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Most circulations calculate "normal" conditions as being the thirty years comprising the prior three decades. Nowadays, the decades used encompass 1991-2020. This made a certain amount of sense back in the day, when there weren't reliable records for many stations. For example during the 1960's, the period 1931-1960 may have been as far back as possible. While going back a century for averages may be a bit ambitious, how about, say, sixty year records would cover 1961-2020. I would hazard a guess that such records would be obtainable.
The thirty-year periods have shown some anomalies. For example, since I have been following weather since the late 1960's the thirty year average for late July has had the mean jump up from 77° to 78° and back again one or two times, and has now risen again to 78°. The January mean has jumped around from 33° to 32° and back to 33°. While I am a global warming skeptic I don't believe there's been a measurable decline in temperatures either. Rainfall figures have been even more choppy. A sixty-year base would smooth a lot of this out.
I can see the reasoning for preferring longer data sets due to multi-decadal oceanic oscillation cycles (AMO is about 75 years?). However, when you consider the magnitude of the impact these have on climate, vs the rate of climate change (about 0.5C/century in my area?), you don't want overly long time scales either since those can get out of date. 150 years would be too much for example.
I think for a lot of sites I would be ok with 50-100 years, but for locations that saw major UHI expansion like Pearson Airport I think I would rather use shorter timescales. Pearson has clearly seen a much stronger warming trend than other southern Ontario/Golden Horseshoe weather stations, whether those are sites that have been urbanized for over a century (ex Downtown Toronto), or sites that were and still are largely rural (ex Uxbridge, Hamilton or Waterloo Airport), or that have maintained a fairly consistent semi-urban character over the decades (ex Hamilton RBG).
I prefer a longer period of time for data like 75 years. Last decade was outlier and could skew things.
Every decade can be an outlier for something, weather, for warmth, cold, rain, or snow. For example, in the NYC Area the winters of the beginning of the 2000s through spring 2016 in the New York area had a rather snowy cast, with certain years such as 2001-2, 2007-8, and 2011-2 being exceptions. The summers of 2001, 2005, 2010, 2011 and 2012 all pierced 100. Longer base periods smooth out those anomalies.
I wholly agree on the 60 year averages. Note that I use the word averages, and NOT 'normals'-- as I understand, these 'normals' are subject to smoothing and other such mathematical fluff...the Bureau does not use 'normals', and as such Australian site averages get updated each and every month. So we instantly get access to the new averages as they are updated constantly with raw data only. Yet another reason why Australia is the best country to live in.
I can see the reasoning for preferring longer data sets due to multi-decadal oceanic oscillation cycles (AMO is about 75 years?). However, when you consider the magnitude of the impact these have on climate, vs the rate of climate change (about 0.5C/century in my area?), you don't want overly long time scales either since those can get out of date. 150 years would be too much for example.
I think for a lot of sites I would be ok with 50-100 years, but for locations that saw major UHI expansion like Pearson Airport I think I would rather use shorter timescales. Pearson has clearly seen a much stronger warming trend than other southern Ontario/Golden Horseshoe weather stations, whether those are sites that have been urbanized for over a century (ex Downtown Toronto), or sites that were and still are largely rural (ex Uxbridge, Hamilton or Waterloo Airport), or that have maintained a fairly consistent semi-urban character over the decades (ex Hamilton RBG).
I don't find UHI interesting. All it does is make a place look warmer than it is.
I don't find UHI interesting. All it does is make a place look warmer than it is.
UHI is important in order to keep in mind what you mention. That's why KEWR is notoriously unreliable, surrounded by 12 lanes of I-78 and NJ Turnpike, and the parking lots and tarmacs themselves.
UHI is important in order to keep in mind what you mention. That's why KEWR is notoriously unreliable, surrounded by 12 lanes of I-78 and NJ Turnpike, and the parking lots and tarmacs themselves.
On the topic of heat island, I think it might surprise some on this thread that for my area of expertise of the Southeast US, urban stations are actually closest to what the natural climate of the area would be like - at least in terms of low temperatures.
Some of you may have noticed airport stations in the Southeast US often have shockingly large diurnal ranges and surprisingly short frost free seasons, quite the contrary to what would be expected of a low elevation, humid climate. This is actually rather artificial: airport stations are cleared of the natural pine forest cover and so cool off far, far more at night than they would if they were forested due to enhanced sky exposure.
It's not like the urban heat island would produce overreading either: most Southeast US cities are not very urban at all despite those cold airports so if anything urban heat island may underread compared to a properly forested station for places like say Valdosta. I've mentioned all the above plenty in the first/last frost threads but hadn't gone quite so into it, so here's a discussion I had on Palmtalk about it: https://www.palmtalk.org/forum/topic...en-cold-spots/
I guess this means that urban heat islands might bring my area of expertise closer to what they would be without the artificial cold instead of making them warmer than they would otherwise be.
And as for how this relates to a longer base than 30 years, that's a problem for the Southeast US. The official stations have started out in good downtown locations but end up getting moved to those very cold airports, meaning the normals can only go back so far before you end up mixing up 2 different places.
Perhaps longer normals will become more feasible many years into the future. But for now it's something I won't necessarily do.
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