27% of world temp stations are in Urban areas.???
Gosh, it depends on HOW YOU DEFINE urban
Some cases are easy. Hong kong, LA, NYC.
But even in cities we know there are cool locations.. yup, areas that are COOLER than rural sites.
Still the question remains what is rural what is urban?
So, a while back I took a look at that
here are the criteria we looked at
1. The population: at 1km resolution and 5 minute (8-10km) resolution.
We then did sensitivities. That means we classified stations as urban or rural based on a rule
that we changed ( if population less than (LT) 1 people per km, its rural, then LT 5,10, 15, 20, etc)
so since there is no well defined cut off you look at ALL cutoffs
2. Urban extent measures. Is the site within the administrative boundary. basically city limits type
metric
3. Land class metric: Did a satellite say it was a city?
4, Nightlights? Are there lights on at night? here too wee have to do a sensitivity.. 0 lights to just a few
5. Impervious surface. Did the DSMP satellite think there was asphalt there.. How much? here we
do a sensitivity.
6. Modis land cover? did the modis sensor see any asphalt? within 5km of the site? 10 km? 20 km?
how much?
7. Airports. was the site close to an airport?
in short. I didnt just make an arbitrary decision 5000 people is urban.
I didnt do what hansen does ( nightlights below 15 is rural )
Nope.
I created a process for testing THOUSANDS of definitions of rural and urban
What if I define rural as less than 10000? 5000? 4000? 3000, 2000, 1000, 500, 250, 100, 50, 10, 5, 0
What if I define rural as less than 20 nightlights, 18, 15, 12, 9, 5, 3,0?
What if I define rural as 20% ISA, 19% 18% etc
What if I combine all these filters? no people, no asphalt, no airports.
That “classifer” then gets used to divide the stations into two piles.
urban and rural.
Oh ya,, then there is the three pile version… rural urban and cant decide.
Oh ya, then there was the version where a rural % was created from 0 to 100%
Anyway, you end up with thousands of ways to divide the stations into various test piles
And then you test one pile versus the other.
Try to find a UHI signal in global data. Its really hard.
Try doing it thousands of times varying the definition of rural.
Still hard to find.
when you finish doing that people will still say.. I cant believe you didnt find it.
Hypothesis: If you just look at rural stations the warming will disappear.
Tested that.
Opps, its still is warming. about as much as the ocean which has no UHI?
Hypothesis busted.
next comes the moving of goal posts.