Human activity and vegetation can have marked effects on the climates of local areas. Eventually mans activities could affect larger areas and ultimately whole continents.

It has been known for years now that urban areas and industrial complexes have an influence on climate. Atmospheric pollution is increased, for example, and the radiation balance is thereby altered. This change affects the daily maximum and minimum temperatures in cities, where they tend to be generally higher than in nearby suburbs. A higher concentration of hydroscopic condensation nuclei in cities results in an increased number of fogs. Also, with the greater heat source found in cities, increased convection gives rise to greater amounts of cloudiness and precipitation. An apparent benefit of this increased heat is a slight decrease in severe weather occurring in large cities (Chicago, for example) as compared to adjacent areas.

Areas of heavy vegetation generally have distinct climates which may differ considerably from climates of nearly open areas. Falling precipitation caught in trees before reaching the ground may be evaporated, but precipitation which reaches the ground does not evaporate or run off readily. Heavily forested areas can absorb and store considerable quantities of water. Snow in forests can be protected from direct insolation by the trees and may stay on the ground for much longer periods than snow on open, exposed sur-faces. In forests, temperature maximums and minimums are higher than over open land at the same latitude. Relative humidities are also higher and wind speeds are considerably lower.

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