Meteorological and Climatological Scales

Scales of distance and area in meteorology and climatology—One dictionary definition of scale is “A progressive classification, as of size or amount.”  In meteorology and climatology, the usual scale classification terms ranging from small to large are micro-, local-, meso-, and macro-scale.  On the very first page of his 1987 book, “Boundary Layer Climates,” T.R. Oke, lists the dimensions of these scales as:

          Micro-scale   10^-2 to 10³ m

          Local-scale    10² to 5 × 10⁴ m

          Meso-scale    10⁴ to 2 × 10⁵ m

          Macro-scale  10⁵ to 10⁸ m

In our everyday thinking, the micro-scale might be as small as a leaf and as large as a city block, but typically the size of a suburban house lot.  Local-scale could range from three or four house lots to half of Baltimore, but more commonly, an area the size of downtown Baltimore where the tall buildings are.  Meso-scale encompasses an area at least the size of the whole Baltimore region up to an area that would extend from New York City to Washington, DC.  Macro-scale is similar to the area on a weather map of the whole Northeastern United States or larger.  In weather forecasting; macro-scale is often referred to as the synoptic scale. 

Along with the horizontal distance scales there are associated vertical scales.  For example, in considering micro-scale conditions of a leaf, the vertical scale would be a shallow layer of air over the leaf, its boundary layer, perhaps only a millimeter or two thick.  The boundary layer thickness is the depth of air that is significantly influenced by the surface.  If a leaf were very rough and hairy with large veins, the boundary layer would usually be deeper than the boundary layer of a very smooth leaf.  A city with tall buildings or scattered tall trees has a deeper boundary layer than a field with a short smooth crop.  During the day, the surface of the Earth is warmed by the sun and in turn warms the air just above it, which becomes buoyant and rises to increase the boundary layer depth.  At night surface cooling leads to a shrinking of the boundary layer, but in cities, the meso-scale boundary layer may remain warmer and deeper than rural boundary layers.

Meteorology is the study of processes in the atmosphere over short time periods.  At the meso- or macro-scale; meteorology is essentially the study of weather.  Weather is described by the state of the atmosphere, primarily in the lowest level of the atmosphere called the troposphere.  Variables that describe weather include air temperature, humidity, air movement (wind), clouds, and precipitation.  Measurement of fluxes at the surface of the Earth from tall towers, for example fluxes of water (evaporation), heat, and carbon dioxide are usually considered to be micro- or local-scale meteorological investigations. 

Climatology is the study of the averages, maximums and minimums of weather variables and the timing and spatial distribution of the variables.  The word “climatology” can be modified to indicate the size of the area being studied.  For example, microclimate is the study of climate in small spaces.  At the micro-scale, some researchers have studied the climate of rabbit warrens.  For a large city, the generally warmer temperatures than rural areas are a reflection of the meso-scale climate.

Contributed by BES Co-PI Gordon M. Heisler