Lower pressure the higher you go, now it all makes sense!

Thunder · Unread post by **Thunder** » Fri 22/10/2004 23:36

Hi

I was reading a guy's website called downunderchase.com made by Anthony Cornelius which I'm guessing a few if not most of you know who he is (I know who know him John). I like reading the guide to forecasting storms which is packed full of info.

Anyway, as I was reading I came across a good point which is simple but helps in explaining a few things. It's to do with the higher you go the pressure drops on average 1hpa per 10 meters. So this lead me to believe if you up in Oxford (300m ASL or there abouts) when a SLP 1030 hpa high is over us you can presumably expect the pressure to be around 1000 hpa, now that's just bloody brilliant isn't it!!

So would another reason be that these storms form inland in summer during highs is the pressure decreases with height? This also brings into question the fact that temperature decreases with height, so wouldn't Oxford or inland Alexandra be colder? As we all now that is wrong, so that's another good thing, the temp doesn't decrease with height as we go inland and is infact most times warmer than at the coast, another plus considering the lower pressure inland. Winter is a bit of a different story though.

And I guess what also helps as Steven W has pointed out is that the central Otago / Southland area is more exposed to upper cold pools, another plus!

So the next factor is moisture. Can someone tell me if typically it is drier in inland NZ in terms of the dewpoint or is it easier for the dewpoint to be higher inland than it is at the coast?

This all applies for the inner North Island aswell and just becasue upper cold pools wouldn't be as frequent, the higher dewpoint and temp at ground level would make up for this, mainly the higher dewpoint I'm guessing.

Does what I'm saying make sense? Would appreciate some input!

Dean. · Unread post by **Dean.** » Fri 22/10/2004 23:43

NZ Thunderstorm Soc · Fri 22/10/2004 23:49

Any anticyclonic disturbance would have to be a feature of anything above and below the anticyclone.
an upper level trough, say ...or
Strong dew points at surface levels, triggering ..um something ?

Today has nice Cu develpment under a reasonably high pressure day

JohnGaul
NZTS

Thunder · Unread post by **Thunder** » Sat 23/10/2004 00:05

Good points people.

I believe Alexandra is about 150m asl (minimum) after looking at various stuff on the web. So imagine a SLP 1020 hpa high over NZ (SI more directly) (not huge but a high!

), that's going to be 1005 hpa in Alexandra, not bad!

Yes, I agree Deano. Higher dewpoints near the coast due to sea breezes and typically drier inland. So, does anyone know what type of air mass / wind direction would you need to deliever high dewpoints to inland areas like inland Canterbury / inland Otago? Is it just a strong NE near the coast so it penetrates inland?? I guess inland Canterbury it would have to come from the NE direction as it's the only option it's got for humid air.

Agree?

Yes John, some ok Cu out ther tody despite the high pressure. They dissapeared though as the dewpoint dropped. I noticed some nice Cu last night aswell when going to Steven G's over the Waimak area and inland towards the Rakaia Gorge area. They didn't go anywhere though.

Dean. · Unread post by **Dean.** » Sat 23/10/2004 00:20

Humidity is increasing here,dewpoint this evening is 21.3,its been hot for us ever since arriving here in July,but only now are the locals saying its starting to warm up again and we are breaking out of another winter.

Thunder · Unread post by **Thunder** » Sat 23/10/2004 00:29

That's a big dewpoint! Storms everywhere if we had that here with an equivelent or slightly higher air temp. I saw those storms over you the other night, should be more soon.

Thunder · Unread post by **Thunder** » Sat 23/10/2004 00:59

Just some interesting info.

I went through the wunderground site and by the looks of things the highest dewpoint the Chch airport has got from 1996 till now is 18 C. Has it got past that at some point for Chch? It's quite high but not near Deano's current 21.3!!

Dean. · Unread post by **Dean.** » Sat 23/10/2004 01:31

At 16.87 degrees south of the equator the dewpoint is around the norm,looking forward to the heights of summer (ie wet season).
Looks like a brief wintery outbreak for mainlanders from Sunday arvo.
Later.

Manukau heads obs · Sat 23/10/2004 08:11

aron, i dont think your reasong on the barometer thing with altitude thing is correct.....
but, what it might mean is the local heating from a warming ground surface, if that ground is hitgher than say a local inversion layer, then its free to keep rising, that warmer air!
yeah, the barometer drops as you go up in altitude like slowly towards the alps,,,,i.e the barometer is then acting like a altimeter.....and so you then need to keep adjusting it the MSL...
but, mind you, i did watch a program about a samll village in south americ on the equator in the Andes mountains,at some really high altitude (i cant remember), and the strong equatorial sun and the thin air and then rocky terain did strange things to the local weather!

TonyT · Unread post by **TonyT** » Sat 23/10/2004 10:28

Sorry Aaron, but you're on the wrong track here. Lower air pressure does not, of itself, have any influence on local stability and hence potential for convection. However, unstable air is more likely to be found in or near a trough or depression, which is why there appears to be a conection between low pressure and storm development. But the lower air pressure at higher altitudes is nothing to do with troughs and depressions, simply the result of less air weighing down. And as Brian points out, you should always deal with MSL corrected pressure readings anyway!

Temperature falls with height (usually, not in a temperature inversion) which fact is what makes convection possible in the first place! Take a reading at 1.5m and climb up a pole and take a reading at 100m (alright, its a very tall pole!) and it will be lower. But the 1.5m temperature at Oxford may well be warmer than the 1.5m temperature at Rangiora, despite the difference in altitude of the two sites, due to differences in local topography - eg Rangiora is more likely to be experienceing a breeze off the sea, whereas winds at Oxford are more likely to be light and variable. However, go 100m above Rangiora and 100m above Oxford and your readings will still be colder, and usually by about the same amount.

As for moisture, well what you need is a source - and the ocean is the biggest source around. So, coastal sites usually get higher humidity than inland sites, but of course wind direction can play a part - high humidities are possible inland when a sea breeze reaches there, and low humidities are possible on the coast when a wind gets there from inland (eg a nor'wester). The higher humidity along the coast is not so helpful for storm generation there becuase it usually has a breeze associated with it which disrupts thermal convection currents. The topography of inland basins gives them a significant advantage in generating convective currents, so when the sea breeze arrives there, the convection is already well established.

spwill · Unread post by **spwill** » Sat 23/10/2004 13:19

Higher altitude usually=less inversion.
Inland locations in summer are usually warmer and this rising warm air often drags in moist sea air from the Coast and can cause local convergence, all of this aids Convection cloud development .
Very moist air is usually imported on to NZ from areas well to the North. Canterbury is well protected from this High dewpoint air.

Thunder · Unread post by **Thunder** » Sun 24/10/2004 00:34

Oh well..

I'll get there!!

Thanks for the great info people.

Have a read from the link below if one's willing..

http://downunderchase.com/storminfo/sto ... ide09.html

Here's a segment of it:

So what's so important about height? Well - what happens as you ascend? The pressure drops! In the bottom 1-2km, the pressure drops at approximately 1hPa every 10m. So say the pressure is 1000hPa at the surface, if you ascend 100m then the pressure is now 990hPa. If you ascend 500m the pressure is 950hPa and if you ascend 1000m the pressure is 900hPa. Remember how we used Normand's Theorem to plot our Skew-T, and we started at the surface? Normally the surface is taken to be around 1000hPa, but what if it were say 950hPa? We would then have to plot our temperature and dewpoint commencing from the 950hPa level! Normally as you ascend, the temperature and dewpoint decrease (in fact, this should be at around 1C/100m), but that isn't actually the case over land! For instance, at an elevation of 600m, about 20km south of Toowoomba on November 11, I measured a temperature of 29 degrees. The sea level pressure at the time was around 1010hPa, which meant the actual pressure I was experiencing was around 950hPa. The dewpoint was hovering around 15-16C. Essentially, at 600m it was the same temperature and dewpoint as plotted at 0m (sea level) on the above Skew-T. So we're going to get a good idea on how increasing the altitude can really help increase the instability!

Or is what said above only good for Oz? Perhaps I haven't properly grasped what he's trying to say.

Perhaps a storm tomorrow if the sun is allowed through and the southerly comes around 3 or 4pm with a dewpoint of 10! Now that'll do it! The midday skew t should reveal all.

Manukau heads obs · Sun 24/10/2004 08:15

could be something in it...central NI might falkl ointo this catergory?

TonyT · Unread post by **TonyT** » Sun 24/10/2004 10:19

Aaron J Wilkinson wrote:Or is what said above only good for Oz? Perhaps I haven't properly grasped what he's trying to say.

I think he's trying to make something out of nothing - sure you get good convection off the sides of mountains, but consider that a 30-45 deg mountain slope is getting much more direct solar radiation than a flat plain - I think I am right in saying the radiation would be proportional to the sine of the angle of incidence - so you always get stronger convection currents off of NE facing slopes in the mornings, and NW facing slopes in the afternoon. Thats why places like Mt Peel generate such good convection late morning - they have had direct radiation for several hours and so long as the atmosphere is unstable things will go upwards. It really doesnt matter what the surface pressure is.

Aaron J Wilkinson wrote:Perhaps a storm tomorrow if the sun is allowed through and the southerly comes around 3 or 4pm with a dewpoint of 10! Now that'll do it! The midday skew t should reveal all.

Quite often when you make these predictions Aaron I notice that you focus mainly on temperatures and dewpoints and so on, these are not the main factors which will dictate how much convection takes place - you have to look at instability as the primary one. I gave you a pointer to the total totals charts which are a good rough guide to instability. If the instability is there then these other things come into play, but the atmosphere has to be unstable first. I think perhaps that is the key message you can take from downunderchaser's piece.

Thunder · Unread post by **Thunder** » Sun 24/10/2004 20:04

Thanks Tony

I'm actually looking at those tot tot charts now and will be alot! And yes I looked at it yesterday for todays posibilities but just didn't mention it in my post above. Yes, I guess I play things a bit too optimistically and will try to be more reserved for next time.

Good stuff below

but consider that a 30-45 deg mountain slope is getting much more direct solar radiation than a flat plain - I think I am right in saying the radiation would be proportional to the sine of the angle of incidence - so you always get stronger convection currents off of NE facing slopes in the mornings, and NW facing slopes in the afternoon. Thats why places like Mt Peel generate such good convection late morning - they have had direct radiation for several hours and so long as the atmosphere is unstable things will go upwards. It really doesnt matter what the surface pressure is.