Climate is the set of characteristic temperatures, humidities, sunshine, winds, and other weather conditions that prevail over large areas of space for long periods of time. Microclimate refers to a climate that holds over a very small area. Microclimates usually are slight modifications of the main background climate altered by features in the landscape. A forest creates a microclimate within the canopy of trees which is cooler, wetter, and has altered soil chemistry compared to the area outside the forest. The altered climate found within forests can support organisms that cannot survive in the surrounding grassland. Similarly a large city has altered wind flows due to the presence of tall buildings, increased overall temperatures, and a very different type of ground cover than the surrounding plains. All these factors contribute to a microclimate characteristic of an urban area. Microclimates frequently support unique ecosystems. Mountain meadows, river valleys, tidal marshes, and crop lands have one or several microclimates which help determine the amount and type of organisms that thrives in these locations.
Microclimates are parts of a complex web of climates that exist on Earth. The general global climate of Earth can be thought of as a collection of many smaller scale climates that coexist like patches in a quilt. These subclimates are further divided into smaller scale climates each with its own distinctive features. For example the continent-wide climate of North America, called the macroclimate, is distinctly different than that of South America. Within the North American macroclimate are several distinct mesoclimates which extend over distances much smaller than the continent. These include plains, mountains, and deserts. Making up each of these mesoclimates are smaller climate zones called local climates. Some distinct local climates are forests, croplands, and large cities. The smallest scale sub-climate is the microclimate defined as the climate that holds over a distance (in any direction) of less than 328 ft (100 m). Distinct microclimates include a small cornfield, a forest clearing, and the canyon formed by several tall city office buildings. Microclimates thus form the smallest building blocks of the overall global climate.