More on Urban Temperatures

A very interesting NASA study has been reported entitled Keeping New York City “Cool” is the Job of NASA’s “Heat Seekers”

Thus study illustrates that the González et al tropical study that was discussed on the weblog on January 29th, also applies to midlatitude cities. The NASA study also shows that the deliberate modification of the urban landscape can alter the temperatures in this area.

Excerpts from the NASA report state,

“The ‘heat is on’ in New York City, whether it’s summer or winter. This is due to a phenomenon called the urban heat island effect that causes air temperatures in New York City and other major cities to be warmer than in neighboring suburbs and rural areas. And, in a big city, warmer air temperatures can impact air quality, public health and the demand for energy. “

‘We need to help public officials find the most successful ways to reduce the heat island effect in New York. With ever-increasing urban populations around the world, the heat island effect will become even more significant in the future,’ said Stuart Gaffin, an associate research scientist at Columbia University, New York, and a co-author of the new NASA study. ‘The summertime impacts are especially intense with the deterioration of air quality, because higher air temperatures increase ozone. That has health effects for everyone. We also run an increased risk of major heat waves and blackouts as the heat island effect raises demand for electricity.’

“In large cities, land surfaces with vegetation are relatively few and are replaced by non-reflective, water-resistant surfaces such as asphalt, tar and building materials that absorb most of the sun’s radiation. These surfaces hinder the natural cooling that would otherwise take effect with the evaporation of moisture from surfaces with vegetation. The urban heat island occurrence is particularly pronounced during summer heat waves and at night when wind speeds are low and sea breezes are light. During these times, New York City’s air temperatures can rise 7.2 degrees F higher than in surrounding areas. “

‘We found that vegetation is a powerful cooling mechanism. It appears to be the most effective tool to reduce surface temperatures,’ Gaffin said. ‘Another effective approach is a man-made approach to cooling by making very bright, high albedo, or reflected light, on roof tops. These light-colored surfaces, best made using white coatings, reflect the sun’s light and thereby, its heat. Interestingly, more area is available to create the lighter surfaces than to add vegetation in a city such as New York.'”

There are two cavaets to t his study, however. First, while the planting of vegetation can reduce the surface air temperature from what it otherwise would be, the addition of water vapor from transpiration can increase the humidity. This can make summer days more stressful as the heat index is elevated (e.g. see Segal, M. and R.A. Pielke, 1981: Numerical model simulation of human biometeorological heat load conditions – summer day case study for the Chesapeake Bay area. J. Appl. Meteor., 20, 735-749.).

Secondly, if the vegetated surface is darker, the solar insolation that is received at the surface will be greater than in the absence of this surface type. The result would be greater heat added to the surface, even though a fraction of the heat would be involved with the transpiration of water vapor. The need to account for the diverse consequences of land surface change for climate manipulation is discussed in Pielke Sr., R.A., 2001: Carbon sequestration — The need for an integrated climate system approach. Bull. Amer. Meteor. Soc., 82, 2021.

With these cavaets, however, the NASA study provides an effective summary of how we can positively alter the local climate for the benefit of society. The research group at the Lawrence Berkeley Laboratory directed by Hashem Akbari
is a leader in the investigation of such urban climate mitigation.

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