Experiment measuring pre-dawn urban air temperature in north Canberra compared to BoM Canberra Airport

Since 9th Feb I have been noting down the simultaneous temperature of pre-dawn air coming in my front door compared to Canberra Airport ~13km away as a crow might fly.

We are not that far from Hall, near farmland to the west and about 100m higher than the airport so in theory we might expect to be 0.64°C cooler. The front door faces east into a suburb and it so happens that most mornings a noticeable breeze comes in the door when opened.

I expected the vagaries of weather would produce a few days when we were cooler than the airport. It turned out that only on one day 16 March were we 0.2 cooler than the airport and that morning was blowing a warmish nor-wester at 16C – the wrong direction for the front door so I took a reading in the yard in full breeze.

Anyway – here is the chart, of suburban air temperature read in the dark minus the Canberra Airport reading off the www. Days of greater contrast tend to have clear skies – rainy days tend to show the least contrast. Average contrast over the 75 days – 3.7 degrees.

I wonder if readers ever try similar comparisons in their local region.

Note my thermometer is a little battery powered “thermistor” gadget with a metre of wire, digital readout to 0.1deg – which I had calibrated informally against two liquid in glass thermometers.

3 thoughts on “Experiment measuring pre-dawn urban air temperature in north Canberra compared to BoM Canberra Airport”

  1. warwick great to see your interest in weather

    hills are cooler than valleys during the day, but warmer at night as inversions form in the valleys.

    you have picked that out well when you say the contrast is higher on days when there are clear skies – that means that there is stronger nighttime radiation and a stronger inversion.

    on the day that there was a negative difference there was a strong wind that kept the atmosphere well mixed

  2. The pre-dawn temperature difference to the airport is surprisingly large. It sounds like you get displaced air when a valley wind forms at nigh. The valley wind at night would be downslope and cooler air, but the displaced (warmer) air has to go somewhere. I’ll guess you are not on a valley bottom, but closer to a ridge line.

    Airports; of necessity, are built on flat ground and not subject to valley winds, generally.

    Does the breeze blow in the opposite direction late afternoon and what is the temperature differential to the airport at that time?

  3. Warwick:
    I remember about 25 years ago a case SW of Sydney on the Liverpool plains. The chap there was recently from Germany and was so annoyed by the difference in what he felt in the early morning in winter and what was reported as the minimum at Liverpool, that he got a (roughly) half metre alcohol thermometer and mounted it on a pillar of his back verandah, on the inside under cover and facing an unheated portion of wall. He calibrated the thermometer in ice & water for the low end.

    His business required an early start so he was leaving home around sunrise. Having come recently from southern Germany he claimed to feel a difference much greater than reported. He said that he was getting readings anything up to 7℃ below those reported (which at that time were around -3 to +1℃). It was a new sub-division so semi-rural, but I wasn’t aware that there was a great height difference to the BoM site.

    Western Sydney forms a ‘hollow’ surrounded by hills/mountains. Obviously formed by river(s) as Parramatta is one of the outlets, the Nepean-Hawkebury another. In winter it seemed that cold air from the Blue mountains would slide down into this hollow. Working at/near Plumpton (near Rooty Hill) the morning temperatures were distinctly cooler than reported for Parramatta (sometimes only -1 but often -3 to -5℃ below forecast).
    This was a paint factory so many thermometers and people who could use them.

    Lastly, last year I recorded differences between my house and forecast and reported Mt. Barker SA minimums. I gave up as results variable. House is about 13 km. from Mt. Barker and higher in elevation, and on slight rise above the river valley so can see frost on farmland below when there is not always any on my block. Results most often lower than Mt. Barker as expected. For some reason local ex-farmers claimed minimum would be 2℃ below Mt. Barker but “figures were all over the place these days”.

    I don’t know if this helps but it does seem that placing bets on BoM getting its forecasted minimum right would be a chancy business.

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