What is the term used to define the lower region of the atmosphere in which we live which lies directly above the surface of the earth multiple choice question?

Altitude, like elevation, is the distance above sea level. Areas are often considered "high-altitude" if they reach at least 2,400 meters (8,000 feet) into the atmosphere.

The most high-altitude point on Earth is Mount Everest, in the Himalayan mountain range on the border of Nepal and the Chinese region of Tibet. Mount Everest is 8,850 meters (29,035 feet) tall. The urban area of El Alto, Bolivia, is the highest-altitude city on Earth. All 1.2 million residents live about 4,150 meters (13,615 feet) above sea level.

Altitude is related to air pressure. In fact, aviators and mountaineers can measure their altitude by measuring the air pressure around them. This is called indicated altitude, and is measured by an instrument called an altimeter.

As altitude rises, air pressure drops. In other words, if the indicated altitude is high, the air pressure is low.

This happens for two reasons. The first reason is gravity. Earth's gravity pulls air as close to the surface as possible.

The second reason is density. As altitude increases, the amount of gas molecules in the air decreases—the air becomes less dense than air nearer to sea level. This is what meteorologists and mountaineers mean by "thin air." Thin air exerts less pressure than air at a lower altitude.

High-altitude locations are usually much colder than areas closer to sea level. This is due to the low air pressure. Air expands as it rises, and the fewer gas molecules—including nitrogen, oxygen, and carbon dioxide—have fewer chances to bump into each other.

The human body struggles in high altitudes. Decreased air pressure means that less oxygen is available for breathing. One normal effect of altitude is shortness of breath, since the lungs have to work harder to deliver oxygen to the bloodstream. It can take days and even weeks for a body to adjust to high altitude and low air pressure.

People who spend too much time in high-altitude locations risk more serious symptoms of altitude sickness. These may range from headaches and dizziness to much more serious consequences, such as brain or lung damage. Above about 8,000 meters (26,000 feet), the human body cannot survive at all, and starts to shut down. Mountaineers call this altitude the "death zone."

To prevent severe altitude sickness, mountaineers bring supplemental (extra) supplies of oxygen and limit their time in the "death zone."

Different regions have different air pressures, even at the same altitude. Factors such as climate and humidity impact local air pressure. Air pressure also decreases around the poles. For this reason, if Mount Everest was located in the U.S. state of Alaska or the continent of Antarctica, it could never be summited without supplemental oxygen—the pressure would make the altitude seem 914 meters (3,000 feet) higher.

Astronomical Altitude

In astronomy, altitude has a somewhat different meaning. It describes the angle between the horizon and some point in the sky. For example, if a star is directly overhead, its altitude is 90 degrees. If a star has just set or is just about to rise, it is right at the horizon and has an altitude of 0 degrees.

The North Star, Polaris, does not rise or set because Earth's axis passes directly through it. It thus has a constant altitude when viewed from anywhere in the Northern Hemisphere. This makes it incredibly useful in celestial navigation.

The envelope of gas surrounding the Earth changes from the ground up. Five distinct layers have been identified using...

• thermal characteristics (temperature changes),
• chemical composition,
• movement, and
• density.

Each of the layers are bounded by "pauses" where the greatest changes in thermal characteristics, chemical composition, movement, and density occur.

Exosphere

This is the outermost layer of the atmosphere. It extends from the top of the thermosphere to 6,200 miles (10,000 km) above the earth. In this layer, atoms and molecules escape into space and satellites orbit the earth. At the bottom of the exosphere is the thermopause located around 375 miles (600 km) above the earth.

Thermosphere

Between about 53 miles (85 km) and 375 miles (600 km) lies the thermosphere. This layer is known as the upper atmosphere. While still extremely thin, the gases of the thermosphere become increasingly denser as one descends toward the earth.

As such, incoming high energy ultraviolet and x-ray radiation from the sun begins to be absorbed by the molecules in this layer and causes a large temperature increase.

Because of this absorption, the temperature increases with height. From as low as -184°F (-120°C) at the bottom of this layer, temperatures can reach as high as 3,600°F (2,000°C) near the top.

However, despite the high temperature, this layer of the atmosphere would still feel very cold to our skin due to the very thin atmosphere. The high temperature indicates the amount of the energy absorbed by the molecules but with so few in this layer, the total number of molecules is not enough to heat our skin.

Mesosphere

This layer extends from around 31 miles (50 km) above the Earth's surface to 53 miles (85 km). The gases, including the oxygen molecules, continue to become denser as one descends. As such, temperatures increase as one descends rising to about 5°F (-15°C) near the bottom of this layer.

The gases in the mesosphere are now thick enough to slow down meteors hurtling into the atmosphere, where they burn up, leaving fiery trails in the night sky. Both the stratosphere (next layer down) and the mesosphere are considered the middle atmosphere. The transition boundary which separates the mesosphere from the stratosphere is called the stratopause.

Stratosphere

The Stratosphere extends around 31 miles (50 km) down to anywhere from 4 to 12 miles (6 to 20 km) above the Earth's surface. This layer holds 19 percent of the atmosphere's gases but very little water vapor.

In this region the temperature increases with height. Heat is produced in the process of the formation of Ozone and this heat is responsible for temperature increases from an average -60°F (-51°C) at tropopause to a maximum of about 5°F (-15°C) at the top of the stratosphere.

This increase in temperature with height means warmer air is located above cooler air. This prevents "convection" as there is no upward vertical movement of the gases. As such the location of the bottom of this layer is readily seen by the 'anvil-shaped' tops of cumulonimbus clouds.

Troposphere

Known as the lower atmosphere almost all weather occurs in this region. The troposphere begins at the Earth's surface and extends from 4 to 12 miles (6 to 20 km) high.

The height of the troposphere varies from the equator to the poles. At the equator it is around 11-12 miles (18-20 km) high, at 50°N and 50°S, 5½ miles and at the poles just under four miles high.

As the density of the gases in this layer decrease with height, the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height in response. As one climbs higher, the temperature drops from an average around 62°F (17°C) to -60°F (-51°C) at the tropopause.