"Subtropical"

[J07]

I saw a chap on the TV last night going for a swim off the Scilly Isles, to demonstrate the existence of the gulf stream. Unfortunately for him, he chose to do it in Spring.

It had me thinking about the sort of climate that those little islands experience. So in the fashion of the lazy internet generation I consulted google. I could only find one weather station:

http://www.wunderground.com/NORMS/DisplayI...&Units=both

To summarise, February is 7C/8C whilst August is 14/18C. As expected, the climate is very moderate and max/min temperatures do not vary much.

In terms of sunshine hours:

http://www.scillyonline.co.uk/weather.html

Giving us 1805 hours per year (if my calculator works correctly). This is good for this latitude, and puts it almost on a par with places like Eastbourne.

That page also tells us the minimum temperature recorded is -5C, lower than I would have expected, to be honest.

Whilst looking up this climate I came across all sorts of travel pages raving about the "subtropical" climate of the Scilly Isles. Now, I have also come across these claims for the Torbay area.

As far as I can see from the statistics, the climate of the Scilly Isles does not strike me as "subtropical"- which is a word I've always associated with the lights of Brisbane and Orlando.

The summers are quite cool, but sunny. Winters, yes, are extremely moderate and frosts are rare. So, it comes down to:

How do we define "subtropical"?

I have heard varying definitions. Some that require a frost free area, whereas others, like the Koeppen definition look at the temperatures and rainfall. We can probably assume that the Scilly Isles very rarely get frosts.

The Koeppen definition asks for the coldest month to average over 0C. OK, basically *everywhere* in the UK satisfies this! It also needs constant year-round rainfall. I don't know rainfall stats for these isles but we can probably assume that they are met. However, we also need the warmest month to average over 22C. But, there is nowhere in the UK that comes anywhere near to this. It's hard enough finding somewhere that has daily maxima averaging over 22C. And, if anything, the Scilly Isles, with a 24hr mean of 16C, are much further from this 22C boundary than many other places.

Apparently, they have palms growing on the islands, along with other tropicals and subtropicals. Obviously, there's no doubt that these plants survive, but I seriously wonder whether they thrive. We see palms all over the place outside of the tropics. I suppose it depends on the species, as some can tolerate very cold temperatures, others can not.

So, what does subtropical really mean? Is the above definition fairly standard? Is "frost-free" a better requirement? Or, is it the case that there is no real standard? I just see that tourist boards can pick up on this, and that they can advertise their place as being subtropical, then the tourists come along one early afternoon in mid-August to be greeted by 14C and heavy rain.

Basically, is subtropical a weasel word, or does it have real meaning to meteorologists and climate scientists?

[SP1986]

I would say it probably needs to satisfy the following:

- Winter Temperatures not going below 0C or rarely going below 0C

- Summer Temperatures above 15C

- Year round humidity above 60%

- Sea Temperatures above 16C year round.

[J07]

I would say it probably needs to satisfy the following:

- Winter Temperatures not going below 0C or rarely going below 0C

- Summer Temperatures above 15C

- Year round humidity above 60%

- Sea Temperatures above 16C year round.

By summer temperatures, do you mean max, min or average?

I suppose the last point rules out anywhere in the UK though by a long way.

BTW, what's the highest sea temperature the UK waters will get to? About 17/18C?

[BARRY]

we have got overbecks just 3 miles as the crow fly's, which is full of sub tropical plants which survive and thrive.

[J07]

we have got overbecks just 3 miles as the crow fly's, which is full of sub tropical plants which survive and thrive.

How do you define "subtropical plant"?

[SP1986]

By summer temperatures, do you mean max, min or average?

I suppose the last point rules out anywhere in the UK though by a long way.

BTW, what's the highest sea temperature the UK waters will get to? About 17/18C?

I mean Max, but then again a place with temperatures below 10C as a summer average min it would be hard to class as subtropical.

The highest temperature for offshore waters is about 18C, then highest for inshore waters might be about 20-22C in shallow water

The other problem is that can you only label subtropical to areas in the subtropics (15 deg N/S of the tropic of cancer/capricorn)?

I think we can safely say that the UK has potential subtropical style weather, but will never be classed as subtropical.

However that doesnt mean that subtropical plants (USDA zones 9-10) cant be grown in this country.

However you could argue New Zealand is subtropical! (very similar climate the the Scilly Isles)

[J07]

I mean Max, but then again a place with temperatures below 10C as a summer average min it would be hard to class as subtropical.

The highest temperature for offshore waters is about 18C, then highest for inshore waters might be about 20-22C in shallow water

The other problem is that can you only label subtropical to areas in the subtropics (15 deg N/S of the tropic of cancer/capricorn)?

I think we can safely say that the UK has potential subtropical style weather, but will never be classed as subtropical.

However that doesnt mean that subtropical plants (USDA zones 9-10) cant be grown in this country.

However you could argue New Zealand is subtropical! (very similar climate the the Scilly Isles)

The upper limit of approx 30* latitude sounds quite sensible.

I have always marvelled at the US hardiness zones, we can get placed in quite a good zone because of our maritime climate, but I think we lack the real summer heat that allows subtropicals to truly thrive.

Off the top of my head, an example might be passionfruit. I know they will grow here, but will they fruit consistently? IME, they won't. In theory I believe we could grow avocadoes in certain areas, although the recent long period of frosts might cause them to drop their foliage and possibly never recover. Again though, I don't know that they would fruit. Likewise, tamarillos (the world's tastiest fruit!), although I believe they are even more marginal. The list goes on with bananas, tomatoes and eggplants. To greater or lesser extent, they can all grow here, but when it comes to thriving and fruiting, you may get lucky with some small bananas, and really I think we need more sunshine to properly ripen tomatoes and more heat in general for eggplants.

I should not complain too much as it's really great to see some of the things we can grow in our gardens here. The last 10 days may have really tested a number of the more underprepared specimens though!

I wouldn't really class New Zealand as subtropical, though the tourist brochures often disagree with me! I remember one region being described as such...then in winter I was crunching around on frost on the beach. This happened during the following spell of weather:

http://www.metservice.com/default/index.ph...eature_aug_2007

An interesting read; it was this that resulted in the NZ winter minimum of -15.4C and multiple frozen lakes for people to go curling.

Northland would be a classic subtropical area I suppose, with the very Northern tip having an annual mean in the region of 16.1C-18C. Though most towns there miss out on the Koeppen (the warmest month mean would only be about 20C) their winters are classic, averaging highs of something near 16C with mild nights and a lot of rain. In other words, the sort of wonderful wintry weather that the likes of WiB desire

I'm not sure how it compares to the Scilly Isles. I'm sure there's a random island somewhere off the coast that matches it very closely, but likely it wouldn't have a weather station!

So in piecemeal fashion :

If we're going on averages alone (Scilly annual mean is about 11.3C), then temperature wise the Chatham Islands (off the east coast at about 43S) would be almost spot on, but are not sunny enough.

Scilly summers seem about the same as those in Invercargill. Most populated NZ areas have average maxes above 10C in winter, you have to go inland and upwards to get single digit maxima but then you'd lose the humidity. Therefore I'd hazard that Scilly minima are similar to Auckland, with the maxima more like Fiordland (both are humid and wet).

[SP1986]

The upper limit of approx 30* latitude sounds quite sensible.

I have always marvelled at the US hardiness zones, we can get placed in quite a good zone because of our maritime climate, but I think we lack the real summer heat that allows subtropicals to truly thrive.

Off the top of my head, an example might be passionfruit. I know they will grow here, but will they fruit consistently? IME, they won't. In theory I believe we could grow avocadoes in certain areas, although the recent long period of frosts might cause them to drop their foliage and possibly never recover. Again though, I don't know that they would fruit. Likewise, tamarillos (the world's tastiest fruit!), although I believe they are even more marginal. The list goes on with bananas, tomatoes and eggplants. To greater or lesser extent, they can all grow here, but when it comes to thriving and fruiting, you may get lucky with some small bananas, and really I think we need more sunshine to properly ripen tomatoes and more heat in general for eggplants.

I should not complain too much as it's really great to see some of the things we can grow in our gardens here. The last 10 days may have really tested a number of the more underprepared specimens though!

I wouldn't really class New Zealand as subtropical, though the tourist brochures often disagree with me! I remember one region being described as such...then in winter I was crunching around on frost on the beach. This happened during the following spell of weather:

http://www.metservice.com/default/index.ph...eature_aug_2007

An interesting read; it was this that resulted in the NZ winter minimum of -15.4C and multiple frozen lakes for people to go curling.

Northland would be a classic subtropical area I suppose, with the very Northern tip having an annual mean in the region of 16.1C-18C. Though most towns there miss out on the Koeppen (the warmest month mean would only be about 20C) their winters are classic, averaging highs of something near 16C with mild nights and a lot of rain. In other words, the sort of wonderful wintry weather that the likes of WiB desire

I'm not sure how it compares to the Scilly Isles. I'm sure there's a random island somewhere off the coast that matches it very closely, but likely it wouldn't have a weather station!

So in piecemeal fashion :

If we're going on averages alone (Scilly annual mean is about 11.3C), then temperature wise the Chatham Islands (off the east coast at about 43S) would be almost spot on, but are not sunny enough.

Scilly summers seem about the same as those in Invercargill. Most populated NZ areas have average maxes above 10C in winter, you have to go inland and upwards to get single digit maxima but then you'd lose the humidity. Therefore I'd hazard that Scilly minima are similar to Auckland, with the maxima more like Fiordland (both are humid and wet).

I'd agree with all the above, and particularly about NZ. That said I think subtropical is difficult to define and may need to sectored into 3 groups such as: warm subtropical/average subtropical/cool subtropical. I think North NZ would fit into the latter.

Then there is actually subtropical or subtropical-like.

I think the Scilly Isles and Chatham Islands could put a strong case in for being subtropical-like.

It pains me to see that the Chatham Islands Nikau palm may struggle to survive if they keep getting lower temperatures. Maybe its time to introduce it to the Scilly Isles on a large scale, if to safeguard its future, and I think the Scilly Isles are the only place it will grow in the UK mainland (even though some where wiped out in the 80's), and have the best chance.

Scilly Isles rarely go below 6C in the most potent cold spell.

Tresco would be an ideal place to harbour some of NZ subtropical species. I would say Nikau is a subtropical species, as it is only hardy to winter minimums of -3C which I believe is Zone 10 (and it doesnt mind being in the dark conditions)

[J07]

I'd agree with all the above, and particularly about NZ. That said I think subtropical is difficult to define and may need to sectored into 3 groups such as: warm subtropical/average subtropical/cool subtropical. I think North NZ would fit into the latter.

Can you think of any examples? Say, North NZ is cool....maybe Sydney then average, and Brisbane warm. But then I think the gap between North NZ and Sydney is maybenot big enough. Perhaps, North NZ/Sydney would be cool subtropical, Brisbane average subtropical and Rockhampton warm subtropical.

Then there is actually subtropical or subtropical-like.

I think the Scilly Isles and Chatham Islands could put a strong case in for being subtropical-like.

I think most NZ'ers would scoff at that. Chatham Islands' weather is something of a minor joke (along with Southland and Coastal Otago). If I have my facts right, it's less sunny than London. Regardless of moderate temperatures, I just can't see somewhere with sunshine hours of 1400 claiming to be subtropical-like.

It pains me to see that the Chatham Islands Nikau palm may struggle to survive if they keep getting lower temperatures. Maybe its time to introduce it to the Scilly Isles on a large scale, if to safeguard its future, and I think the Scilly Isles are the only place it will grow in the UK mainland (even though some where wiped out in the 80's), and have the best chance.

Are their temperatures going down? Interesting...

Scilly Isles rarely go below 6C in the most potent cold spell.

Sounds reasonable, but their lowest recorded temperature of -5C is quite amazing. How did that happen?

Tresco would be an ideal place to harbour some of NZ subtropical species. I would say Nikau is a subtropical species, as it is only hardy to winter minimums of -3C which I believe is Zone 10 (and it doesnt mind being in the dark conditions)

It would be good, but maybe not truly ideal, though certainly the best place in the UK. I still don't really understand why they are so fussy in their native land. They seem to cluster in groves, and everyone talks about how hard it is to grow them and that they mostly don't bother. I heard this just 2 miles from where there was a fantastic stand of them. And it was flat, coastal plains around a North facing bay, sheltered by ranges to the south, east and west. Plenty of rain, plenty of sun, mild winters and warm summers. I just don't get it. I begin to wonder whether it has something to do with soil type, or perhaps in some places they just get crowded out as they are such slow growers.

[SP1986]

Can you think of any examples? Say, North NZ is cool....maybe Sydney then average, and Brisbane warm. But then I think the gap between North NZ and Sydney is maybenot big enough. Perhaps, North NZ/Sydney would be cool subtropical, Brisbane average subtropical and Rockhampton warm subtropical.

I'd hate to suggest something that is just a theory and has no bearing on reality, but I'd class Florida west keys as warm subtropical, the Azores in average subtropical, and well for the sake of argument lets keep North NZ as cool subtropical. There are lots of reasons to argue its not a valid way of dealing with the definition of subtropical though.

I think most NZ'ers would scoff at that. Chatham Islands' weather is something of a minor joke (along with Southland and Coastal Otago). If I have my facts right, it's less sunny than London. Regardless of moderate temperatures, I just can't see somewhere with sunshine hours of 1400 claiming to be subtropical-like.

True enough, but the strength/quality of the sunshine when it does get it probably helps - much better than ours in the winter

Are their temperatures going down? Interesting...

Im sure I read some data which showed that the average winter minimums in NZ as a whole have been declining recently, but it was a few months ago and Ive forgotten already! Best being critical about that one.

Sounds reasonable, but their lowest recorded temperature of -5C is quite amazing. How did that happen?

I have no idea, I can only assume it was a very deep cold plunge, where the -5C 850hPa line was into Iberia and that coupled with a clear night and no wind cause temperatures to drop very fast. I would imagine on the same night though that other islands might have been above freezing - St Marys where the observations are taken are by far the biggest islands - I wonder whether the same temperatures where recorded in Bryher?

It would be good, but maybe not truly ideal, though certainly the best place in the UK. I still don't really understand why they are so fussy in their native land. They seem to cluster in groves, and everyone talks about how hard it is to grow them and that they mostly don't bother. I heard this just 2 miles from where there was a fantastic stand of them. And it was flat, coastal plains around a North facing bay, sheltered by ranges to the south, east and west. Plenty of rain, plenty of sun, mild winters and warm summers. I just don't get it. I begin to wonder whether it has something to do with soil type, or perhaps in some places they just get crowded out as they are such slow growers.

Well you only have to look at there closest relative in the palm world Cocos Nucifera, to understand why they are so fussy. They are an endangered species, thatswhy I think its important to replicate the coditions it grows in, in another country.

[J07]

True enough, but the strength/quality of the sunshine when it does get it probably helps - much better than ours in the winter

Doesn't growth stop in winter though? And also, they can grow in places that get no sun in June/July.

Im sure I read some data which showed that the average winter minimums in NZ as a whole have been declining recently, but it was a few months ago and Ive forgotten already! Best being critical about that one.

No, you might be right about this. I remember reading something about how SSTs around NZ had warmed throughout the 20th century, as expected, but in the last decade they have been cooling slowly. So this could link to declining minima, especially on small islands like the Chathams.

Well you only have to look at there closest relative in the palm world Cocos Nucifera, to understand why they are so fussy. They are an endangered species, thatswhy I think its important to replicate the coditions it grows in, in another country.

Ah, I didn't realise they were that closely related. "Nikau" apparently means "Empty cocount". The first polynesian settlers must have been pretty upset!

At the moment, their southern limit is maybe 43S, and the country stretches right up to about 34S. If SSTs did continue falling, I think that the groves on the west coast would be more insulated against this as the Tasman Sea is warmer than the Pacific anyway.

Are they really classed as an endagered species?

[J07]

Just to come back to the "quality of light" you mentioned. What, specifically do you mean by this? Actually, I find this an interesting subject in its own right. We all know about the "softness" of the British light, especially apparent at this time of the year. But if you go to NZ in July, you would not be able to judge the season by the light, as it's still very sharp and the colours well-defined. I presume this is what you meant by quality?

But, the UV in an NZ winter is still going to be quite low, and the growth of the Nikau may have already shut down for the winter anyway (I don't know this for sure, not familiar with the seasonal growing habits of temperate palms). But since it's UV that is required for photosynthesis, I don't see how an index of 2 (NZ winter) will make that much difference over a UV index of 1 (UK winter). Of course, it's greater (double, I presume?), but they are still both very low and surely not sufficient for any real kind of growth.

However, an argument could be made for improved "quality" of light in NZ in summertime, over ours. Again, it really depends on how we define quality. Looking at the UV, there is a study which showed that the UV at an NZ site is approximately 41% greater than at a site of equivalent latitude in the US. Apparently, this relationship also holds true for Europe. The study also claimed roughly a 2% increase in UV for a 1 degree decrease in latitude.

Just for a quick estimation, using these figures for the capitals. Wellington is at about 41S, London 51N. Accounting for the latitude difference and the 41% anomaly we end up with a UV difference of 73%. This sounds astonishingly high to me, I hope I did my sums right. As the UV Index scale is apparently not logarithmic (thank God) this would mean, say, for a UV index in high summer in London of 8 we would have Wellington at 13 or 14- assuming the same day, weather conditions etc etc. If this sort of difference persisted for a few months, then I would hazard a guess that it would have a vast effect on the growth of plants. Roughly speaking, Wellington, with a latitude similar to New York City, would have a UV more akin to the Florida Keys, roughly 2000km further south. But this is based on an extrapolation from data taken from latitudes 40N-47N. I've no idea if the relationship remains linear as we approach the tropics.

Just to clarify, what I've stated above is just a rough working out based on thin scatterings of knowledge. I don't know enough about plant biology to be sure about my claims of increased growth due to increased UV. For all I know, beyond a certain level it may become a limiting factor. I also just did the UV calculation based on the index itself, not the actual power per unit of wavelength, the information to which I do not have access of course.

So getting back to it; there is then the quality of light issue in summertime. Without a scientific basis, and just on personal experience, I believe that UV and QoL are separate entities. Even with an index of only 1 or 2 in winter, on a cloudy day, I always found that it was not possible to look above the horizon without squinting torturously or putting on sunnies. This became a real pain in the I have a problem to be honest. This was independent of the height of the sun or the direction in which I was looking. Hence I believe this QoL does not rely on UV. I suppose it has something to do with fewer particulates and pollution in the air in general, as opposed to the relative lack of O3 the SH is known for.

Still, my presumption is that the QoL difference is greatest in winter (as you originally stated). I am not sure if there is such a big difference in summer, although I believe that one does exist.

Well, anyway, hope I'm not miles off the mark with what I've said. It's an interesting discussion for sure. You know a heck of a lot more about meteorology and plants than I do so hopefully you can help fill in the gaps in my knowledge!

[SP1986]

UV is an interesting one, in fact everything in NZ seems to be a fossil of its own long history. The Nikau was more widespread thousands of years ago than it is now, so we can assume the NZ landmass is moving south? (not sure for certain). Most interestingly is the colour of the naturalised (or semi naturalised) NZers they tend to be quite dark skinned, so I'd suggest the pattern in the growth and existence of Nikau palm species is mirrored through those relative of Maori people that may still exist in NZ today.

Like you said the UV isnt that impressive on the whole in the south with a UV of 2/3 but perhaps if you take quality (clearness of air, and lack of nucliide particles in the atmosphere), a sharpness of light may help the Nikau survive, and perhaps help the Maori descendents keep there browner skin?)

This would work if you took London (a very polluted atmosphere) with a very low UV, the quality of light is very poor, and I wonder whether the quality of light further north (in Scandinavia) in cleaner areas is better than London?

Also I wonder if NZ become an overpopulated, overpolluted country, would that have any effect on that light quality?

I do believe that the UV in summer in NZ is so high because of its excellent air quality, if you take the Mediterranean on a similar lattitude they have UV of around 9-10 in high summer.

Palms generally are classed as an endangered species because there are very few palms growing in the wild. However with a little help from temperature countries around the world, e.g. UK, NZ, East USA, I'm sure we can bring their numbers back up.

I often wonder also regarding UV, if western UK had a UV of 2-3 in winter, how often would the temperature go below 0C - I'd hedge my bets on rarely if ever!

UV is a changeable band all around the world, interacting with intraspecific conditions, but I think generally once you move past 20 degN/S the average UV doesnt change much annually.

[J07]

UV is an interesting one, in fact everything in NZ seems to be a fossil of its own long history. The Nikau was more widespread thousands of years ago than it is now, so we can assume the NZ landmass is moving south? (not sure for certain). Most interestingly is the colour of the naturalised (or semi naturalised) NZers they tend to be quite dark skinned, so I'd suggest the pattern in the growth and existence of Nikau palm species is mirrored through those relative of Maori people that may still exist in NZ today.

NZ is moving North along with Australia. I think there's a fair number of fair people (with "English skin" as they put it) over there, but you're right about those who do tan. The tan can persist throughout winter, and by the end of September/ start of October the UV is already hitting about 5 or 6 again. So it makes sense that they can retain darker skin tone.

Like you said the UV isnt that impressive on the whole in the south with a UV of 2/3 but perhaps if you take quality (clearness of air, and lack of nucliide particles in the atmosphere), a sharpness of light may help the Nikau survive, and perhaps help the Maori descendents keep there browner skin?)

I think about 95% of Maori live on the North Island anyway. UV there is above 10 from November until February, roughly. Though I believe there are few "true" Maori around now, just like most races there has been interbreeding.

Also I wonder if NZ become an overpopulated, overpolluted country, would that have any effect on that light quality?

Interesting one....

I guess we'll never find out!

I do believe that the UV in summer in NZ is so high because of its excellent air quality, if you take the Mediterranean on a similar lattitude they have UV of around 9-10 in high summer.

On the note of air quality, some cities issue air pollution warnings in winter and track the air pollution on a daily basis. This is just because of their prevailing local weather conditions and also because open fires are still so common! I've managed to find a weather forecast showing this (attached). You can see the air pollution readings they take at Christchurch. Conveniently, this forecast is for the last day of winter- and it states a total of 13 high air pollution nights for that winter in total. You can also see the effect the main divide of the southern alps has on the weather there.

Another danger with the sun in NZ is the relatively low air temperatures. As an example, yesterday Wellington only got to 15C, pretty poor for the time of the year, meaning people may not adequately prepare themselves for sun exposure. (The UV index was 11). I'm not sure I subscribe fully to this argument, as to me it's abundantly clear when my head is getting caved in by the sun, even if it may be cool standing in the shade. But if you are born and raised in such an environment, it's likely that you won't notice how strong the sun actually is.

Having a look at this http://www.sunsmart.org.nz/uv-radiation--i...ndex-(uvi).aspx , the sun is currently ranging from 10 in the South to 12 in the North.

It's nothing compared to Australia though:

http://www.bom.gov.au/weather/national/charts/UV.shtml

16 in Melbourne!