Mountain Flying
In the mountains, general principles common to other areas will be vastly different. You must be prepared to adapt your flying techniques as the need arises, for the peculiarities of the region and the type of aircraft. In other words, have not only Plan A, but Plan B, C, etc. up your sleeve, because, very often, once you’ve looked at a site and gone round for finals, you will find a cloud has got there before you! You cannot afford to assume that a particular situation is the same as, or similar to, any other you might have encountered previously. You can also expect fog, especially in the early morning, which will often stick to the sides of valleys for quite some time. Sometimes, you can create your own clouds, by pulling down warm air from an inversion above.

In UK, mountainous areas include Scotland and Wales, the Lake and Peak Districts, and generally any hilly country above 1500 feet amsl, although a geologist would probably expect to see 2000. In many other parts of the world, these would be considered as just foothills, as one description of mountain flying includes a reference to 5000 feet Density Altitude. Actually, it reads:
"experience in maneuvring a helicopter at density altitudes of over 5000 feet to include numerous takeoffs and landings in situations indicative to difficult mountainous terrain. This terrain consists of abrupt, rapidly rising terrain resulting in a high land mass projecting above its surroundings, wherein complex structures in which folding, faulting, and igneous activity have taken part. These mountainous areas produce vertical mountain winds, turbulence associated with mountain waves, producing abrupt changes in wind direction often resulting in up flowing or down flowing air currents. In the contiguous United States, the boundaries of these mountainous areas are defined in the aeronautical information manual."
In Canada, look out for Designated Mountain Areas, which naturally include the Rockies, extending into the USA (don't blame me for the English above - it was written by the US Forestry people).
However, air behaves the same way round pointy bits of ground whatever its height above sea level, and this includes flying around buildings (we will look at laminar flow shortly). The big difference here is that you have less power to play with, hence the mention of density altitude.