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Questions and Answers

Unread post by NZ Thunderstorm Soc »

I thought I would start a new topic on questions and answers to do with the weather.

My first question is :-

Why do you get NW Arches in some NW conditions and not in the others, like today?

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Re: Questions and Answers

Unread post by NZstorm »

The lack of a NW arch points to dry air aloft. The flow was more westerly today by the looks , but very dry anyway.
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Re: Questions and Answers

Unread post by Michael »

Does the south island get arches on the westcoast in southerlies? Is the westcoat towns too close to the alps unlike christchurch.I seen them in Napier in NW.
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Re: Questions and Answers

Unread post by NZ Thunderstorm Soc »

Michael wrote:Does the south island get arches on the westcoast in southerlies? Is the westcoast towns too close to the alps unlike christchurch.I seen them in Napier in NW.
Yes, they do. Mike :smile:

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Re: Questions and Answers

Unread post by AndyWX »

Interesting question John. I am thinking that the incident angle of the airflow approaching the Southern Alps has alot to do with it. To get a perfect arch I think the air has to cross the mountains at pretty much a right angle to the centreline of the range. As Steven says the airflow was more westerly today which would result in leewaves less suitable for the formation of altocumulus lenticularis that produces the arch effect. However I don't think this would be the full story and Steven would also be right about the dry air aloft. The altocumulus forms in the crests of the leewaves when air rises and cools below the dewpoint but if there is insufficient water vapour present the air will start to descend towards the next trough before condensation takes place resulting in no cloud. These are just my thoughts, I may be wrong so will be interested to hear what others think.
Last edited by AndyWX on Sat 03/10/2009 14:55, edited 1 time in total.
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Re: Questions and Answers

Unread post by Hypno »

How do you have to go until you are unlikely to get snow. when its snowing to sea level is it likely to be snowing on the top of mt Cook?
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Re: Questions and Answers

Unread post by AndyWX »

We have a similar situation again today here in Canterbury, wind from the west but only cloud on and just to the east of the mountains. Looking at the sat pic loops for the morning it can be seen that the air approaching the mountains is coming from down south and is unstable (plenty of large cumuloform clouds) making it cooler and drier (cool air holds less water vapour). So it does seem likely that the lack of altocumulus lenticularis is indeed due to a combination of the airflow not crossing the mountains at right angles to their centreline (ie from the northwest) and the fact that the air is dry. The instability of the air may also affect the wavelength and amplitude of the leewaves which would have an effect. The 2330z Vis sat pic shows that the edge of the cloud on the mountains is well defined and it is completely clear way out into the Pacific Ocean to the east. It looks like that the moisture being dropped on the mountains is enough to dry out the air to a point where there is insufficient left to form cloud downwind in the leewave crests.
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Re: Questions and Answers

Unread post by Storm Struck »

Hey Andy your from John Gauls favourite spot :lol: Ladbrooks area where a tornado touched down on October 27th 2002.
Great spot there for storms in November- mid January.
Canterbury, home of good rugby and severe storms
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Re: Questions and Answers

Unread post by Richard »

A interesting topic you have raised here John,here's what i think.
It appears to me that these arches are created by air subsidance within the leading warm front section of weather systems while moving over NZ and then dissipate or move to NE with the encroachment of the more active cold front part of the system.
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Re: Questions and Answers

Unread post by AndyWX »

Yes, you would be right there Richard in that the wind direction before a cold front is often northwesterly which then backs to the southwest after the front has passed. The northwest wind before it gets to the mountains is warm and moist as its coming from up north and this coupled with the fact that it crosses the Alps perpendicular to the main divide line means that plenty of orographic cloud (which creates the arch) is formed. After the front passed the air is crossing the main divide at an angle creating less pronounced leewaves and it is also colder and drier so less water vapour is available for cloud formation. An interesting thing that can happen in ChCh during a northwesterly is the formation of whats called a lee trough northeasterly. The northwest wind basically partly flows around the mountains and through Cook Strait and curves back around off Kaikoura and comes in at ChCh as a northeasterly which wedges in under the northwesterly. On these occasions there can be a northwest arch but a moderate northeasterly blowing in town.
Jason, I missed that tornado. Murphy's law but I have been trying to spot one for many years but always end up being in the wrong place at the wrong time.
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Re: Questions and Answers

Unread post by NZ Thunderstorm Soc »

Thanks for all your comments,fellas, on this question.
As Steven W from Auckland pointed out, it could be of the dry air aloft. I noticed that today, the air was possibly drier as any cloud forms that did eventuate, as they drifted across, seemed to dissapate from the NW or west/NW wind that did blow across today.
What clouding that did occur, the glaciation process from any colder air aloft seemed to result in possibly showers offshore, as noted by Ben (Tich)

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Re: Questions and Answers

Unread post by RWood »

I remember years ago Alex Neale or someone from the Met. Service wrote a paper on 3 types of northeasterly on the Canterbury coast - one of which was the "return flow" type described very well above.
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Re: Questions and Answers

Unread post by AndyWX »

Yes Rupert, there has been a reasonable amount of research done on the topic, a significant amount by the Geography Dept at the Uni of Canty. The 3 types of NE you referred to are called the Lee Trough NE (which you called return flow), the Seabreeze NE, and the Gradient NE (which is due to mesoscale weather systems). This is why the NE in ChCh is the dominant wind as there are 3 ways it can happen, and they can reinforce each other. During the summer you will often get a northwesterly situation that produces a lee trough NE at ChCH which is then further enhanced by the seebreeze effect. It can get quite strong and annoys people because it dramatically cools down a day that starts off well and promises to be nice and warm. Nice web site. I noted that one of your photos (Blenheim to Picton Road) shows some nice altocumulus lenticularis which is the same type of cloud that forms the NW arch in Canterbury.
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Re: Questions and Answers

Unread post by Richard »

AndyWX wrote: The northwest wind basically partly flows around the mountains and through Cook Strait and curves back around off Kaikoura and comes in at ChCh as a northeasterly which wedges in under the northwesterly. On these occasions there can be a northwest arch but a moderate northeasterly blowing in town.
I do agree with what you are saying Andy in that its generally accepted that Southern Alps create a blocking effect,if you take into account that the Alps height starts to descend from Arthur's Pass northwards,this is why North Canterbury is renowned for being an area that experiences strong and sustained periods of NW winds that the rest of eastern Canterbury, other than areas immediately down wind from the main rivers gorges dont experience to the same degree.This would also explain why South Canterbury being directly down wind from the highest section of the Southern Alps receives far less NW winds than the rest of eastern South Island, except for the Kaikoura Coast line which is sheltered by Kaikoura Rangers.
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Re: Questions and Answers

Unread post by RWood »

AndyWX wrote:Yes Rupert, there has been a reasonable amount of research done on the topic, a significant amount by the Geography Dept at the Uni of Canty. The 3 types of NE you referred to are called the Lee Trough NE (which you called return flow), the Seabreeze NE, and the Gradient NE (which is due to mesoscale weather systems). This is why the NE in ChCh is the dominant wind as there are 3 ways it can happen, and they can reinforce each other. During the summer you will often get a northwesterly situation that produces a lee trough NE at ChCH which is then further enhanced by the seebreeze effect. It can get quite strong and annoys people because it dramatically cools down a day that starts off well and promises to be nice and warm. Nice web site. I noted that one of your photos (Blenheim to Picton Road) shows some nice altocumulus lenticularis which is the same type of cloud that forms the NW arch in Canterbury.
Thanks Andy! I take it (to perhaps be blindingly obvious) that you have a present or past background with Met Service or NIWA? Are you a member of the Met. Society? Those 3 "NE titles" certainly ring bells, in fact I was going to hazard a guess at remembering their names but lacked the confidence.

Re the Blenheim-Picton Road pictures, the expected happened in the Sounds - slight gusty NW with rather lowish cloud, then out in the Strait, a really great sunset which I found difficult to capture. The shots taken at Ashburton, all the way down to Cook/Aoraki, were on a day when I couldn't see a single cloud from dawn at Christchurch till late in the afternoon, when a small finger of ragged stuff was showing near Mt Sefton. A wonderful "big sky" day.
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Re: Questions and Answers

Unread post by AndyWX »

Hi Richard
Yes the complex terrain of the Southern Alps has a major effect on the northwester (as it does on all winds) and will result in different areas experiencing different effects. Although some of the airflow does go around the mountains a great deal still goes over the top. The height of the main divide (as you say) and the shape and orientation of the valleys will enhance the wind in some places and diminish it in others. If you had a close look at the nature of the terrain west of North Canterbury you may see, in addition to the lower main ridge height, other terrain factors that would enhance the airflow in that region. I had a quick look on Google Earth but haven't made any conclusions yet.
Hi Rupert
Yes I did work for the Meteorological Service as a technician and technical officer back in the eighties and have done some research work at Canterbury Uni on surface winds. If you are interested there are a number of papers around on the nature of the winds in Canterbury. There is alot that is still not known though of course.
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Re: Questions and Answers

Unread post by RWood »

Thanks Andy!
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Re: Questions and Answers

Unread post by Weetbicks »

Hi all, I'm new to the forum and am eagerly trying to learn as much about weather and forecasting as I can via online resources. I would put myself at a beginner level. I'm still trying to "understand" some of the basics (because unless I fully understand it I feel I won't progress).

One question that I've always had, and never been able to explain is to do with air pressure. I've read quite a bit so far on air pressure and so forth, but one thing is just stumping me so here goes:

On tv, when you see a low pressure, the isobars around the low pressure area have low pressure values, ie 980, 970mb and so on. The number sounds low, which would make sense going along with it being a "low" pressure. Similarly, a high pressure, the isobars have higher numbers ie 1000+mb.

Now, given what I've read and my experience, lows are associated with cold weather, and highs with warmer weather. Cold air is more dense than warm air. If cold air is more dense, my assumption is that MORE cold air can be present within a given area above the surface. Conversely, a high pressure being associated with warm weather, means the air around a high is less dense, and thus LESS air can be present within a given area above the surface. If there is more air in a low, wouldn't that mean HIGHER air pressure, and if there is less in a high, wouldn't that mean LOWER air pressure?

Also, a low is associated with a sinking motion of cooler air, wouldn't that put more air pressure on a given surface point?

Am I totally confusing myself here? My probe is I can't seem to attribute low barometric pressure readings with what I know of a low pressure system, and the same goes for a high.

Can someone please set me straight in very basic terms why the isobars have lower pressures in a low and most importantly WHY the pressure is lower than found in a high? (basically why does a low pressure have low pressure and vice versa).

Sorry if this is so basic for most of you, but it's something I really want to understand and have a good foundation on before I go any further,

Thank you for your time.
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Re: Questions and Answers

Unread post by David »

Well, I am no expert, but I can help with some basics :)
In a low pressure system, air spirals clockwise (in the S Hemisphere) towards the low center whilst rising (hence lower pressure at the surface). In an anticyclone the air spirals anticlockwise away from the center, and is generally descending (hence higher pressure at the surface)
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Re: Questions and Answers

Unread post by NZstorm »

The air in a low pressure system is rising whereas the atmosphere in an anticyclone is subsiding. This is what creates the suface air pressure. And every weather system brings its own thermodynamics and so don't get locked into the idea that low pressure systems are cold and highs are warm.
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Re: Questions and Answers

Unread post by Michael »

Lows have a warm sector(mid lat ones) and have a cold and often a warm one is where the warm air comes in then the cold air often sits in the core and dragged in with the eastern side of the low also a low can fill ie become shallow when the warm front overtakes the cold one and is occluded? and the low becomes often rainy in the centre and winds slacken,in a high the air subsides and can form cloud or clear skies,In winter a high is often cooler than a low.In a tropical low its warm air makes it go and has strong winds close to the centre and light winds in the centre.hope it helps,others can elaborate.
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Re: Questions and Answers

Unread post by Weetbicks »

Great, thanks for the responses all! So basically I think my problem is I'm associating highs with warm temps and lows with cold but that is not necessarily the case? The fact that air is rising in a low and subsiding in a high does make sense to me now in terms of the air pressures associated with both types of cyclones.

Am I correct then in assuming that since in a Low, air rising is what causes it to condense and form lots of clouds and thus rain, while in a high, subsiding air suppresses cloud formation which causes characteristic lack of clouds and rain with highs?

One final question too, if air rises in a low pressure, does this have anything to do with that air being warmer than it's surroundings and thus rising? (vice versa for high pressure) ?
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Re: Questions and Answers

Unread post by NZstorm »

The rising air in a low pressure system is a dynamic process caused by upper level divergence. Also, the fact the air is rising means cooling does take place and yes, cooling leads to cloud and rain formation.

If the surface air of a low is warmed sufficiently, then clouds can form thermodynamically. This rising air is convection. We see this sometimes with cold airmasses over our relatively warm seas in winter or afternoon heating over land in summer.
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Re: Questions and Answers

Unread post by AndyWX »

Hi Weetbicks
You're on the right track now, lows are often due to warm air rising due to it being more bouyant than surrounding air. As you say, warm air becomes less dense, and it will move upwards until it reaches a level where the surrounding air is the same density. As it rises it cools because it expands, due to the decreasing atmospheric pressure as you go up, and at a certain height the dew point will be reached and cloud will form. The amount of cloud depends on how moist the air is to start with but in general you get plenty of cloud with a low pressure system. When the rising air gets to the level where it is in equilibrium with the surrounding air it will spread out sideways, eventually cool and then sink back down to the surface at other locations where highs form. The cool sinking air within a high is being compressed and therefore warms (have you pumped up a bicycle tyre?) and decreases the relative humidity and dries up any cloud. Because the air is cool to start with and it previously dumped most of it's moisture when it rose, not much warming has to occur to completely disperse any cloud. Hence highs are associated with clear skies most of the time (not always). Now because in a low the air has expanded and some has moved sideways the atmospheric pressure at the surface is decreased because there is less air above that point and therefore less weight force pushed down. The opposite occurs in a high, there is more air above. Don't forget that the weather maps you see on TV are mean sea level maps and only indicate conditions on the surface. All these processes occur in the troposphere, the lower layer of the atmosphere. You may say the air is warm on a fine day but it is only the air near the ground that is warm. You observed correctly that lows are often associated with cold weather. This is because they often form at the frontal boundary between advancing cold air from down south and warmer air to the north. The isobars are arranged so the the wind is from the south. You were confused in thinking that it is the air within the low itself that is the main cause of the cold.
Hope this helps. I have used your question as an excuse to see how much weather theory I am able to remember.
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Re: Questions and Answers

Unread post by Weetbicks »

Wow, that is an awesome response thanks Andy, you've really made it very easy for me to understand, cheers !
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