Annual averages of daily maximum and daily minimum temperatures. Annual number of 'cold' nights and 'hot' days.
There is much interest in reporting the annual surface temperature across Australia. However, it is likely that the maximum and minimum temperatures will not necessarily change at the same rate as the average. Therefore, it is appropriate to report on both the maximum and minimum temperatures.
Cropping and other activities can be influenced by the frequency of especially cold nights and especially warm days, so trends in these figures are also of interest.
Torok and Nicholls (Internal link1996) produced a homogenised or 'high-quality' dataset of annual mean maximum and minimum temperature series for Australia. The primary purpose of this dataset was to enable reliable analyses of climate trends and variability at annual and decadal timescales. Each station record was adjusted for discontinuities caused by changes in site location and exposure, and other known data problems. Such discontinuities can be as large, or larger than, real temperature changes (eg. the approximate 1°C drop in maximum temperatures associated with the switch to Stevenson screen exposure) and consequently confound the true long-term trend. Generally the high-quality records were homogenised from 1910, by which time most stations are believed to have been equipped with the current standard instrument shelter.
Della-Marta et al. (Internal link2004) updated the Torok and Nicholls dataset, using most of the homogeneity techniques employed by Torok and Nicholls (1996), as well as a number of new methodologies to assess whether a record was likely to have been contaminated by urban warming, and to use available comparison observation data in cases where a station had moved.
There are five sites in Tasmania in the annual temperature dataset: Hobart, Launceston Airport, Low Head, King Island Airport and Strahan Airport. Hobart is assessed an 'urban' site, and the others 'non-urban'. All of these sites (apart from Launceston Airport) are coastal.
An Australian high-quality daily temperature dataset has also been developed, by Trewin (Internal link2001). This involved detection and removal of gross single-day errors and examination of available metadata for evidence of inhomogeneities. Rather than making homogeneity adjustments in mean temperatures, the daily temperature records were adjusted for discontinuities across their distributions. This makes the dataset particularly useful for examining changes in the occurrence of extreme temperature events, such as numbers of hot and cold days per year.
There are seven sites in Tasmania in the daily temperature dataset: Hobart, Launceston Airport, Grove, Cape Bruny, Low Head, Eddystone Point and Butlers Gorge. Of these, only Hobart and Launceston Airport had any daily temperature data digitised for the years prior to 1957. Low Head's earlier data has since been digitised, but has not been corrected. Butlers Gorge also has no data after 1992, so as in the annual temperature dataset there is little detail on inland areas.
For the two sites in both datasets – Hobart and Launceston Airport – there are some differences, as the methods of correction are different. In general this does not affect the overall trends, but it does highlight the complexities of the correction process, and indicate that neither data series should be taken as absolute 'truth'.
Both the annual and daily temperature datasets are available from the External linkBureau of Meteorology's web site.
The annual datasets of both mean daily maximum and mean daily minimum for the five sites in Tasmania through to the end of 2006 were extracted and processed.
The daily datasets for the seven sites in Tasmania contain several gaps, or months where a large number of daily values are missing. Months were excluded from further analysis if less than 60% of the daily observations were present. This especially affects Eddystone Point, Cape Bruny, Low Head and Butlers Gorge.
For each year and for each month, the number of days with minimum temperature below a set threshold was counted. This was divided by the number of valid observations in the month, and then multiplied by the number of days in a complete month to give a frequency in days. The results for the month were discarded if less than half of the days in the month had valid observations. This assumes that the missing data have a similar distribution to the valid data. Annual figures were calculated, but only if all 12 months had sufficient data. These annual figures are used to indicate the number of 'cold nights' in each year. A similar analysis was done for daily maximum temperatures above a set threshold, to determine the number of 'warm days' in each year.
For all statistics, a 'normal' value was calculated as the mean for the 30 years from 1961 to 1990. This benchmark period is widely used in climatology, and is recommended by the World Meteorological Organization (Internal link1988).
A recurrent problem in analysis of climate datasets is smoothing, so that long-term trends can be isolated from shorter-term variability. A common 'smoother' is the simple linear trend line, generally a least-squares fit to the data. One difficulty with this is the implicit assumption that there is indeed a linear trend in the series. Another problem is the tendency for the viewer to extrapolate from the linear fit to forecasts values into the future.
Another common smoother is a 'moving average', where the arithmetic mean over a certain span of time is calculated. By careful selection of the span, long-term variations can be made apparent whilst short-term ones are removed. Doing this can shift the phase of peaks and troughs in the series, so care must be taken when using such results.
One smoother that preserves the phase is the 'loess' or 'local estimator'; this has been chosen to smooth the series presented here. The details of the smoother can be found in R Development Core Team (Internal link2006). Problems remain at the start and end of the series, where the smoother can 'look' in only one direction. There are also possible problems when there are large gaps. However, the loess smoother provides a good overall indication of the underlying trends in the data.
In the following graphs, a loess smoother was applied with a span equivalent to 50 years; that is, the smoothed value is influenced by observations within about 25 years either side. This level of smoothing was chosen to give the a good visual display of the underlying trends rather than any other reason.
Annual mean daily maximum temperature
Interannual variability in mean daily maximum temperature for sites in Tasmania is typically less than 1 °C; that is, most years are within half a degree of the long-term average. This is, of course, much smaller than the variability seen from one day to the next, or even one month to the next.
Most of the sites in the annual mean daily maximum temperature dataset show a gradual rise in temperatures from the mid-20th century on. The size of this increase is typically 0.3 to 0.6 °C, less than the interannual variability.
Mean daily maximum temperature at Hobart
Mean daily maximum temperature at Hobartinternal SOE link to larger image
For Hobart, the smoothed curve shows a general increase from a relatively cool 1940s to a warmer 1990s and early 2000s, with the rise through that time about 0.4 °C. Although 1992, 1995 and 1996 were relatively cool years, they are more than balanced by the warm years 1988, 1993 and 2000. Interannual variability over the most recent two decades is generally greater than in the early part of the record.
Mean daily maximum temperature at Launceston Airport
Mean daily maximum temperature at Launceston Airportinternal SOE link to larger image
Launceston Airport shows a similar pattern to Hobart, with perhaps a stronger increase (around 0.7 °C) over the last 50 years.
Daily maximum temperatures in the early part of the 20th century appear to be relatively higher in Launceston, and comparable to those at the start of the 21st century. King Island Airport and to a lesser extent Strahan Airport and Low Head all show a similar pattern, which is not apparent at Hobart.
Mean daily maximum temperature at Strahan
Mean daily maximum temperature at Strahaninternal SOE link to larger image
Strahan's mean daily maximum temperatures show less overall trend than the other two sites, with recent decades very similar to those earlier. The second half of the Strahan record shows much more variability than the first half, with (for example) a very large swing from a warm 1988, 1989 and 1990 to a cool 1991 and 1992.
Annual mean daily minimum temperature
Interannual variability in mean daily minimum temperature is generally a little smaller than in mean daily maximum temperature. In the annual datasets, most Tasmanian sites show a minimum around the late 1940s. Hobart, Low Head and Strahan all show rises from then, but both Launceston Airport and King Island show a maximum around the early 1980s followed by a decline.
Mean daily minimum temperature at Hobart
Mean daily minimum temperature at Hobartinternal SOE link to larger image
Minima at Hobart are about 0.7 °C warmer in the later part of the record than the early part, and interannual variability appears to have increased slightly in the same period. At least some of the changes can be attributed to the increasing size and density of the surrounding city.
Mean daily minimum temperature at Launceston Airport
Mean daily minimum temperature at Launceston Airportinternal SOE link to larger image
At Launceston Airport, a significant feature is the peak in daily minimum temperatures in the late 1970s, followed by a decline in more recent decades. The late 1970s were more than 1.2 °C warmer than the later 1940s, but the mean daily minimum appears to have declined by about 0.6 °C in the last two decades. King Island Airport displays a similar pattern.
Mean daily minimum temperature at Strahan
Mean daily minimum temperature at Strahaninternal SOE link to larger image
Strahan's overnight minima have risen by around 0.6 °C since the 1960s. There have been no especially cold years in the most recent decade, exacerbating the upward trend.
Frequency of cold nights
Shifts of a few tenths of a degree in mean minimum temperatures may not appear important at first glance. However, small changes in the mean can be caused by marked changes in the extent or magnitude of extremes. Certain activities are very sensitive to cold nights. Some fruit crops require 'winter chilling' for success, whilst others can be adversely affected by frost. Also, domestic energy use for heating is much greater on cold nights.
Some parts of the state have many more 'cold' nights than others. The Central Plateau is at high elevation and is well away from the sea, so experiences many cold nights: Butlers Gorge averages over 90 nights each year with a minimum temperature below 0 °C. Locations on the northeast coast, on the other hand, are close to the warm waters of the Tasman Sea, and only rarely have cold nights: Eddystone Point only rarely drops below 0 °C.
As the expectation of low temperatures is different in each location, different threshold temperatures are used for each site as an indicator of a 'cold night'.
Number of nights each year 2 °C or colder at Hobart
Number of nights each year 2 °C or colder at Hobartinternal SOE link to larger image
The number of nights where the temperature dropped to 2 °C or lower in Hobart varies markedly from year to year, having ranged from less than five to over 20. Imposed on this variability is a general decline since the 1940s. Before the 1970s, most years had at least 15 nights 2 °C or colder; since the 1990s, it has become rare to have so many cold nights in a year. However, in 2006, there were 15 nights dropping below 2 °C in Hobart.
Number of nights each year -2 °C or colder at Grove
Number of nights each year -2 °C or colder at Groveinternal SOE link to larger image
Some part of the change in cold night frequency in Hobart can be attributed to the increase in the city surrounding the observing site. Grove, also in the southeast, should not be affected in this way. Grove also tends to experience more cold nights than Hobart. The number of nights falling to –2 °C or lower at Grove has fallen from around 12 each year in the 1960s to around 5 in the early 2000s.
Number of nights each year -2 °C or colder at Launceston Airport
Number of nights each year -2 °C or colder at Launceston Airportinternal SOE link to larger image
The average frequency of nights of –2 °C or colder at Launceston Airport is a little higher than at Grove. Launceston Airport shows a decline in cold night frequency from over 15 in the 1940s to around 10 by the late 1980s, but the more recent decades do not indicate a continued decline in cold night frequency. As at the other sites, 2006 was exceptionally cold at Launceston Airport too, with 19 nights reaching –2 °C; this gives rise to the upward trend in the smoothed line at the end of the series.
Frequency of warm days
Just as cold nights can affect some activities, so can warm days. Certain crops only become viable when there is sufficient heating during their growing season. On the other hand, energy use for building cooling increases sharply on hot days. Days over 35 °C are quite rare in Tasmania, especially along the north coast.
Number of days each year 30 °C or warmer at Hobart
Number of days each year 30 °C or warmer at Hobartinternal SOE link to larger image
The number of days reaching 30 °C in Hobart each year has increased since the 1950s, although not by as much as the number of cold nights has decreased. There is also very great variability, with 1999 having just 2 such hot days but 2003 having 14 of them.
Number of days each year 30 °C or warmer at Grove
Number of days each year 30 °C or warmer at Groveinternal SOE link to larger image
At Grove there also appears to be a slight increase in days reaching 30 °C in recent decades, but much of this is apparent rise is explained by the 17 such days in 2003. Overall, there is little consistent trend.
Number of days each year 30 °C or warmer at Launceston Airport
Number of days each year 30 °C or warmer at Launceston Airportinternal SOE link to larger image
Despite having a higher summer mean maximum temperature than Hobart or Grove, Launceston Airport experiences fewer 'hot' days. In many years there are no days that reach 30 °C, although the long-term average is around 4 days per year. There is no clear trend in such extreme days at Launceston Airport.
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