Guyra Warmer than Armidale?

01st Oct 2024

Well, certainly not if we just look at the average daily maximum temperature. According to the BoM figures, the Guyra Hospital weather station has an average maximum of 17.8 degC while the figure for the Armidale Tree Group Nursery in Mann St is 20.4 degC. So the average daily maximum temperature is 2.6 degC cooler in Guyra compared to Armidale.
However, when we look at the average daily minimum temperature, there is a hint of something interesting going on. The average for Guyra Hospital is 6.6 degC, compared to 6.3 degC for Armidale Tree Group Nursery. So Guyra’s average daily minimum temperature is 0.3 degC warmer than Armidale’s.
Since it’s the difference in average of daily minima that is unexpected, let’s look at the winter temperatures. The average daily minimum temperature in winter (June - August) for Guyra Hospital weather station is 1.3 degC, and for Armidale Tree Group Nursery it is 0.1 degC. Again, Guyra’s average is warmer than Armidale’s average, this time by 1.2 deg C.
With Guyra being some 300 metres higher than Armidale, we would expect Guyra’s winter minimum to be colder than Armidale’s on average. Perhaps it’s something to do with the wind, so let’s look at the temperatures and wind together.
We can use the 9am observations published by the BoM, which include the wind direction and speed. A bubble plot is handy for showing the overall picture hidden in a lot of data. In the bubble plot pictured, each bubble represents a winter’s morning in 2023 or 2024. For example, the blue bubble near the bottom with an arrow pointing to it is the 1st of August, 2024.
At 9am on that morning, the temperature at Guyra Hospital was 3.6 deg C (follow the horizontal grey dotted line) and at Armidale Tree Group Nursery it was 7.0 deg C (follow the vertical grey dotted line). The size of the bubble is proportional to the wind speed at that time and the colour shows the direction. For our example morning of the 1st of August 2024, the wind speed at Guyra Hospital was 26 km/h, coming from due south. So the bubble representing this morning is moderately large and blue. With this way of plotting the morning temperatures, all the bubbles above the 45 degree line (the pale pink area) are the mornings when Guyra was warmer than Armidale. All the bubbles below the 45 degree line (the pale blue area) are when Guyra was colder than Armidale. The further a bubble is away from the 45 degree line, the greater the temperature difference between Guyra and Armidale.
Straight away we can see that the mornings when Guyra is colder than Armidale tend to have strong winds from the west round to the north (the large yellow, brown and red bubbles below the 45 degree line. The mornings when Guyra is warmer than Armidale tend to have light winds or calm conditions (the small bubbles and squares above the 45 degree line).
There are two possible reasons for these differences. The first reason is what is known as adiabatic cooling and heating. When the wind comes over the Guyra plateau from the west or northwest, the air cools due to the decrease in pressure with increasing altitude. Then, as the air descends from Guyra down to Armidale, it heats up again, due to the increase in pressure with decreasing altitude. This explains why Guyra is cooler than Armidale on windy days. However, on still nights or early mornings, there is a second reason that comes into play. Known as cold air drainage, on still winter nights, the cold air close to the ground drains off the higher parts of the landscape, and down into the valleys. This leaves the higher parts, like the Guyra plateau, warmer than in the valleys, like Armidale.
So, is Guyra warmer than Armidale? Not on average, but on most still winter nights up round the Guyra Hospital weather station, it certainly is.
Ian Reeve
Black Mountain

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Guyra Warmer than Armidale?

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