Trees and vegetation help to cool buildings and neighborhoods in a variety of ways. Here’s a quick overview of how these methods work:
The water cycle helps to explain how trees generate local and regional cooling through evapotranspiration. Photo credit: Mike Thomas, International Society of Arboriculture.
Evapotranspiration includes the evaporation of water from soil and transpiration--the process by which plants absorb water through their roots and release it as vapor through their leaves. Both of these liquid-to-gas processes use heat from the surroundings and thus cool the air.
A Berkeley Lab study found that trees can transpire 100 gallons (378.5 L) of water every day, which has the same cooling effect as 5 standard air-conditioning machines running for 20 hours.
Shaded areas are protected from direct sunlight, which reduces temperatures by 20-45°F (11-25°C) relative to peak temperatures in unshaded areas. Trees planted strategically around buildings and homes can significantly cool the indoor air. Shading windows, for example, is a very effective way to block direct sunlight from entering the building. A study in Davis, California found that cars parked in a shaded parking lot in the summer had interior air temperatures that were 45°F (25°C) cooler than cars parked in direct sunlight. Something as simple as vines covering a west-facing wall can shade the wall from direct sunlight, reducing the heat transferred into the house and indoor temperatures up to 36°F (20°C) in the summer.
Note Cities in cooler climates should consider that shading has the opposite effect in winter, when more sunlight and heat are desirable indoors. For most places however, the benefits from reduced cooling costs in the summer will outweigh the costs of increased heating in winter. This is because the sun is weaker in the winter, and deciduous trees shed their leaves, allowing a greater proportion of sunlight to filter through.
Increased urban vegetation can have varying effects on wind speeds. While a row of trees can block cooling breezes in the summer, they can also shield the house from cold winds in the winter. However, larger expanses of trees – such as in urban parks – can help increase local air circulation, generating cooling breezes that have a community-wide cooling effect.
 EPA. 1992. Cooling our Communities: A Guidebook on Tree Planting and Light-Colored Surfacing. US Environmental Protection Agency, Office of Policy Analysis, Climate Change Division. p32.
 Scott, K., J. Simpson, E. McPherson. 1999. Effects of Tree Cover on Parking Lot Microclimate and Vehicle Emissions. Journal of Arboriculture 25(3):129-142
 Sandifer, S. and B. Givoni. 2002. Thermal Effects of Vines on Wall Temperatures—Comparing
Laboratory and Field Collected Data. SOLAR 2002, Proceedings of the Annual Conference of the American Solar Energy Society. Reno, NV.
 Spronken-Smith RA and Oke TR. 1999. Scale modeling of nocturnal cooling in urban parks. Boundary Layer Meteorology 93: 287-312. Accessed via American Planning Association. How cities use parks for climate change management. http://www.planning.org/cityparks/briefingpapers/climatechange.htm