In mountain areas, powerful forces are unleashed in the atmosphere on a windy day. For example, most keen hillwalkers and pilots are familiar with the gusts and lulls that occur, which are a symptom of the turbulence generated as the air flows over hills and ridges. Higher up, something more remarkable occurs in the right conditions: the formation of mountain waves.
These are largely invisible, but sometimes clouds form in the rising air around the crest of each wave. Known as lenticular clouds, they are often elongated and smooth-faced like a lens, and there may be a series heading off downwind, roughly at right angles to the upper wind direction, and more or less stationary relative to the ground. In the setting sun, the rounded glowing shapes sometimes resemble a cartoon-book version of a spaceship, and are no doubt responsible for at least some reported sightings of UFOs.
Typically mountain waves are most likely when a strong stable airflow passes across a mountain range, with less stable air above the peaks. The waves then set up downwind (on the lee side) and sometimes extend for many tens of kilometres. To visualize how this might appear, consider the water waves that sometimes occur downstream of a rock or stone lying just beneath the surface in a river, or the waves (known as whelps) that follow a tidal bore.
On the front of each wave, powerful air currents often rise smoothly towards the crest, and glider pilots have long exploited mountain waves to achieve remarkable heights and distances. For example, pilots have flown more than 1000km in a day in Scotland in mountain waves and reached heights of over 30,000 feet. Considerable skill and experience is needed though due to the extreme turbulence sometimes encountered at low levels, and an oxygen supply is often required as well.