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
