Tuesday, 22 May 2007

Global warming and heat island effects

Since writing my last blog on climate data available from the Bureau of Meteorology I’ve had some correspondence with Wilma Keppel of the US based www.managingwholes.com website. Wilma highlighted the impact of what are termed ‘heat islands’ on temperature and climate. In short, heat islands are built up areas such s towns, cities and even road-ways where the temperatures in and around them tend to be above that of the surrounding areas. It’s not hard to conclude that these must play a role in increased air temperatures more widely than just in those areas.

Here’s an extract from Wilma’s last email that I thought was worthy of sharing more widely.

“The heat island effect can be significant. The northernmost native persimmons that John ever found were in the heat island of central Omaha, Nebraska (almost exactly in the middle of the continental U.S.). This city of about half a million is 3-4 degrees C warmer than the surrounding areas.

As you can imagine, heat island effects greatly increase people's perceived need for air conditioning. Using air conditioning increases outdoor air temperatures -- both by pumping heat outdoors, and by running compressor motors. And when outside air temperature goes up, air conditioner efficiency goes down.

A big part of the problem is pavements and roofs that absorb heat. Installing high-reflectance roofs can have a HUGE effect. (On chicken coops, too -- see my chicken tractor article on Managing Wholes).

Another significant effect comes from big trees that shade roofs and pavements. This is also a significant effect on heat buildup in the countryside. I well remember driving through eastern Iowa, which used to be home to the western edge of the eastern hardwood forest that covered most of the east half of the temperate parts of North America. Sensible states like New Hampshire allow trees to grow right up to the edge of the roads, shading them and keeping them cool. Iowa has a policy of cutting trees within 10 meters or so of the road, which means no shade. And since most of the country has been plowed into farms with huge fields, cutting down the roadside trees has gotten rid of a large proportion of the remaining strip woodland windbreaks, so soil erosion has gone up. Cutting the trees also drastically decreases habitat for the remaining native wildlife. All this so drivers going too fast on icy road D/S won't spin off and hit a tree. (New Hampshire has narrower, steeper roads, and gets more snow, but expects people to drive sensibly.)”

Wilma’s comments also reminded me of a conversation I had not so long ago with a mate, Garry Coates, who used to (and probably still does) tour widely throughout Australia on a motor bike. When riding during the heat of the day out on the western plains areas throughout New South Wales he found that the temperature difference between heavily treed and open areas was significant. In the open areas he’d find himself suffering, and close to heat exhaustion. In the treed areas he was cool and not sweating at all. Garry’s view is that a lot of the Australian areas with large treed canopies (such as around Heallsville, Narbethong, Toolangi etc) are naturally drought resistant. Plants and animals survive there OK because the moisture gets locked in. Take away the large trees and the moisture goes as do the animals.

With regard to Garry’s observations Wilma mentioned that Peter Donovan (also of the US based www.managingwholes.com website) told her that in researching tropical rain forests, he discovered that they typically occur in at least somewhat brittle climates, and it's the tree canopy that keeps the forest underneath moist throughout the dry season.

Wednesday, 9 May 2007

Bureau of Meteorology On-Line Climate Data

Today Jan gave me a copy of “The Australian Fruit and Vegetable Garden” book which was published in 2006 by members of The Digger’s Club. This is an excellent publication, and details the types of plants that are suited to various areas of Australia based on different climates. We will use the book as part of planning what vegetables, and fruit and nut trees to grow on Ochre Arch.

From a temperature perspective the book refers to the following ‘growing range of food plants by air temperature’:
* Tropicals CZ 11, 12. Growing range; +5 degrees C to +35 degrees C
* Sub-tropicals CZ 10, 11, 12. Growing range; -1 degrees C to +35 degrees C
* Temperate fruits CZ 8-9a. Growing range; -15 degrees C to +25 degrees C
* Hardy annuals. Growing range; +5 degrees C to +25 degrees C

The book also uses the terms:
* ‘growing days’ which are defined as where the maximum temperature is equal to or greater than 15 degrees C
* ‘hot days’ which are defined as where the maximum temperature is equal to or greater than 30 degrees C
Among other things it is also important to look at minimum winter temperatures.

The above prompted me to figure out what climate zone Ochre Arch fell within, in the context of the various bands that the book discusses. I ended up getting onto the Bureau of Meteorology website to see what data is available, and it just happens that they’ve added an excellent new feature which they’ve titled ‘Climate Data On-line’. To quote directly from the website: “Climatology maps, prepared using data collected over a long period of time (generally 30 or more years), show the variations in climate across the country. Other maps show a range of weather, hydrological and climate information.” In short, one can source average, mean and specific (e.g. minimum and maximum) information for each month of the year since recording commenced by location.

The 3 nearest weather recording locations to Ochre Arch are at:
~ Grenfell (410 metres above sea level, and approximately 20 km [direct] south west of the farm),
~ Forbes (240 metres above sea level, and approximately 35 km [direct] north of the farm) and
~ Quandialla (250 metres above sea level, and about 45 km [direct] south-west of the farm).

The following data was gleaned from the Bureau of Meteorology website:

Long term data – Month by month since Bureau of Meteorology records commenced:
The Bureau’s database records for the long term statistics start from 1965 for both Grenfell and Quandialla, and 1873 for Forbes.

1. Highest ever recorded temperature in degrees C
Grenfell = 43.9 on 15 January 2001, Quandialla = 45 on 31 January 2001, and Forbes = 47.8 on 11 January 1882.

2. Mean number of days (in a year) where the temperature is equal to or greater than 30 degrees C
Grenfell = 66 days, Quandialla = 90, and Forbes = 92

3. Lowest ever recorded temperature in degrees C
Grenfell = -6.5 on 15 June 2006, Quandialla = -5.8 on 21 July 1983, and Forbes = -5.6 on 11 July 1958

4. Mean number of days (in a year) where the temperature is equal to or less that than 0 degrees C
Grenfell = 11.6, Quandialla = 21, and Forbes = 20

5. Mean annual rainfall in mm:
Grenfell = 609, Quandialla = 509, and Forbes = 521

Annual data – past 12 complete months i.e. 1 May 2006 to 30 April 2007
6. Number of days during the period where the temperature is equal to or greater than 15 degrees C
Grenfell = 313, Quandialla = 331, and Forbes = 329

7. Number of days during the period where the temperature is equal to or greater than 15 degrees C (See point 6 above) LESS Mean number of days (in a year) where the temperature is equal to or greater than 30 degrees C (from point 2, above):
Grenfell = 247, Quandialla = 241, and Forbes = 237

The figures show some (to me) significant differences, especially between Grenfell and the two other towns of Quandialla and Forbes.

Quandialla and Forbes have almost twice as many frost nights (where the temperature is equal to or below zero) each year than Grenfell. Grenfell’s higher altitude (410 metres V 240 & 250) could be one of the key factors, as could the fact that the location of the Grenfell weather station is on the top of the main hill in the town whilst (the topography of) Forbes and Quandialla are quite flat. Cold air is heavier, so the temperature on the top of hills is as a rule warmer than in the gullies. Curious that Grenfell has actually recorded a lower minimum temperature than Forbes and Quandialla (and that is occurred only last year), presumably due to the higher altitude?

Quandialla and Forbes both have almost 50 % more hotter days (greater than 30 degrees C) than Grenfell. My theory is that altitude has the biggest impact. Forbes and Quandialla have both recorded higher maximum temperatures than Grenfell. It’s also interesting to note how hot it was at Forbes in 1882 (and a pity I don’t have comparable data for the same year for Quandialla and Grenfell).

I’m not sure whether it is valid to do the calculation I’ve done in point 7 above (deducting ‘hot’ days from ‘growing’ days to in effect come up with a ‘net annual growing days' number), but if it is then the result effectively means that all 3 locations have about the same number of net growing days. Grenfell is renowned as one of the best dryland cropping regions in Australia. The above statistics would suggest that this is driven by comparative altitude (less hot days to knock the crops around), comparable growing days and higher mean annual rainfall.

Ochre Arch’s altitude varies from around 315 metres to 415 metres above sea level. This provides us with some excellent opportunities to site different species of fruit & nut, and vegetables to suit their natural requirements. For example, we’d place plants needing more frost on the lower areas near the gullies; and those plants that do not like frost toward or on top of the hills. From a ‘human habitation’ angle it would seem that given the house is on the lowest part of the property it will be subjected to the coldest nights and hottest daily temperatures. Perhaps we need to relocate it to the hills! It’s also my gut-feeling that the amount of rain that falls on our higher country is higher than on the lower. In time I hope to prove or disprove this as we now take rainfall measurements at both the house and near one of the high points on the property.