# Planet Earth/4f. Why are Mountain Tops Cold?

## Mountain Tops are Cold

The summit of Mount Kilimanjaro near Moshi, Tanzania, is ice capped and cold, despite being near the equator.

It would seem that the tops of high mountain peaks would be warmer, given that the sun is closer to the tops of mountains. So why are the tops of mountains on Earth colder than lower elevations? This strange relationship between elevation and temperature is a fascinating aspect of Earth’s atmosphere. The temperature profile of the atmosphere changes with altitude because of the absorption of atmospheric gasses that occupy different layers of Earth’s atmosphere, and the absorption of sunlight on the Earth’s surface.

Imagine an experiment with a lamp, representing the sun, shining on a surface below it representing the Earth. If the surface was a perfect reflector of the light (albedo = 1), such as a mirror, then the closer you move toward the light bulb in the lamp, the warmer it would be. The coldest place would be next to the mirror, the furthest distance from the lamp.

Example of heat absorption resulting in a gradient of temperatures below a lamp. If the surface is a perfect reflector, then the coldest zone would be above the surface, if the surface absorbs some of the heat the coldest zone would be somewhere between the lamp and the surface, this cold zone is called a pause.

Now imagine that the surface is rather a grayish color (albedo = 0.12), with only 12% of the light reflected back into space. In this case, the surface will absorb the energy from the light, and the surface will warm over time. So that the coldest point between the lamp and the surface would no longer be the surface, but be somewhere above the surface. This cold point or inflection point is known as a pause. A hill on this gray surface will cool with elevation as you approach this colder layer (or pause) above the surface. The moon is an example of this simple style of a planetary body that lacks an atmosphere, it has a low albedo of 0.12. The surface of the moon oscillates between extremely cold and hot temperatures, with nighttime temperatures dropping below −150 degrees Celsius, below the freezing point of dry ice, and daytime temperatures well above 106 degrees Celsius, higher than the boiling point of water. This great oscillation and range of daily temperatures is a result of the lack of any absorption of sun light by gasses above the surface of the moon. When the sun is shining on the surface of the moon temperatures soar, and when the moon turns from the sun, temperatures plunge in the dark. Earth would have a similar great oscillation of temperatures if not for its atmosphere.

Oxygen plays an important role in Earth’s atmospheric temperatures. Oxygen, Nitrogen and Argon, are the most abundant gasses on Earth. Oxygen blocks ultraviolet (high energy) light from the sun. Like a second surface above the ground, these gasses are heated by the sun’s high energy ultraviolet rays, near the top of the stratosphere (50 km above the Earth). Ozone blocks more of the incoming ultraviolet sun light resulting in a warmer layer. This warm layer above the surface of the Earth is known as the Stratopause, and marks the top of the Stratosphere. The air is denser below the Stratopause. Temperatures are cooler above and below this layer, resulting in two cold layers below and above this heated zone high in Earth’s atmosphere. If the atmosphere lacked oxygen, or other gasses that absorb high energy ultraviolet rays, the temperature profile would lack these two distinct cold layers, and have only a one cold layer. The presence of oxygen and ozone in the atmosphere results in temperatures increasing below the Stratopause, within the high Stratosphere of the atmosphere. Note that the temperatures in the Stratopause are still pretty cold compared to Earth’s surface, reaching around −15 degrees Celsius, but this is much higher than the two very cold layers in the atmosphere. The Tropopause, which is about 15 kilometers above the Earth’s surface, dips to −51 degrees Celsius, while the Mesopause, above the Stratosphere at 80 kilometers is even colder −100 degrees Celsius. These three layers, the two cold layers, the Mesopause and Tropopause, and single warmer layer, Stratopause, divide the atmosphere into four layers; the lowest layer is the Troposphere, then Stratosphere, Mesosphere, and highest Thermosphere, with the highest temperatures, but lowest pressures of Earth’s atmosphere.

Thermal profile of Earth’s atmosphere, showing the major divisions based on temperature.

## Troposphere

The Troposphere is the cloud rich zone of Earth’s atmosphere.

As the nearest layer of the atmosphere, and the layer we breathe and live our lives within, the troposphere is the layer that contains most of Earth’s weather. The term tropos, means change, and this lowest zone of the atmosphere experiences much change due to weather. It is also one of the warmest layers of atmosphere because of its proximity to the surface of the Earth. The troposphere is between 8 to 14 kilometers thick depending on where you are on Earth. Bulging near the equator and is thinner near the poles. As the air that we breathe, the air is densest within this layer in the atmosphere, especially near sea level. Composed of 78% nitrogen, 21% oxygen and 1% argon, the air in the troposphere also contains water vapor, carbon dioxide, methane and other Green House gasses. These heavier gas molecules are more abundant closer to the surface of the Earth. Because these gasses absorb infrared light, they work to retain much of the heat emitted by the Earth’s surface. Clouds, and water vapor is highest within the troposphere, and clouds rarely form above 15 kilometers above the Earth’s surface. Temperatures in the troposphere decrease with elevation from averages at the surface of 15 degrees Celsius decreasing to −51 degrees Celsius near the top. This decrease results in mountain peaks being considerably colder than valleys. Air within the troposphere is the only air that animals can breathe, including humans.

## Stratosphere

Lacking thick clouds the Stratosphere is the zone of the Earth’s atmosphere above the cloud rich Troposphere.

If you have traveled by airplane, most commercial aircraft maintain a cruising altitude near the base of the Stratosphere, between 33,000 feet to 42,000 feet above the ground, or 10 kilometers to 13 kilometers, just above the cloud covered world below. Temperatures at this altitude are chilly, with average temperatures dipping nearly to −60 degrees Celsius. Air pressure drops dramatically in the Stratosphere, from 226 mm Hg (30 kilopascals) to 1 mm Hg (0.146 kilopascals) at the top of the Stratosphere. Air pressure is so low that most gas molecules in the Stratosphere are so widely spaced that the air is unbreathable for humans and even birds. Clouds do not form within the Stratosphere, because of these low pressures, as most water vapor is confined to the Troposphere below. Temperatures however increase with altitude within the Stratosphere, because it is an important layer for Ozone formation from oxygen. Oxygen, Nitrogen and Argon gas make it up to these high altitudes above Earth, and the top of the Stratosphere marks the important absorption point of ultraviolet light from the sun. Temperatures are warmed at the very top of the Stratosphere because of the absorption of these high energy wavelengths of light from the sun.

## Mesosphere

The mesosphere is 35 kilometers thick, about 22 miles, with air pressure dropping to a minuscule 0.17 mm Hg (or 0.02 kilopascals) of pressure, with even lower pressures near the top of the Mesosphere. Few gas molecules are found this high in the atmosphere. Light molecules of Helium (He) and Hydrogen (H2) gas becomes more abundant in the Mesosphere, but also atoms start to ionize at these very low pressures. Oxygen, typically bonded in pairs, become isolated as ions of O−2, as does Nitrogen N−3. Temperatures drop with altitude in the Mesosphere, becoming extremely cold near the top. Because there is so little gas in the Mesosphere to scatter light, the sky looks much darker, almost as dark as outer space. “Meso” means middle, and there is still another thick layer of atmosphere even higher above Earth.

## Thermosphere

The thermosphere is what we normally think of as outer space. The air density in the thermosphere is so low that it resembles the darkness of outer space. Most scientists place the boundary with space about 100 kilometers above the Earth, which is within the thermosphere. The thermosphere is the thickest layer examined so far, with a thickness of 90 kilometers (56 miles). The air pressures are so low, that this layer is near a vacuum, similar to outer space, but it does contain many charged ions of gasses. Temperatures rise within the Thermosphere because these ions interact with the incoming high energy ultraviolet and gamma rays from the sun. During the day, sunlight warms the upper Thermosphere up to 2,000 degrees Celsius, and during the night temperatures drop to 500 degrees Celsius near the top of the Thermosphere. The top of the Thermosphere is heated because of the interaction of these charged ions at the edge of Earth’s atmosphere. The Thermosphere is also a very dangerous place. With such high temperatures, space craft can easily be burned and damaged. For example, the space shuttle Columbia upon re-entry into the thermosphere these high temperature atmospheric gasses were able to penetrate the heat shield and destroyed the internal wing structure, and the shuttle broke apart, killing its seven crew members on February 1, 2003.

Gasses high in the thermosphere become ionized by x-ray, gamma and ultraviolet light from the sun, which results in electrons leaving atoms. These highly charged particles circulate in near vacuum like pressures in the thermosphere. These charged particles (isolated electrons, protons, and neutrons) collide and are excited into higher energy states, releasing photons. This emission of photons of light can be observed as colorful auroral displays from the surface of the Earth. Given the electromagnetic attractions of these charged particles, they tend to collect near the magnetic poles of the Earth, high above the magnetic north pole and magnetic south pole.

The Aurora borealis or northern lights are caused by the high Thermosphere of Earth’s atmosphere.

On some dark nights enough of these charged particles will be excited emitting photons, which can be observed from Earth’s surface as spectacular displays of greenish light in the night sky. The Aurora borealis or Northern Light, and the Aurora australis or Southern Light, can be observed from high latitudes on the surface of the Earth, especially on cloudless nights following high solar activity.

Some atmospheric scientists refer to the Thermosphere, as the Ionosphere, because it is the region in the atmosphere where ionized particles are found. The Thermosphere/Ionosphere is important to study, since the ionized particles common in this layer can affect long range radio transmissions. Observations of the changes in the ionized particles within the Thermosphere is often referred by the media, as Space Weather.

## Exosphere

NASA astronaut Nicholas Patrick, outside the International Space Station which orbits Earth within the Thermosphere at 409 kilometers above Earth.

The Exosphere is the term used to define the outermost layer where the density is so low that gas molecules and ions do not interact with each other, and the pressure is a near vacuum, similar to space. The Exosphere likely is composed of mostly Helium and Hydrogen, although little is known of this region. It begins about 600 kilometers (374 miles) from Earth’s surface. Low Earth Orbiting space satellites span across the Thermosphere and Exosphere boundary, with altitudes from 180 to 2,000 kilometers above Earth. The International Space Station, with a low orbiting height of 409 kilometers above the Earth, actually orbits the Earth from a position high in the Thermosphere. Most GPS satellites orbit higher within Medium Earth Orbiting space, about 20,200 kilometers (12,552 miles) above the Earth. Geosynchronous orbiting satellites orbit the Earth by matching the Earth’s spin, such that from Earth’s surface, a satellite in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Geosynchronous satellites are important for global radio and cellular telephone communications, first proposed by science fiction author Arthur C. Clarke, who proposed the idea of high-altitude satellites in the 1940s. The first phone call using geosynchronous satellites was made on August 23, 1963 between U.S. President John Kennedy and Nigerian prime minister Abubakar Tafawa Balewa, both men would later be assassinated while in office. Most Geosynchronous orbits have an altitude around 35,786 kilometers (22,236 miles) from the Earth’s surface.

A very special type of geosynchronous orbit is the geostationary orbit, in which satellites match the rotation of the Earth. Communications satellites are often placed in a geostationary orbit so cell towers do not have to rotate to track these satellites, but can be pointed permanently at the position in the sky where the satellites are located in sync with the Earth’s rotation. The farthest observing space satellites are located much further into space, in the High Earth Orbit, which extends above 35,786 kilometers, all the way toward the distance to the moon, 384,000 kilometers away. High Earth Orbits are infrequently used because of the costs involved, but have been used to monitor nuclear bomb testing by hostile nations. The United States Vela satellite network, launched in 1963, successfully identified nuclear testing in the Indian Ocean by the Israeli military in 1979. Satellites at these high orbits can be affected by the gravity of the moon, and are only used in missions requiring very long distances above Earth. Such as the IBEX High Earth Orbiting satellite which maps particles that trail behind the motion of the solar system as it travels through interstellar space.