Ž . Geomorphology 35 2000 127–143 www.elsevier.nlrlocatergeomorph Experimental study of wind directional variability in the vicinity of a model valley J.E. Bullard a, ) , G.F.S. Wiggs b , D.J. Nash c a Department of Geography, Loughborough UniÕersity, Loughborough, Leicestershire, LE11 3TU, UK b Sheffield Centre for International Drylands Research, Department of Geography, Sheffield UniÕersity, Western Bank, Sheffield, S10 2TN, UK c School of the EnÕironment, UniÕersity of Brighton, Cockcroft Building, Lewes Road, Brighton, BN2 4GJ, UK Received 18 January 2000; received in revised form 15 April 2000; accepted 10 May 2000 Abstract This paper reports the results of a preliminary study examining the effects of valley presence on airflow direction and focuses on valleys situated in areas of low relief. Wind tunnel experiments are used to visualise near-surface airflow over a series of models representing valleys oriented at different angles to mean airflow. Results indicate that the presence of a valley has no significant influence on airflow direction when the approach angle of the wind is perpendicular to the valley strike. However, any wind approach-angle less than 908 is found to cause marked deflection of airflow to become more parallel to the valley. Analogies can be drawn between the surface streamlines recorded on the negative terrain of the valley models and those around positive terrain such as hills. A discussion of air pressure perturbation over both positive and negative terrain concludes that valley structures can have a significant effect on overlying synoptic windflow. The nature and magnitude of this effect is dependent upon a complex range of environmental parameters including valley morphometry, wind speed and the thermal stability of the airflow. The findings outlined have implications for the development of aeolian bedforms and processes of aeolian sedimentation in the vicinity of valleys. q 2000 Elsevier Science B.V. All rights reserved. Keywords: airflow; topography; wind tunnel 1. Introduction The modification of airflow by local topographic factors such as hills, valleys and escarpments is Ž widely recognised Jackson and Hunt, 1975; Bradley, 1980; Mason, 1986; Hunt et al., 1988a; Hunt et al., . 1988b; Finnigan et al., 1990 . Such modification has ) Corresponding author. Tel.: q 44-1509-22-2792; fax: q 44- 1509-22-3930. Ž . E-mail address: j.e.bullard@lboro.ac.uk J.E. Bullard . implications for understanding local meteorological Ž . patterns Weber and Kaufmann, 1998 , pollutant dis- Ž . persal Beniston et al., 1989 and aeolian processes Ž Goossens and Offer, 1997; Offer and Goossens, . 1994 . This paper reports the results of a preliminary study examining the effects of valley presence on airflow direction. There are numerous existing stud- Ž ies of wind flow over valleys Whiteman and Doran, . 1993; Guardans and Palomino, 1995 but, with few exceptions, studies of airflow modification have con- centrated on valleys within mountainous terrain. In this paper, we are concerned with areas of low relief 0169-555Xr00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž . PII: S0169-555X 00 00033-7