Thursday, December 17, 2009

Analysis of Australian Temperature - Part 6 - Relationship between cloud cover and temperature

One would expect that a greater amount of cloud cover would result in a lower temperature and a lower amount of cloud cover would result in a greater temperature. We should previously that the amount of cloud cover in Australia has been decreasing since 1950, and we also showed that temperatures in Australia have been increasing, especially during the middle of the day, and temperatures have not been increasing during the night.

So what influence does the amount of cloud cover have on Australian temperatures during certain parts of the day and night? Shown below are the graphs of cloud cover anomalies verses temperature anomalies since 1950.

















So what is quite clear from the above graphs is the clear relationship between cloud cover and temperature during the day. All relationships were significant apart from 9pm and Midnight, where no significant relationship was found. Interestingly, the relationship between cloud cover and temperature at 3am and 6am was positive, in that greater amounts of cloud cover was related to greater temperatures.

This is not that surprising, as cloud cover at night works as a blanket, helping to keep the temperature in. From 9am until 6pm, the relationship is undeniably negative, in that lower amounts of cloud cover is related to higher temperatures, and greater amounts of cloud cover is related to lower temperatures.

This is summarised in the graph below:



Here we can see that the strength of the relationship increases until Noon/3pm and decreases after/before this.

So it is clear that cloud cover amounts have a very strong relationship with temperature, as we previous hypothesised. As we showed previously that cloud cover levels in Australia, especially during the day, have been decreasing, what effect does this have on temperature?

Can we adjust the temperatures for this decrease in cloud cover? What effect will that have on the long term averages on Australian temperature? We will find this out in the next post.

21 comments:

Ben said...

Most of us knew instinctively and rationally that less clouds means higher temps during the day and lower temperatures during the night.
What's interesting is that apparently over time, the daily cloud cover over a large mass of land (Australia) has declined over the past what, decades?. A clear indication that it's a natural phenomenon unless cloud cover can be shown to be inversely related to CO2.

Jonathan Lowe said...

yes I'm not sure if cloud cover and CO2 are related, they might well be. Either way, cloud cover is decreasing in Australia and its highly related to temperature:

Jonathan Lowe said...

Interestingly, Croke et al. (1999) determined that the mean cloud cover of three regions of the United States (coastal southwest, coastal northeast and southern plains) rose from 35% to 47% from 1900 to 1987, while global mean air temperature rose by approximately 0.5°C. Likewise, Chernykh et al. (2001) determined that global cloud cover rose by nearly 6% between1964 and 1998.

Rafe said...

When you write "From 9am until 6am, the relationship is undeniably negative", I think you meant to say "9am until 6PM".

Jonathan Lowe said...

thanks for that rafe, I shall correct.

Anonymous said...

You say these are significant but I didn't see an R^2 above 0.55. How is this? I thought only R^2 above 0.8 was significant.

Jonathan Lowe said...

Hi Anonymous,
significance is based on a lot of things, and one of the most important is sample size. There is no specific R^2 value that proves significance. A significant value is one where the probability of a false positive is less than 5%.

Anonymous said...

I had no doubt that less clouds during the day would mean warmer day temperatures, more clouds duing the day cooler day temperatures. This follows from more radiation falling on the surface during clear days and less during cloudy ones.

Similarly at night, its colder on clear nights and warmer on cloudy ones. You can observe this on a daily basis, in the absence of wind.

You say "Can we adjust the temperatures for this decrease in cloud cover?"

Why should you adjust the temperatures? A hot day is a hot day and a cool one a cool one. The temperature tells us so.

The reason for this maybe more or less cloud cover or cool winds from the south or warm ones from the north, but the temperature records should not be tampered with.

In the case of Australia, decrease in cloud cover is probably due to it getting drier, which should result in hotter days and cooler nights.

Is this happening? And how does one average the temperature over the day? Is this averaging accurate?

Regards - Richard

Anonymous said...

I think that is specious reasoning. You could probably prove that winter is cloudier than summer, but the logic of this comment suggests that winter is colder because of increased cloud cover, and not because clouds form at colder temperatures.

davidmhoffer said...

I am curious if you are going to extend this study to include analysis of cloud cover effect seasonaly? I live in an extreme north temperate zone where it is common knowledge that more cloud cover means cooler summers and warmer winters. Or to be more accurate, increased cloud cover means summers that are less hot and winters that are less cold. But you have to be far enough north (south in your case) for this to become significant. Since a lot of long term temperature reconstruction is being done from tree ring data, and the trees being used come from Siberia in a lot of studies, I always wondered if the growing season was reflective of the annual mean since increased cloud cover would have opposite effects on temp in summer vs winter for similar reasons to day vs night.

Jonathan Lowe said...

Hi David,
you say you live north, so we ar talking broome-darwin-carins? This is Australian data we are talking about. I understand that the winters up there have lots of clouds and unstable temperatures during such time. And yes I will do a seasonal analysis of cloud cover. thansk for the comment.

knowledgedrift said...

lol... I meant WAY north. A whole hemisphere. Winnipeg, Canada.

I any event will be very interested in your results.

Jonathan Lowe said...

thought you may have. I only have Australian data from the Bom that I am analysing.

knowledgedrift said...

tx. doesn't matter where the data is from as long as the latitude is high enough that the sun's inclination would reverse cloud cover effects from cooling to warming based on season. that's the effect I am interested in and how it relates to annual temps.

Kmbh Gordon said...

Having just discovered your blog, I wonder how comparable the noon temperature is from standard time to daylight saving. BoM records the time of the max wid-gust at "local" time, which can vary according to which side of the Tweed you are on. My vague recollection of schoolboy meteorolgy in GB is that we always worked on GMT, but what does the BoM do?

Jonathan Lowe said...

work on AEDT. SO if there is daylight savings, then its included, if there isn't, it's not.

Tim Curtin said...

JL: Excellent work, I much enjoyed reading all 6 parts. I have done some work on similar lines, but regressing min & max temps on solar radiation/insolation and atmopsheric CO2 amongst other variables. I'd be glad to share it with you if you contact me at tcurtin "at" bigblue.net.au

Kmbh Gordon said...

Time of record.
So a noon reading in NSW/ACT/VIC suddenly flips to 1100 AEST on one day of the year, and continues there for summer, flipping back to noon AEST at the end of summer (QLD having carried on with AEST throughout). Is this significant to your study?
"Adjustment" of temperature records seems to be the norm in climate science, but I would hope to see a raw reading set alongside the "improved" reading and a clear justification for the adjustment.

Jonathan Lowe said...

hi tim, this isn't the final analysis of the australian temperature record.

And Kmbh, yes this is correct. At the moment the analysis is based on what you said, so no adjustments. You might want to read the summer vs winter thread which would talk about the day light savings issue.

ABC NEWS WATCH said...

How do I get in touch? Would like to see a graph comparing Melbourne CBD, Laverton and a station by the name of Durdidwarrah, west of Laverton to highlight growing UHI effect.

MarcH

Jonathan Lowe said...

you can get that data and most of everything else data here for free: http://www.bom.gov.au/climate/data/index.shtml

Link