It is a foregone conclusion of the author that the only variable is "global climate change."
Urban Heat Islands
An urban heat island (UHI) is an urban area or metropolitan area that is significantly warmer than its surrounding rural areas due to human activities. The temperature difference usually is larger at night than during the day, and is most apparent when winds are weak. UHI is most noticeable during the summer and winter. The main cause of the urban heat island effect is from the modification of land surfaces...As a population center grows, it tends to expand its area and increase its average temperature.
Most people can visualize the size of a two car garage. They have roofs that typically measure 25' by 25'.
Washington DC averages 3.6 inches of rain each month May-through-August, inclusive. That means that 187 cubic feet of water falls on that 25' by 25' patch each month. If paved, that water runs into a storm drain rather than being available for trees and grass to transpire.
That rain lost to run-off denies that 25' by 25' footprint of 11 million BTUs of heat sink due to transpiration for each month of the summer. 11 million BTUs is the equivalent of burning one hundred gallons of gasoline. Every month of the summer. For each 25' by 25' patch of pavement in the DC metro area.
(The math: The latent heat of evaporation for water is 970 BTU/lb X 64 pounds per cubic foot for 62K BTU per foot of water. 62K * 187 ft^3 = 11.6 million BTU. A gallon of gas has 114,000 BTU per gallon. 1.16e+6/1.14e+4= about 1e2 or one hundred.)
Another factor involves convective heat transfer. Sunlight that is converted to heat in the top of a tree's canopy is not heating up the air at ground level. The wind velocity is higher 40' above ground and there is more mixing and churning as the wind whistles through the tree tops. The temperature rise impacts a much deeper blanket of air than the heat coming off a blacktopped parking lot and consequently the maximum air temperature is much lower.
The reporterette has likely never heard of such stuffReplyDelete
During the past few years I recruited a team of more than 650 volunteers to visually inspect and photographically
document more than 860 of these temperature stations. We were shocked by what we found.
We found stations located next to the exhaust fans of air conditioning units, surrounded by asphalt parking lots and roads,
on blistering-hot rooftops, and near sidewalks and buildings that absorb and radiate heat. We found 68 stations located at
wastewater treatment plants, where the process of waste digestion causes temperatures to be higher than in surrounding areas.
In fact, we found that 89 percent of the stations – nearly 9 of every 10 – fail to meet the National Weather Service’s own
siting requirements that stations must be 30 meters (about 100 feet) or more away from an artificial heating or radiating/
reflecting heat source.
Good point on the evaporation. I didn't know that.ReplyDelete
The way streets and buildings are arranged makes a big difference in how heat builds up