While sometimes used interchangeably, “altitude" and "elevation" are often distinguished in the earth sciences, the former commonly referring to vertical distance above ground and the latter to the height of a particular point of the Earth’s surface in relation to sea level. Under average conditions, the atmosphere displays certain weather characteristics with mounting altitude. For the observer on the ground, changes in elevation can reflect these trends as well as others dependent on the vagaries of topography.
In the lowest layer of the atmosphere, the troposphere, within which virtually all of Earth’s weather transpires, temperature generally declines with altitude. This is a function of decreasing air pressure as altitude increases; higher up, the molecules of atmospheric gas are more spread out and slower moving, producing less heat energy. This is the basic reason why the traveler ascending from lowlands into mountains normally experiences progressively cooler temperatures.
A temperature inversion occurs when the normal cooling of air with altitude is reversed: cold air near the ground is overlain with a layer of warmer air, which prevents convection, or air-mass lifting, for the duration of the inversion. Basins enclosed by mountains are particularly susceptible to such inversions, which can concentrate pollutants in the lower atmosphere. During an inversion, a mountain rambler may enjoy sunny, warm conditions while a valley counterpart contends with chilly fog.
As an air mass rises and cools, it has less capacity to hold water vapor. At a certain temperature and altitude, vapor will condense into water droplets, forming clouds; heavy enough droplets will fall as precipitation. High-elevation terrain can thus provoke cloudiness, rainfall and snowfall by forcing a mass of air upward, a process known as orographic lifting. A mountain range may essentially wring clouds dry on its windward side, creating a so-called “rainshadow” in its lee. A classic example is the Cascade Range in the Pacific Northwest, which is set close to a major moisture source in the form of the Pacific Ocean. Maritime weather systems drop heavy rains and snows on the western slopes of the Cascades, promoting luxuriant temperate rain forests. Such systems are much drier once they surmount the Cascade Crest and flow down eastern slopes, a condition enhanced by the drying trend any air mass undergoes as it descends. Thus, leeward slopes of the Cascades are notably drier, defined by more open conifer woodlands and shrublands, while semiarid steppe sprawls in the rainshadow lowlands eastward.
Winds are often stronger at higher altitudes, not least because near the ground they are slowed by frictional drag. Mountains, buttes, spires and other topographic prominences are therefore commonly exposed to significantly fiercer winds than an adjacent lowland, particularly when they’re isolated or especially lofty. High-elevation winds may be enhanced by funneling or compression in response to topographic contours. Mount Washington, the high point of the Presidential Range in the northern Appalachians, experiences infamously tempestuous weather due to its prominence, latitude and proximity to prevailing storm tracks and the North Atlantic. During winter, hurricane-force winds – better than 121 kilometers per hour (75 mph) – assault Mount Washington an average of two days out of three. At 8,848 meters (29,029 feet), Mount Everest – the planet’s highest mountain – rears into jet-stream winds for much of the year; a significant obstacle for mountaineers, they may exceed 300 kilometers per hour (186 mph). A wispy snow banner off Everest’s tooth is a common signifier of the jet-battered summit.