encrg! 01' tlie \ c i n d ivah finally introduced [hat ;I precise definition nf the
~ I C
ol' iilrhulcncc hccanie possible, hut it m u s t bc strcssed thitt thc rather
loosc description o l an eddy has proved to be the key opening for the
intcrpretation of many aspects of turbulent atmospheric diffusion.
The most complete series of measurements in this early period were made
by the British ..Wind Structure Subcommittee," over a large flat plain at
Cardington (U.K.); seven poles (one of them 150 ft high) were equipped with
Dines ancmometcrs, a modified type of Pitot tube, giving much finer recordings of the wind velocity than a cup anemometer; temperature was measured
by electric thetmomcters on the ground and at the top of the highest pole;
thc range occupied by the poles was 4ooo x 700 ft. The book published by
Ciiblctt (1932) contains a considerable amount of data; from the discussion
of C'. Durst it results that the eddies. over this plain, in the layer 0 < z <
150 in are essentially of two types. Type I: Friction eddies, identical in
structure. but much larger than eddies observed in the boundary layer near a
wall in a wind turinel; the axes of these eddies are oriented at random, in all
directions; their diameter never exceeds 1OOft. If the mean temperature
gradient
1) = - d r f d z
is overadiabatic, /I > /in, where /j0 = I"C,Il(M m is the dry adiabutic kqwrurc, these eddies are very abundant. On the contrary, if the gradient is
subadiabatic (/? < Po) and in particular when there is a temperature inversion (/j < 0), they are rare and are very rapidly damped. Type I I : Convection eddies are like the well-known Benard cells developing in a liquid over a
heated wall; their axes are horizontal; their diameter in the wind direction is
between 3000 and SO00 ft. and normally to this direction, between 600 and
2000 ft,
Many other interesting results are given in this report about the repartition of the turbulent energy of the wind in the direction of the mean wind
velocity and normally to this direction. Scrase (1930) has carried out with a
thrw-dimcnsional vane a series of measurements giving some insight on the
fluctuations or the wind velocity at 1.5 and 19 m above the ground. As an
example, for one series, when the mean velocity at z = 1.5 m was
u =: 4.70 m/sec. he gives the values
z = 1.5 m
57
66
43
cm/sec
z = 19 m
55
40
31
cm/sec
he has also computed the values of the corresponding Reynolds stresses:
.. . .
-PU'W' = 3.62.
-PW'U' = 1.50,
-pt.'u' = 0.58 (Cgs)
~ I C
ol' iilrhulcncc hccanie possible, hut it m u s t bc strcssed thitt thc rather
loosc description o l an eddy has proved to be the key opening for the
intcrpretation of many aspects of turbulent atmospheric diffusion.
The most complete series of measurements in this early period were made
by the British ..Wind Structure Subcommittee," over a large flat plain at
Cardington (U.K.); seven poles (one of them 150 ft high) were equipped with
Dines ancmometcrs, a modified type of Pitot tube, giving much finer recordings of the wind velocity than a cup anemometer; temperature was measured
by electric thetmomcters on the ground and at the top of the highest pole;
thc range occupied by the poles was 4ooo x 700 ft. The book published by
Ciiblctt (1932) contains a considerable amount of data; from the discussion
of C'. Durst it results that the eddies. over this plain, in the layer 0 < z <
150 in are essentially of two types. Type I: Friction eddies, identical in
structure. but much larger than eddies observed in the boundary layer near a
wall in a wind turinel; the axes of these eddies are oriented at random, in all
directions; their diameter never exceeds 1OOft. If the mean temperature
gradient
1) = - d r f d z
is overadiabatic, /I > /in, where /j0 = I"C,Il(M m is the dry adiabutic kqwrurc, these eddies are very abundant. On the contrary, if the gradient is
subadiabatic (/? < Po) and in particular when there is a temperature inversion (/j < 0), they are rare and are very rapidly damped. Type I I : Convection eddies are like the well-known Benard cells developing in a liquid over a
heated wall; their axes are horizontal; their diameter in the wind direction is
between 3000 and SO00 ft. and normally to this direction, between 600 and
2000 ft,
Many other interesting results are given in this report about the repartition of the turbulent energy of the wind in the direction of the mean wind
velocity and normally to this direction. Scrase (1930) has carried out with a
thrw-dimcnsional vane a series of measurements giving some insight on the
fluctuations or the wind velocity at 1.5 and 19 m above the ground. As an
example, for one series, when the mean velocity at z = 1.5 m was
u =: 4.70 m/sec. he gives the values
z = 1.5 m
57
66
43
cm/sec
z = 19 m
55
40
31
cm/sec
he has also computed the values of the corresponding Reynolds stresses:
.. . .
-PU'W' = 3.62.
-PW'U' = 1.50,
-pt.'u' = 0.58 (Cgs)