Air flow under strong wind condition in autumn - winter season is seriously perturbed by topography . the perturbation displays as a horizontal vortex covering whole city of beijing and part of mountain area in northwest 秋冬季大风天气的流动受地形的动力扰动作用明显,扰动表现为水平涡旋形态,其影响范围包括北京城区及西北部山地。