Why it is colder up high

Have you seen a mountain with a white, snowy top?

Even in summer, the top can be covered in snow. That is because the higher you go, the colder the air gets.

On a hot summer day, you might look up and see snow on the top of a tall mountain. How can that be? The University Corporation for Atmospheric Research (UCAR) explains that near the Earth's surface, air gets cooler the higher you climb. That is why the peaks of tall mountains can stay snowy even in summer.

On average, the air gets about 6.5 °C colder for every 1,000 meters you climb. So if it is a warm 27 °C at the beach, UCAR says you could expect about 1 °C at 4,000 meters up, almost freezing.

In the lowest layer of the atmosphere, the troposphere, air is warmest near the ground and gets colder with height, which is why tall peaks can be snow-covered even in summer (UCAR). The average decrease, the standard lapse rate, is about 6.5 °C per 1,000 m: UCAR's example takes 27 °C at sea level to around 1 °C at 4,000 m.

The standard rate is an average, not a law for every day. The U.S. National Weather Service describes the lapse rate as the rate of temperature change with height and notes that it varies: the faster temperature falls with height, the steeper the lapse rate and the more unstable the air, with values below about 5.5 to 6.0 °C/km counted as stable and values near 9.5 °C/km as unstable. So on any given day, the real rate of cooling with height can differ from the average.

Air is pulled down toward the ground.

So there is more air low down and less air high up. High up, the air is thin. Thin air is colder.

Gravity pulls air toward the Earth, so the air is packed most tightly near the ground. UCAR explains that air pressure and air density are highest at low altitudes. As you go up, there is more space between the air molecules, and the air gets thinner.

NOAA explains that as the air becomes thinner with height, its temperature drops too. In the lowest layer of the sky, where almost all our weather happens, the temperature falls from an average of about 17 °C near the ground to about −51 °C at the top of the layer.

Gravity holds the atmosphere against the Earth, so air pressure and density are greatest at low altitude and fall with height, leaving more space between molecules higher up (UCAR). NOAA's JetStream guide links the cooling of the troposphere to this thinning: as gas density decreases with height, the temperature in the troposphere also decreases, from an average of about 17 °C (62 °F) at the surface to about −51 °C (−60 °F) at the tropopause.

The troposphere holds about 75 to 80 percent of the atmosphere's mass and almost all weather (UCAR). Its top varies: NOAA gives about 18 to 20 km at the Equator, 9 km at 50° latitude and just under 6 km at the poles, while UCAR gives about 10 km typically, up to 20 km near the Equator and as low as 7 km over the poles in winter. Above it, in the stratosphere, temperature rises with height instead.

As you climb a mountain, the plants change.

At the bottom there are tall forests. Higher up, the trees get small and bent. At the top there are no trees at all, just tiny flowers.

Because it gets colder as you go up, plants and animals change in bands up a mountain, like the layers of a cake. Rocky Mountain National Park in Colorado shows this well.

The montane zone, from about 5,600 to 9,500 feet, has sunny meadows and forests of ponderosa pine, lodgepole pine and Douglas fir, where elk graze. The subalpine zone, from about 9,000 to 11,000 feet, is a dark forest of subalpine fir and Engelmann spruce. Above about 11,000 to 11,500 feet is the alpine tundra: no trees at all, only low plants, strong winds and a short summer.

Temperature, wind and season length change with elevation, so vegetation forms belts. The National Park Service (NPS) describes three in Rocky Mountain National Park, with boundaries that overlap and shift with slope and exposure. The montane ecosystem, from approximately 5,600 to 9,500 feet, mixes meadows with ponderosa pine on dry south-facing slopes, denser Douglas fir and lodgepole pine on moister north-facing slopes, and aspen groves on wet soils.

The subalpine ecosystem, just below treeline between 9,000 and 11,000 feet, is dominated by subalpine fir and Engelmann spruce, with lodgepole pine after fires. The alpine tundra begins between 11,000 and 11,500 feet depending on exposure; about one-third of the park lies above the limit where trees can grow in northern Colorado.

Near the top, the wind is very strong and cold.

Trees grow short and twisted. Some only have branches on one side. Above them, no trees can grow.

Trees grow straight and tall lower down, but near the top of the forest they get shorter and bent. Strong, cold, dry winds kill new growth on the side of the tree facing the wind, so some trees end up with branches on only one side. They are called flag trees or banner trees.

Right at the treeline, little trees can only grow as tall as the rock that shelters them from the wind. They spread sideways instead, forming low, dense mats called krummholz. The NPS says some of these small, tough trees may be several hundred to a thousand years old.

Approaching treeline, Engelmann spruce and subalpine fir become shorter and deformed. Cold, dry winds destroy new growth on the windward side, so permanent growth survives only on the lee side, producing banner or flag trees (NPS). At treeline, seedlings germinate in the lee of rocks and grow only as high as the shelter allows; further growth is mostly horizontal, with new roots where branches touch the soil, forming krummholz.

Snow cover can protect krummholz in winter, but branches that rise above the shelter of rocks or snow are usually destroyed. Well-established krummholz may be several hundred to a thousand years old (NPS). Treeline is thus a transition zone, an ecotone, where the forested subalpine gives way to treeless tundra.

Above the trees is the tundra.

Flowers there are tiny and grow close to the ground. Furry pikas gather hay for winter. Marmots sleep all winter long.

The alpine tundra is a land of extremes. Summer is short, with snow often lasting into June, and in Rocky Mountain National Park July's average high is only about 11 °C (52 °F). Strong winds blow most of the time.

Plants stay low to escape the wind. Some grow in cushions like clumps of moss; others have hairy stems that shield them from the wind, or reddish colors that turn sunlight into warmth. Animals are tough too. Yellow-bellied marmots eat all summer, then hibernate. American pikas gather hay piles to eat under the snow in winter. White-tailed ptarmigans grow white feathers for winter.

The NPS describes alpine tundra as treeless, cold and relatively dry, with long winters and short growing seasons. In Rocky Mountain National Park, snow lingers into June, July's average high is 52 °F (11 °C), and a summer 1980 study measured an average June-August wind of 20.2 mph at the Alpine Visitor Center; winter gusts at an alpine site in 1973 to 1974 reached 98 to 122 mph.

Most alpine plants are perennials, dwarfed but with full-sized flowers; cushion plants avoid the wind, dense hairs give protection from wind and red pigments convert sunlight into heat, and some take two or more years to form flower buds. Lichens can photosynthesize at any temperature above freezing. Animals adapt by hibernating (yellow-bellied marmot), caching hay (American pika), changing plumage (white-tailed ptarmigan) or climbing rocky terrain (bighorn sheep). Tundra is fragile: repeated footsteps can destroy plants, and recovery may take hundreds of years, so visitors stay on trails.