Urban impacts on climate - The Global Climate System: Patterns, Processes, and Teleconnections - page 218

(GEO 2003 ). There are two major seasons. The cool season (November to April) is
dry, and synoptic circulation is generally weak. Anticyclonic weather dominates and
surface inversions are frequent, creating a stable boundary layer atmosphere. The
warm season (May to October) is wet. The trade winds bring instability and con-
vection, with rain occurring most frequently in the afternoon and evening.
G¨ teborg, Sweden
G¨ teborg is a small northern European city located on the west coast of Sweden
(57862 0 N11845 0 E). Its location close to the Arctic Circle means it has about 5 hours
of daylight during the winter and around 15 hours of daylight in the summer.
Surrounded on three sides by a mix of large and small valleys and higher elevations
in between, G¨ teborg has a population of about 750 000. The central part of the city
is divided into the eastern section, containing old, densely packed buildings of
between four and six stories; and the western section, which is newer, with more
variety in building heights and also landscape features, such as parks. Climatically,
G¨ teborg is firmly located in the mid-latitude westerly circulation belt. Alternating
high pressure systems and fronts represent the synoptic weather pattern.
7.3 ESSAY: Variability of urban climates
Sue Grimmond, King's College London
7.3.1 Introduction
Urban areas are locations where there is a dramatic human influence on the
natural environment. Urban construction materials have different thermal
conductivities and capacities; the geometry of buildings and their spatial
arrangement trap radiation and pollutants and create a very rough surface
that influences air flow and dispersion; the heat released by human activities,
from vehicles, industry, etc., supplements natural sources of energy; and
engineering structures (pipes) remove water from the surface and modify
natural topography and drainage networks, thereby altering runoff and
humidity regimes (Oke 1987 ). The net effect is profound changes to the
radiative, thermal, moisture, and aerodynamic characteristics from the pre-
existing landscape, which alter natural budgets of heat, mass, and momen-
tum, resulting in the development of distinct urban climates. The most widely
recognized urban climate effects include urban heat islands (the common
observation that cities are warmer than their surroundings); urban induced
wind circulation; and precipitation enhancement downwind of urban areas
(see detailed reviews in Landsberg 1981 ; Oke 1997 ; Lowry 1998 ).