Pavement and UHI Effect

The USHCN seems to deny that there is a significant UHI Effect, they are UHI Deniers!

Well, how bad is it?

In 1900 there were 10 miles of paved road in the US. Today there are 16 million hectares (61000 square miles) of roads and parking lots. According to the Virginia Highway Department roads get 60 degrees (33^oC) warmer than ambient temperature. But how to compute the actual heat difference over the course of a day? Thanks to the solar lobby we have an answer - the US on average receives 6 Solar Hours (hours at 100% sun) per day.

Lets assume the asphalt replaced trees. Tree lose more than 65% of their heat by evaporation and because they are at ambient temperature lose almost zero heat by thermal radiation. Asphalt at 33^oC is going to lose enormous amounts of heats by radiation and convection.

How much heat will be lost by radiation? Lets first assume a summer day with ground/air temp of 90^oF or 305K (Kelvin). The road is assumed to be 60^oF warmer or about 338K.

At Engineering Expert Witness Blog, a professional engineer (not a scientist but a REAL professional) discusses the convection problem.

First lets cover convection - there is engineering rule for rooftops that the convection loss (unforced) is 10 W/m²/K. The PE is using a rule for unforced flat surface of 15 W/m²/K and assuming an even larger temperature difference - 35K. We will stick with VA Highway number and split the difference heat transfer coefficient and go with 12. We will compute for a unit area.

Heat Flow = 12 * 33 = 396 W/m2

http://ntl.bts.gov/lib/13000/13100/13135/MPC02-136.pdf has a detailed study of asphalt heat emission and we are more or less compliant with it.


From the Stefan-Boltzmann Law: j = εσT⁴
for net emission j = εσ(T₁⁴ - εσT₀⁴
ε - emissivity of asphalt = .9
σ - sigma (SB constant) = 5.6704*10^-8
T₀ = 305K
T₁ = 338K
εσ*338⁴ - εσ*305⁴ = about 224 W/m2

The total loss is 620 W/m² for 5 hours per day .

What about the heat loss to the ground? Well, a worse case analysis would use 4 inches of asphalt, 8 inches of aggregate (1/3 meter for easy computation) and assume ground temperature at ambient. Heat transfer coefficient is 0.7 w/m/k.

HL = .7 * 33 / (1/3) = about 70 w/m2.

How much energy did it take to heat our asphalt (assuming the 4 inches (0.1m) is all at 60F above ambient)
Density = 1.3 g/cm3
Specific heat = 1.3 J/gC

E_h = 1.3 g/cm³ * 0.1m * 1 m² * 1.3 J/gC * 33 C * 10⁴ cm²/m² = 55770 W to raise the temp 60 degrees F. That's about an hour and a half of energy at peak sun.


How much of a difference does this make for the US?
Since the thermal radiation is assumed to be absorbed in the lowest 200 meter of air, and the convection only affects the surface air, all the energy is going into the surface air. So what is the impact of our 16 million hectares of asphalt?

Total = 620 W/m² * 16 * 10⁶ h * 10⁶ m2/h = 9920 TW

Total Yearly energy = 5 * 9920 * 365 = 18.1 eW*h (thats exaWatt*hours)
In 2008 the US produced 4.344 TWh of electricity but produced about 10TWh of waste heat doing it. The paving produced 1.5 million times more heat.

This has nothing to do with CO2 - this is just the heat from replacing trees and grass with asphalt.

Work in Progress...