In my last post I showed how on average temperature changed diurnally across a number of Queensland BOM stations. In this post I will show examples of temperature change at some of these stations. I am using “one minute data’, which despite its name, is really the value at the last second of every minute, in other words, sampling at 60 second intervals.
We know that temperatures spike up and down every few seconds, but these spikes are not captured by the Bureau unless they are the highest and lowest for each minute, and only noticed by a keen observer if the highest or lowest temperature spike so far that day occurs in the same minute (usually on the hour or half hour) as final second temperature reported at the Station Observations page.
Let us begin with this plot of a temperature spike at Maryborough Airport on 15 February, kindly reposted by Anthony Watts. This was one of many examples from different locations around Australia of times when the maximum temperature of the day occurred in the same minute as a half-hourly recording, but exceeded it by a large amount (1.5 degrees in this example).
Figure 1:
Please note that for this plot I only had access to the half hourly data from the Bureau, supplemented with some time offset data from the UK Met Office, usually 10 minutes before the BOM values. With the higher resolution given by one minute data, we can gain a better appreciation of what was happening on this day.
Figure 2:
Note the spike at 13:00. It is just part of the constant fluctuation during daylight hours which is not apparent from the data available for Figure 1.
Let’s have a closer look at the period from 12:00 to 14:00.
With the caveat that we can only guess at the 59 one second values in between the final second samples, we can use the latter values to investigate temperature response by day and night to various influences. Assuming that the intervening one second fluctuations are approximately equally above and below a 60 second de facto mean represented by the value at the final second (as the Bureau’s Fast Facts would have us believe), a centred 5 minute mean of one minute (final second) data would approximate a mean of the complete 300 seconds. I use a centred 5 minute mean to compare with the one minute data, but please understand this is an approximation, a best guess, when applied to short time lengths. Its real value will be with all 115,200 data points- more later.
Figure 3:
Firstly, note how well the five minute centred mean represents most of the larger fluctuations, while considerably smoothing the final second data.
Secondly, note that the day’s maximum, 33.7C, was reached in the final second of 12:59, and was still at 33.7C at some second of the next minute, before falling 1.5 degrees to 32.2C in the final second of 13:00.
Thirdly, note that if this was a station in the USA, where 5 minute means are used, the maximum for the day would have been approximately 32.5C, still 1.2C less than the official value.
The temperature also fell 1.6C in the 60 seconds to 12:53. And here are all the minute to minute temperature changes at Maryborough on 15 February (large outliers circled).
Figure 4:
As shown in the previous post, this is the typical diurnal pattern. Figure 5 shows one minute temperature fluctuation for the whole period, 1 January to 21 March 2017.
Figure 5:
Note the swelling of fluctuation in daylight hours, the constriction at sunset and sunrise as heating/ cooling regimes change, and the outliers: values can change by up to +2.3C or -2.1C in 60 seconds.
And here is an example of how a day’s temperature can change quite naturally, but we have to ask: would a mercury thermometer be able to match this?
Figure 6:
I now turn to other stations. Hervey Bay Airport is about 30km from Maryborough Airport, only a couple of kilometres from the sea. Firstly, how temperature changes from one minute to the next for the whole period.
Figure 7:
Note that the daily increase in fluctuation is much less than at Maryborough. Hervey Bay Airport is only a couple of kilometres from Sandy Strait, and proximity to a water body may be a tempering influence.
Note also the large outlier of -2C in one minute- still less than the 2.2C downwards spike on 22 February in less than a minute, which prompted my first query to the Bureau! What could have caused such an outlier? Here’s the one minute temperature plot for 16 March:
Figure 8:
This outlier was the result of an entirely natural weather event, a sudden cool change, possibly a storm front: 4.4mm of rain was recorded at 09:00 on the 17th. Would a mercury thermometer be sensitive enough to capture that?
And here’s 22 February:
Figure 9:
Note the unusual spiking between about 04:30 and 06:30. Something was going on. Note also that the minimum temperature at 06:00 was far below at 23.2C, 1.6 degrees below any other temperature that day- for one second.
I now turn to Thangool Airport, a few kilometres from Biloela in the Callide Valley, 150km from the coast.
Figure 10:
Note the same shape, and though much further inland, not apparently different range from Maryborough. Most of the change between 09:00 and 15:00 is within the bounds of +/- 1 degree each minute, but there are many outliers.
I shall now look at how temperature changed on a sample of days. Firstly, 31 January shows a typical temperature curve for a clear sunny day.
Figure 11:
Figure 12 shows 7 January, a day with a mid-morning drop. 0.2mm of rain was recorded on the 8th.
Figure 12:
Note how after the sudden plunge the temperature quickly returns to “normal” as if nothing has happened.
28 January shows a late afternoon drop with a smaller recovery until sundown.
Figure 13:
Figure 14:
Note the typical warming curve which lasts until 16:47 when there is a sudden drop of 2.3 degrees in 3 minutes, with continued cooling. I suspect a wind change was the cause.
Figure 15:
This shows a midday weather event, with the rapid return to the “normal” curve. 6mm was measured next morning.
Figure 16:
Note the sudden spike mid-morning. The temperature spikes nearly 4 degrees in a few minutes to a value not expected for another hour or two. This is odd and I cannot think of a natural weather event that could be the cause. Whatever the cause, I doubt a mercury thermometer would track this change.
The final station for this post is at Lady Elliott Island, about 80km off the coast in the Coral Sea. The screen is on white coral sand, about 100 metres from the water to the east. First, one minute change over the whole period.
Figure 17:
Note again the typical shape, but with much smaller daytime range of changes than inland sites. Upward outliers are muted (there is only one instance of a temperature change in one minute of more than one degree). However, downwards outliers are large and occur throughout the 24 hour period.
Here are some plots of several days on a tropic island.
Figure 18:
Note the early morning downward spikes: rain showers.
Figure 19:
Note the sudden drop just before midday: another rain shower. But note how the temperature quickly returns to nearly what it was before.
Figure 20:
Again, morning showers (quite normal near the sea in the wet season).
Now for the largest one minute temperature drop of -2.3 degrees just before midnight on 14 March.
Figure 21:
Now watch the temperature recovery next day.
Figure 22:
So, with a drop of nearly 6 degrees in a few minutes, this was a perfectly natural weather event. Apart from sudden weather generated decreases like those shown above, it seems that there is a floor to minima of about 26C to 27C, due of course to the sea temperature.
While these examples are interesting, what about a day with sunny, fine weather? Here’s the plot for 16 February.
Figure 23:
Note a much more regular daytime curve (with rapid large spikes between 09:00 and 15:00), peaking only just after midday- except for a spike at about 14:30. Here’s a closer look at the time from 12:00 to 15:00.
Figure 24:
The second largest downwards spike (-1.3C) of the whole record occurred at 14:32. This was purely a spike, not due to any weather event. Could a mercury thermometer possibly match this? If not, it would not reach the same maximum (30.8C). On a hot sunny day on a coral island 100 metres from the sea, daytime temperature spikes up and down rapidly by up to a degree (or more) at a very high frequency. Compare this with Maryborough in Figure 3.
This confirms generalisations I made in my last post:
“Temperatures in daylight hours are very volatile, while at night temperatures change very little except in unusual weather events. Fastest and most sustained warming is in the hour after sunrise. Fastest and most sustained cooling is also in daylight hours. Night time cooling is much more gradual. Cooling is on average more rapid than warming. Rapid warming occurs when the sun suddenly appears. Rapid cooling is associated with weather events such as rain storms.”
The Bureau of Meteorology have claimed that their AWS sensors are so designed that they mimic the mercury in glass thermometers they have replaced. They claim a mercury in glass thermometer would track the above fluctuations closely. However they have as yet provided no papers or comparative data to back this up. From analysis of these stations’ data, I find that hard to believe.
Again we say, show us the data.
kenskingdom 