For an airport site near Visakhapatnam, India, and based on 10 years of data for the months of January, April, August and October, values of u0 are given as a function of wind speed, wind direction and Pasquill diffusion category.
The standard deviation of wind direction fluctuation (0,) plays a significant role in diffusion models particularly in complex terrain. Horizontal diffusion in the surface layer is dependent on a, (Hanna, 1981), which can be used to estimate lateral diffusion (Hanna et al., 1977). Pasquill (1976) gave an empirical relation for estimating oY from a, [ a,, = o, * x . f(x)]. But proper care should be taken in estimating o, as it is dependent on wind speed, stability and surface roughness.
The effect ofwind speed and direction on o,has been studied elsewhere (Smith, 1961;Munn, 1964;Sachdev and Rawlani, 1968). In this brief note the dependence of o, on stability, wind speed and direction for four representative months will be presented for an airport site near Visakhapatnam, India, using a lo-year period of data (1958)(1959)(1960)(1961)(1962)(1963)(1964)(1965)(1966)(1967).
The experimental site, instrumentation and the method of computing o,from the wind direction traces have been described earlier (Sadhuram et al., 1983). The site is at an aerodrome and the measurements are made at a height of 22 m above the ground. The roughness elements within 200 m of the tower are as follows: (1) generally flat terrain from east to south; (2) a runway and slightly rugged topography from south to west; and (3) small grass and bushes in the other sectors. The site is far from buildings and houses. In this connection, the wind field in the lowest 20 m is affected only by the surface roughness prevailing a little over 100 m upstream (Panofsky et al., 1983).
In this study, C, D and E of the Pasquill stability classes have been chosen and the wind speed has been divided into three categories: O-3, 3-6 and > 6 m set-i. The months of January, April, August and October have been selected as representative of winter, premonsoon, monsoon and postmonsoon seasons, respectively. The mean values of a, have been computed in different stabilities and wind speed classes for different seasons. Even with the large sample size (years 1958-1967), there were insufficient data in some categories which explains why there are gaps in Tables IA andB.
At constant stability and strong winds, a, values vary little with wind direction and season. Looking next at the effect of wind speed, (1) in the 'E' class, there is no apparent Boundary Layer Meteorology 34 (1986) 99-101.