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surfaces; anthropogenic emissions of sensible heat; and changes
in the air-land exchange of water and the corresponding impact
on the radiation budget. Changes in surface roughness in urban
areas also affect the exchange of heat, mass, and momentum
between the surface and the atmosphere, as well as the depth
of the urban mixed layer. Hydrological processes are altered to
a significant degree by the effect of buildings and pavements on
surface moisture, runoff and streamflow. There are also some
indications
6
that large urban areas may influence the genesis,
intensity, and movement of convective storms and frontal
boundaries.
Weather impacts urban areas and urban residents in many
ways. Heavy rains can cause severe flooding, snow and freez-
ing rain can disrupt transportation systems, severe storms can
cause power failures, and so forth. The major direct impact on
human mortality results from heat waves, and urban areas are
particularly vulnerable because of their high population densi-
ties and because urban areas exacerbate conditions that lead
to heat stress. An analysis of deaths from various weather
conditions in the United States in the twentieth century clearly
showed that heat waves caused more deaths, both on an annual
average basis and from single events, than all other weather
conditions combined.
7
The temperature-mortality relationship
has a strong latitudinal dependence, with mortality rates in
northern cities affected more by higher temperatures, and in
southern cities by lower temperatures.
As the resolution increases in mesoscale prediction models,
it will be ever more important to properly represent urban
influences on the radiation budget, surface moisture, sensible
heat exchange processes, and anthropogenic heat and mois-
ture fluxes. This also means that weather observation networks
will need to be enhanced in order to provide the three-dimen-
sional observations required to properly initialize the models,
but also to provide improved information on weather condi-
tions in cities.
Enhanced urban weather observations and forecasts
As important as weather observations and forecasts are today,
they will be even more critical in the future, as urban popu-
lation growth contributes to changes in global and regional
climates. The importance of observation systems suitable for
tomorrow’s cities is receiving international attention. For
example, the Global Earth Observation System of Systems
(GEOSS) has included a new task in its GEO 2007-2009 work
plan.
8
Task US-07-01, Nowcasting and Forecasting User
Applications, seeks to “facilitate the transfer of advanced
nowcasting and forecasting capabilities from and to major
cities in developed and developing countries [by building]
upon the Helsinki Testbed experience to develop user appli-
cations related to precision weather forecasts, severe weather
Earth’s changing climate and urban weather
Over the past decade, the climate change debate has shifted
dramatically. In the early 1990s, the questions being asked
were: “Is the climate really changing?” and “Is there a percep-
tible contribution from human activities?” Today, the questions
are very different: “Will the climate change gradually or
abruptly?”; “How can anthropogenic impacts on climate be
diminished or reversed?” and “Will the frequency and inten-
sity of severe weather episodes change, and what will be the
socio-economic impacts?”
In 1988, the World Meteorological Organization and the
United Nations Environment Programme established the
Intergovernmental Panel on Climate Change (IPCC). The role
of the IPCC is to assess scientific, technical and socio-economic
information relevant to the risk of human-induced climate
change, its potential impacts, and options for adaptation and
mitigation. The IPCC’s assessments are undertaken by hundreds
of global experts who base their analyses on peer reviewed,
scientific and technical literature. The third and most recent
IPCC assessment report was completed in 2001
2
and the fourth
assessment is scheduled for completion in 2007. The IPCC’s
projected changes in climate have been summarized in Table
2.
3
They predict profound socio-economic implications as a
consequence of higher temperatures; increased heat indices;
more intense precipitation events; increased risk of drought;
and increased severity of tropical cyclones.
The urban weather problem is multidimensional. Weather
has special and significant impacts on those who live in large
urban areas
4
. Conversely, large urban areas can impact the local
weather and hydrologic processes in various ways. And urban
dwellers have different weather information needs than their
rural counterparts, due to the diversity of user groups and
population sectors. These include the following:
• the general public
• air quality management agencies
• water supply and sewage providers
• electric power industry
• fuel suppliers – natural gas, fuel oil, coal, gasoline
• transportation sectors – aviation, marine, and surface
• emergency response agencies
• public safety agencies
• insurance companies and underwriters
• health care providers
• recreation facility providers.
Urban heat islands can yield temperatures that are up to 5
degrees Celsius greater than their rural counterparts, but night-
time differences up to 12 degrees Celsius have been observed.
5
These increased temperatures result from the combined effects
of the thermal and radiative properties of buildings and road
Table 1: Urban indicators
Urban Percentage
Urbanization rate (%)
Doubling time (years)
1950 1975 2000 2030
1950-2000
2000-2030
1950-2000
2000-2030
29.8 37.9 47.2
60.2
0.92
0.81
75
86
54.9 70.0 75.4
82.6
0.63
0.31
...
...
17.8 26.8 40.4 56.4
1.63
1.11
42
62
World
MDR*
LDR*
* MDR = More developed regions *LDR = Less developed regions
Source: UNPD, 2001