Date: Wed, 7 Jun 2000 13:41:13 -0400 From: "Leslie R. Lemon" Subject: Re: aus-wx: snowfalls To: "INTERNET:aussie-weather at world.std.com" X-MIME-Autoconverted: from quoted-printable to 8bit by europe.std.com id NAA29387 Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com Lyle: > > ground. As this process goes on the atmosphere cools and becomes > saturated > > via this process to progressively lower and lower levels until finally > > reaching the surface. > > So your saying that the higher humidity helps snow to the ground? This > contradicts what David said??? I am uncertain about what David said. I am sorry, I have not seen his post. But, yes, higher humidity helps snow reach the surface.......or any other precipitation type for that matter. It really is simple when you think about it. When water substance in liquid or solid form finds itself in an environment that is less than saturated, that water substance looses mass through evaporation or sublimation. The higher the relative humidity the slower the rate of mass loss and the longer that water substance will survive before it transforms entirely into vapor (gas) form. This is related to the magnitude of the vapor pressure over the liquid. When the air is finally saturated, the mass loss through evaporation or sublimation ceases. When saturation is reached, the wet bulb temperature is the same as the air temperature which is the same as the dew point. If that temperature is below freezing, the snow reaches the ground as snow. On the other hand, if that temperature is above freezing, the snow will continue to melt during its decent to the surface and depending on several variables, may reach the surface as very wet snow or rain. I touched on the subject of the "wet bulb" temperature which is independent of evaporation to an extent as explained above. The wet bulb temperature dictates whether the ice (snow, sleet, hail) melts or remains frozen during decent. Here, I will not get into that. I can, if you would like, in a later post. Perhaps David's post dealt with that. Les ************************ Leslie R. Lemon Radar, Severe Storms, & Research Meteorologist Tel. 816-373-3533, 816-213-3237 E-Mail: lrlemon at compuserve.com +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: David Jones To: "Aussie Weather (E-mail)" Subject: aus-wx: snow falls. Date: Thu, 8 Jun 2000 08:46:56 +1000 X-Mailer: Internet Mail Service (5.5.2650.21) Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com Lyle... not surprised you are confused, as higher and lower humidity's can increase the possibility of snow reaching the ground. I guess their are two limiting cases. 1) The air is sufficiently cold at the surface for snow to survive its fall to the ground without melting. In this case higher sub cloud humidity's will reduce evaporation from the flake, and if the sub cloud layer is saturated with-respect-to ice (but not water, as is common), the fake may even continue to grow. Unfortunately for us in AUS, it is very rare for precip to fall when the surface temp is near zero. 2) The other case is where the surface temp is too warm for flakes to survive the fall to the ground (without evaporative cooling). In this case, low humidity can increase the likelihood of flakes reaching the ground. Anyway, if the flake falls into air warmer than 0C, heat transfer from the surrounding air to the flake causes melting, just as you would expect. However, if the air is unsaturated, the heat transferred to the flake can be removed through evaporative cooling. The evaporative cooling slows the melting of the flake (and in fact in the limiting case can even stop it entirely), and it is possible for frozen precipitation to survive much longer in the above freezing environment than is otherwise possible. Typically, if the air is saturated, flakes only manage to remain frozen for a total fall distance of 2-300m, meaning snow will not be seen at surface temperatures above about 2C. In dry air, the evaporative cooling can increase the distance to greater than 500m, meaning it is possible for snow to be observed at the surface with temperatures approaching 6C. Of course, if the air is too dry, the evaporation might be sufficient to entirely evaporate the flake, in which case you won't see any precipitation. Another factor which I haven't touched on and which seemed to be quite important with the May cold-outbreak was the trapping of cold air in alpine/subalpine valleys and the subsequent lowering of the freezing/melting level as continuous frozen precipitation fell into the valley. With the May event, the snow east of Melbourne settled to significant depths to about the 200m level in sheltered valleys, while the west and southwest facing slopes (for example the Dandenongs) really only got significant accumulations above about 500m. Cheers, David. Climate Analysis Section National Climate Centre Bureau of Meteorology Fax : (+61 3) 9669 4678 GPO Box 1289K, Melbourne Ph (work): (+61 3) 9669 4861 Victoria 3001, Australia Ph (home): (+61 3) 9898 4425 email : D.Jones at bom.gov.au +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: "John Woodbridge" To: Subject: RE: aus-wx: An exceptional prolonged cool period at Giles Date: Thu, 8 Jun 2000 09:57:17 +1000 X-Mailer: Microsoft Outlook CWS, Build 9.0.2416 (9.0.2910.0) Importance: Normal Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com Hi Blair, It think this is significant. Giles is a long way from any oceanic influences and thus is representative of the continental air mass. If we add to this the cooler than normal Brisbane summer with lack of thunderstorms and tropical rain events, fewer than anticipated TC's during a La Nina event, and perhaps even the lower than normal springtime Tornado situation in the US midwest... It seems to add up to general lack of heat. I wonder if insolation values are down?? John. >snip Giles, near where WA, SA and the NT meet, has had an exceptionally cool seven months. Over the period from November 1999 to May 2000 (inclusive), temperatures have been 4.1 degrees below average, with negative anomalies of at least 2 degrees in every month, including -6.1 in November and -5.8 in April. Only 39 out of a possible 213 days in this period have had above- average temperatures, including only 2 of the last 53 (an unbroken run of 26 successive below-average days is currently in progress). There were only 33 days over 35 (long-term average 94) and 3 over 40 (long-term average 18). Blair Trewin +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ Date: Wed, 7 Jun 2000 20:34:38 -0400 From: "Leslie R. Lemon" Subject: aus-wx: snow falls. To: "INTERNET:aussie-weather at world.std.com" X-MIME-Autoconverted: from quoted-printable to 8bit by europe.std.com id UAA20577 Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com David wrote: > 2) The other case is where the surface temp is too warm for flakes to > survive the fall to the ground (without evaporative cooling). In this case, > low humidity can increase the likelihood of flakes reaching the ground. > Anyway, if the flake falls into air warmer than 0C, heat transfer from the > surrounding air to the flake causes melting, just as you would expect. > However, if the air is unsaturated, the heat transferred to the flake can be > removed through evaporative cooling. The evaporative cooling slows the > melting of the flake (and in fact in the limiting case can even stop it > entirely), and it is possible for frozen precipitation to survive much > longer in the above freezing environment than is otherwise possible. > Typically, if the air is saturated, flakes only manage to remain frozen for > a total fall distance of 2-300m, meaning snow will not be seen at surface > temperatures above about 2C. In dry air, the evaporative cooling can > increase the distance to greater than 500m, meaning it is possible for snow > to be observed at the surface with temperatures approaching 6C. > Of course, if the air is too dry, the evaporation might be sufficient to > entirely evaporate the flake, in which case you won't see any precipitation. > Lyle - What I said and what David has said, is entirely consistent. We do not disagree. This (2 above) is precisely the case in which I mentioned the importance of the wet bulb temperature in my post. Above, David is addressing the case where the dry bulb temperature - the actual air temperature - is above freezing but the wet bulb temp, that is the temperature the evaporating flake "experiences" is below freezing. In that case, as David has described so well, humidity less than saturation permits the snow to reach the surface, assuming it began its decent with sufficient mass such that it does not completely loose that mass via sublimation as it falls toward the surface. I hope I have not 'muddied' the water still further! LOL Les ************************ Leslie R. Lemon Radar, Severe Storms, & Research Meteorologist Tel. 816-373-3533, 816-213-3237 E-Mail: lrlemon at compuserve.com +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: Blair Trewin Subject: Re: aus-wx: An exceptional prolonged cool period at Giles To: aussie-weather at world.std.com Date: Thu, 8 Jun 2000 10:56:04 +1000 (EST) X-Mailer: ELM [version 2.5 PL2] Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com > > Hi Blair, > > It think this is significant. Giles is a long way from any oceanic > influences and thus is representative of the continental air mass. If we > add to this the cooler than normal Brisbane summer with lack of > thunderstorms and tropical rain events, fewer than anticipated TC's during a > La Nina event, and perhaps even the lower than normal springtime Tornado > situation in the US midwest... It seems to add up to general lack of heat. > I wonder if insolation values are down?? > > John. > I'd interpret it as being (mostly) a particularly notable local manifestation of the increased regional cloud cover associated with La Nina - which is associated with low summer and autumn maximum temperatures through much of northern and central Australia. The most unusual aspect of 1999-2000 has been that the largest temperature anomalies have been in the western half of the country (unlike 1974, when they were in Queensland). Averaged over the country as a whole, mean maximum temperatures for the first five months of 2000 are still running a little above 1974's, although autumn was cooler than 1974. >From a national perspective, the number of TCs was actually slightly above average (11, compared to the long-term mean of 9.4), and not far below the anticipated number (Neville Nicholls' experimental forecast went for 12). What was a little unusual for a La Nina event was that only three of the cyclones affected Queensland or the NT, and that those were all late in the season. (Three more crossed the WA coast only, and five formed in the WA region but never reached land in Australia - although one, famously, made it to Africa). Blair Trewin +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: davidkc at advanceenergy.com.au Subject: aus-wx: Storm2000 News Flash To: aussie-weather at world.std.com Date: Thu, 8 Jun 2000 13:16:54 +1000 X-MIMETrack: Serialize by Router on Domino/Advance(Release 5.0.2 (Intl)|4 November 1999) at 08/06/2000 01:17:00 PM Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com ---------------------- Forwarded by David Carroll/Advance on 08/06/2000 13:16 --------------------------- Storm2000 Weather Advisory on 08/06/2000 13:21:41 To: Storm 2000 Weather Advisory cc: Subject: Storm2000 News Flash Latest storm and tropical weather information from Lowe's Storm2000 Hurricane Central -- http://www.storm2000.com TROPICAL DEPRESSION ONE ADVISORY NUMBER 2 NATIONAL WEATHER SERVICE MIAMI FL 10 PM CDT WED JUN 07 2000 ...TROPICAL DEPRESSION ONE IS STATIONARY AND POORLY-DEFINED... AT 10 PM CDT...0300Z...THE POORLY DEFINED CENTER OF THE TROPICAL DEPRESSION WAS LOCATED NEAR LATITUDE 20.8 NORTH...LONGITUDE 92.8 WEST OR ABOUT 460 MILES...740 KM...SOUTHEAST OF BROWNSVILLE TEXAS. THE DEPRESSION IS CURRENTLY NEARLY STATIONARY. AN NORTHWESTWARD DRIFT IS EXPECTED TO BEGIN LATER TONIGHT OR THURSDAY. MAXIMUM SUSTAINED WINDS ARE NEAR 30 MPH... 45 KM/HR...WITH HIGHER GUSTS. LITTLE CHANGE IN STRENGTH IS FORECAST DURING THE NEXT 24 HOURS. AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT WILL INVESTIGATE THE SYSTEM ON THURSDAY. THE ESTIMATED MINIMUM CENTRAL PRESSURE IS 1008 MB...29.77 INCHES. INTERESTS IN THE WESTERN GULF OF MEXICO...AND THE COASTAL SECTIONS OF NORTHEASTERN MEXICO AND TEXAS SHOULD MONITOR THE PROGRESS OF THIS SYSTEM. REPEATING THE 10 PM CDT POSITION...20.8 N... 92.8 W. MOVEMENT... NEARLY STATIONARY. MAXIMUM SUSTAINED WINDS... 30 MPH. MINIMUM CENTRAL PRESSURE...1008 MB. THE NEXT ADVISORY WILL BE ISSUED BY THE NATIONAL HURRICANE CENTER AT 4 AM CDT...THURSDAY. Please continue to use Lowe's Storm2000 Hurricane Central as your No. 1 resource for Hurricane and Tropical Storm updates: http://www.storm2000.com/ In partnership with the Federal Emergency Management Agency: http://www.fema.gov/ Questions? Let us know how we can help at:storm-owner at lists.hurricanecentral.com To unsubcribe from this list at anytime, send a blank message to: storm-unsubscribe at lists.hurricanecentral.com or visit our website at http://www.storm2000.com/email/ for a form that will allow you to easily unsubscribe from the list. Brought to you by Cox Interactive Media: http://www.cimedia.com/ & Lowe's Home Improvement Warehouse: http://www.lowes.com/ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ X-Sender: jacob at mail.iinet.net.au X-Mailer: QUALCOMM Windows Eudora Pro Version 4.1 Date: Thu, 08 Jun 2000 13:31:16 +0800 To: aussie-weather at world.std.com From: Jacob Subject: aus-wx: Perth min temp differences Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com With the talk about Brisbane min differences on the list lately, its not all different for Perth either. The "city" site changed in 1993 from East Perth to Mount Lawley (which is right next door), but for some reason it has much cooler overnight winter minimum temperatures than the old East Perth site, even though both suburbs border each other and Mount Lawley is in a built up area, as it is in the inner city. Nearly all of our coldest city monthly temperatures from May to September on record have been recorded after 1993 because of this. The media love to report them as record temperatures, even though to be fair prolly none of those would have been records if the site stayed at East Perth Last night's minimum temperatures is a perfect example of how cold the Mount Lawley site can get compared to the rest of the metro sites. Gosnells: 8C (about 10km SE of the city) Jandakot Airport: 2C (about 15km S of the city and usually the coldest spot in Perth) Medina: 5C (about 30km S of the city) Mount Lawley (Perth City): 2.5C (right next to the CBD area) Perth Airport: 8C (about 15 km E of the city) Swanbourne: 8C (right on the coast) This just shows the big differences in minimum temperatures on a clear winters night. Jandakot usually has the coldest temperatures on these nights, like it did last night. You would say Perth Airport should be the coldest, being further inland and also in a less built up area, but for some reason that isnt the case. Swanbourne being right on the coast is usually the warmest. Jacob +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: Blair Trewin Subject: Re: aus-wx: Perth min temp differences To: aussie-weather at world.std.com Date: Thu, 8 Jun 2000 16:29:15 +1000 (EST) X-Mailer: ELM [version 2.5 PL2] Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com > > > With the talk about Brisbane min differences on the list lately, its not > all different for Perth either. The "city" site changed in 1993 from East > Perth to Mount Lawley (which is right next door), but for some reason it > has much cooler overnight winter minimum temperatures than the old East > Perth site, even though both suburbs border each other and Mount Lawley is > in a built up area, as it is in the inner city. > I'm not familiar with the Perth site, but would suspect that local ground surface and the proximity of buildings, asphalt surfaces etc. would be significant. The difference between Mount Lawley and Perth Airport only really shows up, on a systematic basis, on about the coolest 30% of nights, between May and September. There is little difference in summer (in fact, Mount Lawley is slightly warmer), and the difference in extremes is far greater than the difference in means. I had a look at the relative frequency distributions back in 1997 when Mount Lawley was getting its string of 'record' temperatures, and came to the conclusion that the old Perth City record of 1.2 was equivalent to -1.0 +/- 0.5 at Mount Lawley. (The lowest Mount Lawley got in 1997 was 0.0). Perth Airport shows quite a marked warming trend (and it is a trend, not a step discontinuity) for minima over the last 30 years, presumably because of the expansion of the suburban area towards it. Blair Trewin +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: "John Woodbridge" To: Subject: RE: aus-wx: Perth min temp differences Date: Thu, 8 Jun 2000 17:33:56 +1000 X-Mailer: Microsoft Outlook CWS, Build 9.0.2416 (9.0.2910.0) Importance: Normal Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com Hi Jacob, Yes, as noted a day or so ago, I can get a 4.5C difference on a calm night, at two sites physically 100m apart and 15m different in elevation. So very local topography can have a significant impact. I know Mount Lawley is quite hilly (for Perth...) and would be interested in knowing the exact location chosen, and whether or not it is in a dip or lower area. It would also be interesting to note the locale of Perth Airport's instruments, as the readings seem very anomolous and should really be close to those from Jandakot being at a similar elevation and in a similar flat open environment. Maybe if there is a very light Easterly blowing, this influences temps at Perth airport, due to stirring and warming of air coming down the Darling Escarpment, whereas this influence doesn't extend as far as Jandakot which stays dead calm... John. >snip With the talk about Brisbane min differences on the list lately, its not all different for Perth either. The "city" site changed in 1993 from East Perth to Mount Lawley (which is right next door), but for some reason it has much cooler overnight winter minimum temperatures than the old East Perth site, even though both suburbs border each other and Mount Lawley is in a built up area, as it is in the inner city. Nearly all of our coldest city monthly temperatures from May to September on record have been recorded after 1993 because of this. The media love to report them as record temperatures, even though to be fair prolly none of those would have been records if the site stayed at East Perth Last night's minimum temperatures is a perfect example of how cold the Mount Lawley site can get compared to the rest of the metro sites. Gosnells: 8C (about 10km SE of the city) Jandakot Airport: 2C (about 15km S of the city and usually the coldest spot in Perth) Medina: 5C (about 30km S of the city) Mount Lawley (Perth City): 2.5C (right next to the CBD area) Perth Airport: 8C (about 15 km E of the city) Swanbourne: 8C (right on the coast) This just shows the big differences in minimum temperatures on a clear winters night. Jandakot usually has the coldest temperatures on these nights, like it did last night. You would say Perth Airport should be the coldest, being further inland and also in a less built up area, but for some reason that isnt the case. Swanbourne being right on the coast is usually the warmest. Jacob +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------ From: Blair Trewin Subject: Re: aus-wx: Perth min temp differences To: aussie-weather at world.std.com Date: Thu, 8 Jun 2000 18:06:54 +1000 (EST) X-Mailer: ELM [version 2.5 PL2] Sender: aussie-weather-approval at world.std.com Reply-To: aussie-weather at world.std.com > > Hi Jacob, > > Yes, as noted a day or so ago, I can get a 4.5C difference on a calm > night, at two sites physically 100m apart and 15m different in elevation. > So very local topography can have a significant impact. I know Mount > Lawley is quite hilly (for Perth...) and would be interested in knowing > the exact location chosen, and whether or not it is in a dip or lower area. I'll have a look at the station file in the morning (when the person with the key to the relevant cabinet gets in...) Blair +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ To unsubscribe from aussie-weather send e-mail to:majordomo at world.std.com with "unsubscribe aussie-weather your_email_address" in the body of your message. -----------------------jacob at iinet.net.au------------------------------