New PDF release: Agricultural Physics

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By C. W. Rose, W. Ashhurst and H. T. Flint (Auth.)

ISBN-10: 0080118844

ISBN-13: 9780080118840

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Various models have been suggested to describe the structure of such convective motions including "bubbles" and "plumes" pulled along by the wind. This instability is associated with free or thermal convection. It is an additional cause of turbulence, referred to as convective turbulence. The relative importance of convective as against forced turbulence increases with the excess of temperature gradient over the adiabatic lapse value, and decreases with the magnitude of wind speed gradients (Richardson*23)).

Such velocity profiles can be explained if we assume that there is an internal friction between layers of air in relative motion, the force per unit area parallel to the streamlines being equal to the shear stress τ. Newton (c. 1687) made two hypotheses concerning this shear stress, which can be shown indirectly to be valid for many fluids (including air and water), provided the flow is of the streamlined type illustrated in Fig. 16. ) These hypotheses were that the shear stress between adjacent fluid 44 AGRICULTURAL PHYSICS layers is proportional to the rate of shear at the layer in question; and that the shear stress is independent of fluid pressure.

Temperature gradients near the ground under clear sky conditions can be hundreds and even thousands of times the adiabatic lapse rate. However, the contrasting conditions of moderate or high wind under a sky thickly covered with cloud tends to produce a well-stirred atmosphere, and temperature gradients close to the adiabatic lapse rate are then observed. The linear temperature profile corresponding to neutral equilibrium for vertical displacements can be written : T{z) = Το-Γζ, where Γ is the adiabatic lapse rate and T0 surface temperature.

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Agricultural Physics by C. W. Rose, W. Ashhurst and H. T. Flint (Auth.)

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