You can click on this image to get an enormous 3200 x 3200 version.
This is a false color image. The Red is vegetation, while the city is grey and the dirt is brown. The river dividing El Paso, Texas from Ciudad Juarez, Mexico, is that thin line through the middle of the image from upper left to lower right, more or less along the vegetation. I-10 runs near it for much of the distance.
So Why Mention This?
While driving down I-10 at about midnight I noticed something “different”. Ciudad Juarez was golden…
About 20 years ago, while driving past, I’d noticed that El Paso was brightly lit (with a lot of that garish bright Mercury Vapor lighting). Ciudad Juarez was quite dark. Clearly the poor Mexicans could not afford night lights.
Now, while driving past, Mexico has plenty of oil revenue for night lights. Ciudad Juarez is brightly lit in that golden yellow color of High Pressure Sodium from edge to edge.
So I have to wonder:
How much do “night lights” tell use about the “size” of this area, really? Yes, it IS an urban area. Yes, it ought to be flagged as “urban”. But does the change from “dark” to “light” really tell us that Ciudad Juarez has suddenly come to life as an urban center, and that in 1980 it was rural?
It’s been urban the whole time. What the night lights tell us is that it is urban now. But the status in the past would be incorrectly classed based on lighting. Notice those mountains on the left hand side? The cities have be smack up against them for a very long time. Growth has been along the river and freeway (and a little bit back from them toward the desert out back…) How many other places in the world today are urban, but as poor as Mexico was 20+ years ago? How many places today will be classed by ‘night lights’ as rural, that are in fact densely populated poor?
Look At The Middle Top – Airport
OK, so in the middle toward the top we have the airport. All around it is brown dirt. “Night Lights” would tend to class a lot of this area as rural. Yet the airport will be a very hot place (black asphalt / tarmac is like that in the desert sun of West Texas / Northern Mexico…) So ‘night lights’ from a little N.E. of the Airport will tell lies about the airport.
Furthermore, you have city on 2/3 of the triangle of airport land.
There is only one direction from which the wind can blow that does NOT blow the UHI of El Paso / Ciudad Juarez over the airport (and the wind most often blows from the city toward the airport…)
There is just no way you can get a valid “rural” temperature from anywhere inside El Paso / Ciudad Juarez nor at the airport.
The only way to get a valid representation of the area would be to set up a station out in the rural area modestly far from the cities. And we have poor / no records from those places against which to compare.
In this case, Night Lights will indicate rampant growth of an area “changing” from rural to urban in Ciudad Juarez, when in fact all that has happened is that they have gotten enough ‘richer’ to install high efficiency lighting.
I would assert that in many cases the same will be true in the history of the USA as prosperity increased. When I was a kid, most towns had poor / little lighting. I distinctly remember 150 watt incandescent bulbs in ‘tin hat’ fixtures, widely spaced and not very bright. Today that same farm town has very brightly lit streets with very high efficiency lighting (much more light for much less power). The town has “grown” from 3339 to 4009 or so people during the last 50 years. Not exactly gigantic growth, and most of that from more people per house, very few new homes. The farm land around that farm town now glows with hundreds of low pressure sodium, mercury vapor, and High Intensity Metal Discharge lamps. Brightness has increased dramatically, while ‘growth’ has largely been elusive.
So I really do have to wonder: Just how valid IS night light brightness as an indication of ‘growth’ over a decades time scale? Does it not, much more completely, reflect a threshold of prosperity where street lighting becomes affordable? A measure of prosperity rather than physical size? And are not the technical changes much more important that the physical land use? We’ve changed from dim incandescents to very bright HID and Mercury Vapor. Those are now giving way in many places to High Pressure Sodium. (In between, we had the darkish egg yolk yellow Low Pressure Sodium. So won’t the ‘color spectrum’ sensitivity of the sensor be very important too?)
I don’t have the answers here, but it sure looks like there are a boat load of “issues” with using “night lights” as a proxy for urban vs rural temperature profiles.
One final note: Last time I looked, the Marine Air Station at Quantico Virginia was classed as “rural”. Military air bases often conduct night operations without lights (the folks need to be practiced for war time actions). So the airport at El Paso has gone from dark at night (in the old days when prop planes tended to fly Visual Flight Rules and mostly in the daytime) to the modern International Airport with 24 x 7 operations and wall to wall lighting. Yet it’s been a barren place with tarmac and concrete as those transitions happened. It seems to me that “night lights” will be spectacularly prone to failure at comparing places like El Paso airport to Quantico, and Ciudad Juarez to El Paso.
Finally, El Paso is looking more dim to me on this last drive through. Businesses are now ‘conserving’ by shutting off lights at night ( that used to be left on as ‘advertizing’ of sorts). The city looks to have dumped the older more garish lights for a more ‘mellow’ lighting. Does this mean that El Paso is shrinking?
No, I don’t think so James…
So why are we using a “proxy” for urban vs rural at all? There are only a few thousand stations currently in use in the GHCN. Is it really all that hard to hit Google Earth for a couple of hours and just LOOK at them and see what’s urban and what’s rural? Really?
Never use a proxy when the truth is knowable…
Still not convinced Ciudad Juarez is glowing Golden now?
Here’s a picture from NASA….
The bright white diagonal line is Interstate 10, the freeway I just drove through. All that Juarez part WAS nearly black a couple of decades ago, but was full of houses and shops in daylight hours, with a lot of dirt, pavement, and adobe buildings. Urban, but unlit. Now it is highly lit. Though I’d question the value of it. The social decay that has moved Ciudad Juarez from the “over the border night life hot spot” to the “avoid it due to murder capital status” and killed the “night club” industry in the process has not been slowed by the lighting.