Urban Heat Islands in Small Towns - Deniliquin, Australia

This photo shows the position of the Stevenson screen at the back of the Post Office in Deniliquin. The temperature was sampled at this site from 1873-1971. Note the close proximity of buildings and paved areas.

Recent research in Deniliquin suggests that as country towns grow they experience warmer nights. The warming of the nighttime temperature is due to the Urban Heat Island (UHI) effect, which is the result of two main features of urban areas. First, buildings, roads and paved surfaces store heat during the day, which is then released slowly over the evening due to the thermal properties of the surface materials and the building geometry which traps the heat stored during the day. The second contributing factor to the UHI is due to the artificial heat released into the urban atmosphere by combustive processes from vehicles, industrial activity and the heat that escapes from commercial and domestic air conditioning.

The controversy surrounding the role of the UHI on estimates of global warming has focused critical attention on Deniliquin's 136 years of temperature record. Few towns in Australia have a longer history of temperature measurement, hence it is a logical choice for research of the UHI and its effect on our understanding of climate variability.

Scientist's have detected global warming of the earth's atmosphere by various techniques. One technique used in Australia involves looking back at our climate records from many of the Bureau of Meteorology's monitoring stations located over the Australian continent. Unfortunately many of these monitoring stations are not suitable for studying what has happened to our climate over long periods of time due to their short station history. However in Deniliquin the Bureau of Meteorology established a monitoring station in 1858. This station along with many others in Australia are used to evaluate changes in our climate such as global warming and El Nino related climate variability such as drought.

The increase in temperature over Australia is determined by records of regular daily measurements of the air temperature. These are taken at 1.5 meters above the surface in a screen that shields the instruments from direct sunlight, so that the measurement is of the air temperature in the shade. (The Stevenson screens are the white boxes in each image on this page.) The measurements that have been recorded include a temperature measurement for the daytime maximum and the nighttime minimum temperature. Research conducted in Australia has shown that over the past 70 years in most of Australia the greatest increase in temperature has occurred in the nighttime minimum temperature.

Deniliquin's maximum and minimum temperatures were recorded at the Post Office from 1873 to 1971, before the station was relocated to the airport in 1984. The records from the Post Office indicate that in Deniliquin the annual average minimum temperature increased by 2.1 deg C until 1971. However when the monitoring station was moved out of the urban area, the last 20 years of record shows that the nighttime temperature is 0.6 deg C lower than the previous 98 year average. This indicates that the urban area of Deniliquin may be warmer than its surrounding rural regions. To test this, measurements were taken of the air temperature, wind speed and direction at seven locations along a transect on either side and through the center of the town. During February 1995, measurements in Deniliquin showed that on clear and calm nights, the town centre can be up to 4.2 deg C warmer than beyond the airport.

This is the current site at Deniliquin Airport. It represents a well exposed location away from the urban area and any surface materials which act to moderate the natural energy balance and promote the genesis of a UHI.

The recent measurements tell scientists that during the years when the minimum temperature was recorded at the Post Office it is conceivable that the measured temperature includes a positive temperature bias caused by Deniliquin's UHI. If climatologists use Deniliquin's temperature record to examine climate change since measurements first began at the Post Office then what they may actually be seeing is how the urban area has warmed as the town has grown, rather than how the larger scale climate has changed.

What researchers want to know is whether the nighttime temperature record for Deniliquin would show the same level of warming if the monitoring station had always been located outside of the urban area and not at the Post Office.

It is unlikely that the urban area of Deniliquin would be up to 4.2 deg C warmer than the rural area under cloudy and windy weather conditions. So after a series of more measurements, researchers hope to identify how Deniliquin's UHI varies between the seasons of the year and during different sorts of weather events. From these measurements climatologists can then critically examine the temperature record and make adjustments for the effect of the UHI on the observed temperature warming in the climate record. The research is on-going and whilst Deniliquin's temperature record before 1971 may not represent the `real' or larger scale climate, climatologists now need to account for this when they derive estimates of climate change.