The term "microclimate" is often misused: some sometimes refer to the microclimate of a city or that of the Mediterranean coast.
By definition, a microclimate refers to the set of climatic conditions specific to a portion of the atmosphere close to the ground, often defined as a maximum of a few hundred meters. Generally, specialists divide climate into four major scales, with the microclimate scale being the finest that can exist.
The scale of a microclimate is defined quite precisely, both spatially and temporally. According to the work of German climatologist Rudolf Geiger (1894-1981), a pioneer in the field, the vertical dimension of a microclimate concerns the first few meters of the atmosphere, hence its name "near the ground climate."
For Geiger, the study of microclimates focuses primarily on the atmospheric layer between 0 and 2 meters above the ground: this is the environment in which the main land animals, including us, live. The horizontal dimension of a microclimate is highly variable, but generally ranges from a few dozen to a few hundred meters: beyond a kilometer in width, we speak of a local climate and no longer a microclimate.
This explains why the microclimate of a street can be very different from that of a square located only a few dozen meters away, depending on its exposure and width, even if the street and the square belong to the same local climate.
The illustration above shows that the microclimate within a dense forest (left) is very different from that observed at the edge of the forest (right), in particular due to the shading and evaporation effects produced by the trees.
By definition, the concept of microclimate also includes all small-scale climates delimited by vertical or horizontal obstacles (walls, roofs, windbreaks, etc.). Thus, a building produces different microclimates: the same microclimate will not be found between the south- and north-facing facades, nor between the basement and the attic.
From a temporal perspective, microclimates are highly dependent on variations in climatic factors, particularly sunshine, which contributes to accentuating microclimatic differences. Some microclimates last only a few hours, for example, in an area that is sunny for only part of the day.
In the mountains, sunshine profoundly modifies the amount of energy received by each slope, which profoundly alters their microclimate and slope breezes. However, in very cloudy regions or on days with little sunshine, microclimatic differences are greatly reduced, as the amount of solar energy received by each surface is generally uniform.
I hope these few points have helped you clarify the definition of a microclimate. Feel free to consult this article for a deeper dive into the subject!
Sources: R. Geiger, H. Landsberg, M. Yoshino, M. Sorre
Image: C. Gaillard