Abstract
The history of the theory of orographic rain, and recent evidence against the “stable upglide” model, are briefly reviewed. A new model is proposed in which the blocking of low level air by a mountain causes approaching cold air to override the warm air, producing an unstable layer upstream of the mountain. This model is compared with recent observations in the Cascades and San Juan Mountains. The suggestion is that under some conditions it is the blocking action of the mountain, rather than forced ascent, which causes enhanced precipitation.
