As living organisms, we are most acutely aware of three things about
the wind. We know that it exerts a force on us and other objects against
which it blows, it is effective in transporting heat from us, and it is highly
variable in space and time. A fourth property of the wind, less obvious
to the casual observer, but essential to terrestrial life as we know it, is
its effective mixing of the atmospheric boundary layer of the earth. This
can be illustrated by a simple example. On a hot summer day about 10
kilograms (550 moles) of water can be evaporated into the atmosphere
from each square meter of vegetated ground surface. This amount of water
would increase the vapor concentration in a 100 m thick air layer by 100
g m-3 (136 mmollmol) if there were no transport out of this layer or
condensation within it. This is much more water than the air could hold
at normal temperature. The observed increase in vapor concentration in
the first 100 meters of the atmosphere is typically less than 1 g mP3, so
we can see how effective the atmosphere is for transporting and mixing.
A similar calculation (Monteith, 1973) shows that photosynthesis in a
normally growing crop would use all of the C02 in a 30 m air layer
above a crop in a day, yet measured C02 concentrations have diurnal
fluctuations of 15 percent or less. Without the vertical turbulent transport
of heat, water vapor, COz, oxygen, and other atmospheric constituents,
the microenvironment we live in would be very inhospitable.
The influence of the surface on the atmosphere of the earth can extend
from hundreds of meters at night to several thousand meters during days
when surface heating is strong. This depth of influence of the surface
on the atmosphere is called the planetary boundary layer. Through the
depth of this planetary boundary layer, like all boundary layers that form
between moving fluids and stationary surfaces, fluxes of momentum, heat,
andmass decrease with height. The lowest 50 m ofthis planetary boundary
layer is referred to as the surface layer; this is the region of most interest.
In this region fluxes of momentum, heat, and mass are virtually constant
with height and profiles of wind speed, temperature, and concentration
are logarithmic.
In order to determine the force of the wind on, or the rate of heat transfer from living organisms in their microenvironments, it is necessary to
the wind. We know that it exerts a force on us and other objects against
which it blows, it is effective in transporting heat from us, and it is highly
variable in space and time. A fourth property of the wind, less obvious
to the casual observer, but essential to terrestrial life as we know it, is
its effective mixing of the atmospheric boundary layer of the earth. This
can be illustrated by a simple example. On a hot summer day about 10
kilograms (550 moles) of water can be evaporated into the atmosphere
from each square meter of vegetated ground surface. This amount of water
would increase the vapor concentration in a 100 m thick air layer by 100
g m-3 (136 mmollmol) if there were no transport out of this layer or
condensation within it. This is much more water than the air could hold
at normal temperature. The observed increase in vapor concentration in
the first 100 meters of the atmosphere is typically less than 1 g mP3, so
we can see how effective the atmosphere is for transporting and mixing.
A similar calculation (Monteith, 1973) shows that photosynthesis in a
normally growing crop would use all of the C02 in a 30 m air layer
above a crop in a day, yet measured C02 concentrations have diurnal
fluctuations of 15 percent or less. Without the vertical turbulent transport
of heat, water vapor, COz, oxygen, and other atmospheric constituents,
the microenvironment we live in would be very inhospitable.
The influence of the surface on the atmosphere of the earth can extend
from hundreds of meters at night to several thousand meters during days
when surface heating is strong. This depth of influence of the surface
on the atmosphere is called the planetary boundary layer. Through the
depth of this planetary boundary layer, like all boundary layers that form
between moving fluids and stationary surfaces, fluxes of momentum, heat,
andmass decrease with height. The lowest 50 m ofthis planetary boundary
layer is referred to as the surface layer; this is the region of most interest.
In this region fluxes of momentum, heat, and mass are virtually constant
with height and profiles of wind speed, temperature, and concentration
are logarithmic.
In order to determine the force of the wind on, or the rate of heat transfer from living organisms in their microenvironments, it is necessary to