False Temperature Indications

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Significantly different temperature readings between two towns can occur even if both towns are the same temperature in most places. Differences often occur due to the exposures of each Stevenson's Screen and the type of screens used.

Most temperatures are measured in very open areas where there are no trees or buildings nearby. This enables more breeze and heat loss than most areas of town where most people live and work. Also heat is trapped and reflects more from buildings, bitumen and concrete. Hence built up and sheltered areas are often 3 degrees warmer than the official temperature readings for the town.

Tolga's temperatures are measured on a sheltered but open suburban block. Therefore, they would give a true indication of what people feel in the town. However, Atherton has their temperatures measured in an open valley which gives a good indication of the shade temperature in an open valley such as on a farm. However, the readings would indicate cooler temperatures than most of Atherton. Atherton and Tolga are very close together and in the same climate at the same altitude. Therefore if there were two identical screens in the same exposure in both centers the temperatures would probably be almost the same if the weather is the same at both centers. However, the screens are not in similar exposures, therefore it would be more accurate to say Atherton's measurement is a good indication for an open valley in the Atherton or Tolga area while Tolga's reading is a better indication for the higher residential areas of both Atherton and Tolga.

In many ways it is better to have the two measurements in different exposures because it gives a good idea of the range of temperatures in the area. In 1992 the Atherton weather station was shifted from the CSIRO in a suburban location. Hence this would cause a significant change of temperature. It is better to leave the same style screen in the one location to avoid confusion and enable accurate comparisons between past and present data. They were thinking of moving the Atherton station again to another location which would cause more confusion but fortunately they decided not to. From this we conclude that Tolga's temperature average would probably be not much warmer than Atherton and temperatures vary considerably depending on the exposure of the screens they are measured in.

When there is wind and cloud cover there may be no nighttime temperature difference between the hills and the valley where Athertonís temperature is measured. But when the night has a period of calm weather and a clear sky there is a big difference because very cold subsiding air gets trapped in the valley but escapes from Tolga and most places in Atherton. Most places in Atherton would have the same minimum as Tolga. With cloud cover warm air is blanketed in and it remains in the valley. Wind blows cold air out of the valley and it is replaced by warmer air on the slopes.

There is another factor that few people know about. The standard of Stevenson's screens have changed in the last few years. A few years ago most weather stations had the old standard screen, but today most have the new standard screens. Unfortunately the new standard screens are reported to be about one degree warmer when the sun is shining. This makes past and present temperatures for a particular center difficult to compare. Since the new screens were introduced the maximum temperatures would have gone up by about 1 degree on most days. Tolga and Cairns are the only two centers I know of in the local area using the old standard screens. Green Island has a radiation shield which is much smaller than a screen but compares well to the new standard screen. It is in a more sheltered area than the old standard screen in Cairns which is in a very open area at the airport. Hence Green Island is often reported about 3 degrees warmer than Cairns. If both centers had the same screen in the same exposure Green Island would probably have a slightly lower maximum than Cairns because you would think the sea would act as a heat sink. That is if both centers had similar weather.

What is really needed for an accurate comparison is screens placed in exposures comparable to suburban back yards with a few trees where most people live or work. The existing screens should be left where they are. This would mean two readings for some centers like Cairns. Such a move would clarify current confusion about temperature readings.

Click here for some details of screens and their exposures for the different local centers.

Records containing only the minimum and maximum temperatures do not tell you how long the temperature was at what value. Hence two centers may have identical minimum or maximum temperatures but one will be much colder than the other. For example Cairns nights probably have always been warmer than Tolga's but there are times when the Cairns minimum is the same as Tolga's. But Tolga's temperature would hover near the minimum reading for a long time while Cairns would have only experienced the low temperature for a short period. A short calm clear period in the night could be experienced in Cairns but not at Tolga. The coolness will depend on how long it is cool for especially in buildings, since buildings are insulated from temperature spikes.

Also, weather measurements that are taken a couple of times a day may be taken when the variables read totally different than what they read for most of the day. Weather variables vary considerably throughout the day, as you will see from the graphs.

Did you know that in most occasions when the relative humidity is higher it is cooler and often when it is between 90 and 100 percent you have a coat on? There is more information on the humidity page.

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