A Walk In The Grass, on Cement, and Asphalt

Tonight I went for a short stroll outside. The sun was down hours ago as it is now about midnight.

As usual, I was barefoot. I spend as much time as possible unshod. (It’s a long story…)

This has given me ‘educated feet’. I grew up running from white paint stripe to grass patch on the mile walk to the city swimming pool. Shade was cherished. Black asphalt in the sun a known hell. Albedo education by contact…

Tonight I was reminded of that. A first step onto the walkway that had been shaded most of the day, but with some afternoon sun, was pleasant. Just a touch of warmth. A stray step to the side onto grass was cold and a surprise reminder to say on the path… Despite identical sun, the grass was cold, the path warm.

Stepping out into the street, the warmth was greater. Asphalt is like that. Walking in the dark about a block, I could tell were trees had shaded the road, and where it was sunny from both sides. Returning on the sidewalk, under trees the cement was nearly cold. (In a California way… just below room temperature ;-) Again stepping on grass was quite a bit colder. On the sidewalk, exiting from under a tree, the cement warmed. It had had the sun hours earlier.

So what did my feet tell me on this walk?

That the sun matters, a lot. What was warmest was what had the most sun.

That plants matter a lot more. Even in direct sun, grass stays relatively cool. Trees are known to let water out to keep their canopy at about 86 F (factoid remembered from some study on forest canopies with a PDF filed “somewhere” and done “sometime” for “someday” that ought to be today, but isn’t…) At night, plants cool very rapidly. They are water evaporators and water keeps them cool. Plants prevent heat storage as they shade the dirt keeping it cooler and evaporate ground water keeping the air cooler.

Cement, and more so, asphalt, matter as heat catchers, storers, and general warmers.

During the sun, pavement warms more. Plants are busy transpiring water and keeping air temperatures lower. As pavement warms, heat is conducted into the pavement. Then in the evening, when plants are quite cool, pavement radiates and conducts stored heat into the air for hours.

Anyplace that was mostly plants in 1900 would be much much colder than any airport today. Most of our present airports were grass fields then. Now they are pavement. Often with brown dirt as mowing costs more than spraying herbicide. The largest ones an ocean of cement. Not much water evaporating in the sun. Not many leaves shading the dirt. As night falls, foot thick runway cement slabs heating the air well into the night.

That, alone, ought to account for the “higher lows” seen in recent data. IIRC, it was something like 90% of GHCN thermometers were at airports now. All would have been grass in 1900. That ocean of cement can do nothing but raise night temperatures.

For thermometers near cities, the urban jungle is known to be a few degrees warmer. Ask any motorcycle rider… it gets darned cold when you leave town and enter the trees. Summers in California in the ’70s were spent on motorcycles. I would dread passing a peach orchard being irrigated at night. The air would be quite cold. Entering town, the warmth was welcome.

It seems such a simple thing. Grass, trees, cement, asphalt, bare feet and wind in the face. The water cycle cools. Plants cool. Country cools. Dry pavement warms, and keeps on warming into the dark. Cities warm. Asphalt heats a lot. (Ever walk on an asphalt shingle roof barefoot in summer sun? I have. I won’t again…) Cities are covered in asphalt, cement, pavement, roofing. So are airports. We put our thermometers of record in or near cities and airports. They simply can not report what it was like when all was green and evaporating water. We can not compare now to then.

Simply put, the land temperature record can say nothing about long term temperature trends without shouting about human development effects and massive heat islands. The Los Angeles basin is a metroplex that stretches for a few hours at 60 miles / hour. New York City surrounds central park on all sides. Chicago blends into suburbs of pavement and roofing for miles and miles. London… so many buildings and cars and pavement in such close contact. There simply isn’t a place to put a thermometer anywhere near those places that is ‘as it was’ 100 years ago. Or even 50 years ago.

Yet that is where temperatures were taken back then. In the little villages surrounded by nothing but plants and rivers for 100 miles. The giant city in the making but not made yet.

That is what my feet told me tonight. That try as we might, we cannot un-cement and un-pave and re-plant the miles of land around the first thermometer locations and get a reading that is fair to compare to the one from long long ago. That “global warming” is in fact very local warming in the heat (and pavement) islands around thermometers that are never placed where no people live, but mostly put at the concrete jungles of modern jet airports. Places where walking barefoot in the summer sun would be painful at best, but where winter snow is removed by force, chemicals, and jet exhaust. Places where laying on cool grass under a shady tree with a pick-nick basket can not be done. That “global warming” is in fact “local pavement warming”.

https://answers.yahoo.com/question/index?qid=20070719230638AAVVZR0

Best Answer: Millions of hectares of cropland in the industrial world have been paved over for roads and parking lots. Each U.S. car, for example, requires on average 0.07 hectares (0.18 acres) of paved land for roads and parking space. For every five cars added to the U.S. fleet, an area the size of a football field is covered with asphalt. More often than not, cropland is paved simply because the flat, well-drained soils that are well suited for farming are also ideal for building roads. Once paved, land is not easily reclaimed. As environmentalist Rupert Cutler once noted, “Asphalt is the land’s last crop.”

The United States, with its 214 million motor vehicles, has paved 6.3 million kilometers (3.9 million miles) of roads, enough to circle the Earth at the equator 157 times. In addition to roads, cars require parking space. Imagine a parking lot for 214 million cars and trucks. If that is too difficult, try visualizing a parking lot for 1,000 cars and then imagine what 214,000 of these would look like.

However we visualize it, the U.S. area devoted to roads and parking lots covers an estimated 16 million hectares (61,000 square miles), an expanse approaching the size of the 21 million hectares that U.S. farmers planted in wheat last year. But this paving of land in industrial countries is slowing as countries approach automobile saturation. In the United States, there are three vehicles for every four people. In Western Europe and Japan, there is typically one for every two people.

At 61,000 square miles, that is bigger than the state of Georgia but smaller than Wisconsin. If you took all the paved area and made it into it’s own state, it would rank 24th in area. Maine is ranked 39th at 35,385 square miles.

Now add in the area of airports, runways, taxiways, train tracks, and all those roofs… Think it is more than 2/300 ths? That’s 2 K out of about 300 K normal daytime temperature in absolute Kelvin. Somehow I think that matters; and might explain the divergence of satellite temperatures from the land (mostly at airports) temperatures.

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