World Travel City Photography

External effects
Modern cities are known for creating their own microclimates. This is due to the large clustering of heat absorbent surfaces that heat up in sunlight and that channel rainwater into underground ducts.
Waste and sewage are two major problems for cities, as is air pollution coming from various forms of combustion, including fireplaces, wood or coal-burning stoves, other heating systems, and internal combustion engines. The impact of cities on places elsewhere, be it hinterlands or places far away, is considered in the notion of city footprinting (ecological footprint). Other negative external effects include health consequences such as communicable diseases, crime, and high traffic and commuting times. Cities cause more interaction with more people than rural areas, thus a higher probability to contracting contagious diseases. However, many inventions such as inoculations, vaccines, and water filtration systems have also lowered health concerns. Crime is also a concern in the cities. Studies have shown that crime rates in cities are higher and the chance of punishment after getting caught is lower. In cases such as burglary, the higher concentration of people in cities create more items of higher value worth the risk of crime. The high concentration of people also makes using auto mobiles inconvenient and pedestrian traffic is more prominent in metropolitan areas than a rural or suburban one.
Cities also generate positive external effects. The close physical proximity facilitates knowledge spillovers, helping people and firms exchange information and generate new ideas. A thicker labor market allows for better skill matching between firms and individuals. Another positive external effect of cities comes from the diverse social opportunities created when people of different backgrounds are brought together. Larger cities typically offer a wider variety of social interests and activities, letting people of all backgrounds find something they can be involved in.