A day in the life of Perth's summer heat waves and a climatic outlook

The above graph shows the temperature as measured in Perth (blue dots) every half hour over the last 4 days. The spiky one going up to nearly 45 C was today. The hottest two days show that the sun does its work in the mornings at a similar rate but starting at different night time temperatures (i.e. the slopes are the same). All the days show a marked drop off as soon as the sea breeze starts to hit whereby the wind direction flips from east to west and we get some cooler sea air affecting the temperature. There is often the sharpest shift of the day associated with this. The red line shows the Pearce Air Base today (5th Jan) which is currently experiencing thunderstorms. At 2 pm it had a huge hit whereby convection was initiated and clouds formed producing a massive 8 degree fall in just 30 mins. The overall trend in the evenings is one of gradual decline however and it shows how the sun's energy dissipates slower than it is received in the morning due to city concrete radiating it back into the air. We can also see that the nice cool evenings of last week have been superseded by 24C night time minima owing to a new air mass swinging in from the east. This is due to the precession of large air pressure gyres typically going from west to east, producing large-scale change in wind patterns.

The second of my weather-related musings involves a 120 year look at Perth's climate using average monthly data since 1876. See below:

Each dot represents an annual average from a 20 year period. The vertical scale is the same in both plots showing average monthly rainfall for each year in mm which varies considerably from over a metre down to around 40 mm in the most recent decade. The horizontal scale in the upper graph is the average minimum temperature for the year; the lower plot has the average max. The black line with grey dots shows the average of these statistics for a 10 year period, ending up towards the drier end in each case.

Clearly, a significant turning point is reached in the 1930s to 40s where a cooling max and warming min temp switches to a pulse in wet weather. Then the max temp just continues to rise and we get dryer. At the same time the min increases dramatically, then decreases dramatically and now we're on a slight increase again.

So its not a story of all doom and gloom but there we are in two decades of the hottest and driest times it has been since 1876. However it is conceivable that prior to 1876 it was also hotter and dryer, similar to today, according to an extrapolation on the lower graph. 2010 and 2006 were the driest two of the last 120 years. Therefore, if we are indeed entering a new age of global warming (i.e. beyond what can be explained by the early phase of the industrial revolution) then we are in for hot and dry times. The next few decades will be interesting in Perth because we may either see a return to the seemingly cyclical behaviour on a ~150 year scale...or we'll go off on a new path to uncharted and dry territory.

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