URBAN CLIMATOLOGY
Week 1: Solar Radiation
1. Radiation Balance:
Net radiation, Rn = (Q+q) (1-a) + RL -RL , where Q and q are direct and diffuse solar
radiation, a is albedo and RL and RL are long-wave terrestrial and atmospheric radiation.
2. Energy Budget:
Rn + F = LE + H + G, where F is artificial heat added, LE is latent heat, H is sensible
heat and G is ground storage heat.
3. Albedo. With low sun, albedo (reflectance) is greater in country than city. City’s
vertical structures almost normal to sun’r rays. More rays reflected to ground, not to the
sky. The city’s rocklike materials also store heat, making city warmer than rural.
With high sun, near mid-day, country receives more direct sunlight than city-results in
lowered heat island.
4. RL-night. Country now cools faster than city. Unobstructed view means greater
outgoing energy. City-pockets of air trapped, cannot move upward easily and still
receiving heat released from energy stored in walls during day. Less skyview in city,
hence even at dawn, city temperatures higher than rural. Difference in temperature
between city core and surrounding called “heat island.”
5. F. Adding to net energy gained and heat island is heat generated by activities of city
(heating buildings, residences, cars, air conditioners).
6. Pollution. Helps trap RL at night, but can also reduce incoming insolation. (Q + q) 1520% less in cities, but not all wavelengths. Ultraviolet reduced much more than infrared.
City UV 30 less in winter, 5% in summer, on average. In smoggy LA, over 50% less,
while in industrial cities, UV almost absent in winter. Insolation especially reduced with
low sun, ie, winter, high latitudes. Annual reduction: 9% Montreal, 7% Toronto.
7. Sunshine. As pollution increases, less sunshine. London has reversed downward trend
with Clean Air Act, with increased sunshine and disappearance of famous London fog.
8. RL .While RL greater in cities with warmer air and pollution, the warmer city also has
greater RL
But there is even differences between radiation above roof level and from the “urban
canyon”.
9. Street design. Depth and width of canyon determine amounts of direct sunlight and
shade. Walking below skyscrapers with narrow streets produces mostly shaded streets.
10. Orientation. E-facing walls gain remarkable amounts of energy in summer a.m. In
winter, E and S-facing gain most heat in a.m. Nocturnal heat losses highest for E-facing
walls, lowest for S-facing in Japanese study.
11. Anthropogenic heat (F). Inner city has greatest energy use. One Australian study had
ratio of annual energy use in inner city to outlying sector of Sidney at about 360:1. Midlatitude and high-lat. During winter, major source of heat is production of heat by city,
not sun. Heat from industrial activities, house heating important sources. For example,
Manhatten Island, NY for Jan., city heat 2 ½ times greater than insolation reaching
ground. Summer, ratio drops to 1:6.
a. As temp. goes down, home heating increases. Average car burns about 3 gallons/hr.
Combustion = heat of typical home furnace in winter. Could use car to heat home if given
right exhaust system and heat exchanger.
b. People produce heat rate between 100 and 300 Watts, depending on activities. Not
insignificant in a city of 10 million. Estimated that BOS-WASH megalopolis, with its 5060 million, with area of 30,000 km2. Human heat source is about 65 cal./cm/day or about
1/6 the amount received from sun.
BIBLIOGRAPHY
Buffo, J. et al., 1972. “Direct solar radiation on various slopes from 0 to 60 degrees N.
latitude,” USDA Forest Service Research Paper PNW-142, 74 pp.
East, C., 1968. “Comparison du rayonnement solaire en ville et a la campagne,” Cahiers
de Geog. De Quebec, 12, 81-89.
Marsh, W.M., 1991. Landscape Planning: Environmental Applications, 2nd ed. N.Y.:
Wiley.
Marsh, W.M. and J. Dozier, 1981. “The Radiation Balance,” in Landscapes: An Intro to
Physical geography. Reading, MA: Addison-Wesley.
Nunez, M. and T. Oke, 1977. “The energy balance of an urban canyon,” J. Applied
Meteorology, 16, 11-19.
Sellers, W.D., 1965. Physical Climatology. Chicago: Univ. of Chicago Press.
Terjung, W.H. and S. S-F Louie, 1973. “Solar radiation and urban heat island,” Annals,
Assoc of Amer. Geog., 63, 181-207.
Yellott, I., 1978. “Passive solar heating and cooling systems,” ASHRAE Journal.