Forecast Team on 15/03/2011 4:17am

Actually air pressures around the planet are even – so a very intense high needs to be counteracted by a a deep low, or series of low pressure systems, to even it all out.  Some highs – the blocking ones – can last months (causing droughts) but there are also lows that can linger around the Southern ocean or tropics for many days or even weeks.

Cheers

Philip Duncan

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