Tuesday, October 13, 2009

Analysis of Australian Temperature - Part 5 - Cloud cover over time

What effect does cloud cover have on Australian temperatures? Has cloud cover increased or decreased over time? Obviosuly, and especailly during the day, the more clouds that abound, the lower the temperature, and conversely the less clouds during the middle of the day, the higher the temperature.

When the sun is set, the relationship is a little more dubius. Cloud free summer days will have hotter nights if there is cloud cover, however the more days that are cloud free, the higher the atmosphere will be to start off with.

Either way, have Australia’s cloud cover increased or decreased over time? Fortunately, the Australian Bureau of statistics provides good data on this, and we are even able to measure the level of cloud cover as a variable over certain times of the day.

The following are Australia’s cloud cover anomalies from 1950 until current. Note that from about 1996 to current for midnight, and from 2000 to current for 3am, limited data was available.

SO what does this show? Well quite clearly it shows no major change in cloud cover at midnight, but from then on the level of cloud cover decreases dramatically overtime, reaching its decreasing trend peak at 3pm, and then the trend increases, where it actually has a positive value at 9pm, before stabilising.

Take a special look at the last five years of cloud cover from 9am to 6pm. These are dramatic negative cloud cover anomalies, and if you remember what the temperatures looked like during these last few years at the same times, perhaps there is a strong relationship between the two.

It is interesting, that during the heat of the day at 3pm, the cloud cover trend is at its decreasing peak. This is interesting. Scientists do not know much about cloud cover and the reason why its levels vary. However quite clearly here, a pattern arises where the level of cloud cover has decreased over time up until 3pm, before increasing back to a level state. This is highlighted by the graph below.

What happens to Australia’s temperature over time, when we account for the changing values of cloud cover? We will find out in the next article.