雨影是山背风面的干燥区域。由于潮湿空气的凝结和沉淀，它们被抬升到山上，到达背风面时失去水分，形成一个相对干燥的地区。
什么因素影响这个干燥区域的大小，如何影响?< / p >
In this case, sooner or later you will form rivers - and lakes, unless the river meets the sea very quickly (as in the case of Germany and the Baltics). As soon as you have lakes, the rain shadow will be moderated.
Taking yet another example, the Kalaharis: east of Namibian Great Escarpment, and west of Highveld/Drakensberg. Thus Kalahari lies within a rain shadow from both east and west - and despite the river Okawango flowing in it, the climate is not moderated.
So there are uncountably many factors that contribute to your question.
遵循由Roe and Baker (2006)所建立的渐近模型，有四个基本参数支配动力学:
$R_0 \to$迎风空气柱中的垂直积分凝结率。
$\Theta_{W,L} \to$雨滴轨迹坡度与地形坡度的比值。
$\mu \to$山高与水分标高之比。< / p >
$\Psi_{W,L} \to $Ratio of mountain length to the formation length scale (of falling hydrometeors).
What affects the amount of precipitation on the leeward flank is a combination of all these parameters, the full expression is found in Roe and Baker (2006) and it is rather complicated. However, if we consider the limits $\Psi_{W,L}, \Theta_{W,L} >>1$, that is, steep trajectories of falling hydrometeors and large orogen size then asymptotically we can estimate the precipitation on the windward side
$$P_w = \frac{R_0}{\mu}(1-e^{-\mu})$$
and on the leeward side
$$P_L = \frac{R_0}{\Theta_L\mu}(e^{-\mu}).$$
The latter says that, for large orogen size as compared to moisture scale height (large $\mu$), the precipitation on the leeward side vanishes, $P_L \to 0$. This is because on the windward flank, the average precipitation approaches a finite upper bound as it depletes all moisture in the air column.