Winds
Winds can increase your operational ceiling, payload, rate of climb, range and cruise speed. They can also do the opposite, and be very difficult to predict, with formidable up and downdraughts associated with them. When cruising downwind, along a lee slope or not, sudden wind reversals could make you exceed VNE or even take away your airspeed completely.
There are several types of wind, loosely be grouped into prevailing or local, with the latter subdivided into other types, such as anabatic, katabatic, etc., and which are infinitely variable (glacier winds descend 24 hours a day). The prevailing wind is steady and fairly reliable, and starts to affect you from about 6000 feet upwards. Smoke from local fires may be used to detect direction, as can water, but this may only give half the story. For instance, it's not uncommon for the windsocks at each end of Banff airstrip in the Rockies to be 180° out with each other! Indeed, upper winds can come in many directions at different levels, and are usually the opposite of lower winds. Where mountains are concerned, they also acquire a vertical element, which is actually where the boundary layer comes from.
As a guide to speed, whitecaps on water foam at 10 mph. Dark depressed puddles on water are called bearpaws (or catpaws) and are caused by downbursts. The most important thing to watch out for is the funnelling of wind as it progresses down a valley, so although the mean windspeed may be reported as 5 knots or so, you may find it as high as 30 in some places, and not necessarily coming from the expected direction.
In fact, understanding how air moves around terrain is one of the keys to good mountain flying, particularly the demarcation lines between smooth and turbulent air. In general, that moving up is smooth, and that moving down is turbulent. You can visualise the difference if you think of a waterfall, and the state of the water before and after dropping over the edge. Close to the ground, the air moves in laminar fashion (the boundary layer), but the depth of the layer and the gust spread will vary considerably, depending on the nature of the surface and its heating. The flow will be broken if the ground becomes rough, or there are trees, and the wind is strong. Turbulence will occur on both sides, resulting in an updraught close to the leeward side and a downdraught close to the windward side as air is made to curl.
The movement of air over a crest line has a venturi effect, giving an increased windspeed over the summit and a corresponding reduction of pressure, which could cause your altimeter to over-read. On passing over or round an obstacle, the air may become turbulent or have formed into rolls with a vertical or horizontal axis.
The general effect of a series of ridges is to form rolls between the crest lines, possibly causing a dangerous situation where a downdraught can exist on an upslope where an updraught would normally be expected:

As a result, on top of steep ridges there may be an area of nil or reverse winds which is difficult to locate on the first recce. The vertical distance to which a mountain chain will influence the movement of air is about 3-5 times its height, changing with the windspeed.