The Earth Exterior

The Earth Exterior

The natural systems encountered in physical geography operate within the four great realms, or spheres, of the Earth. These are the atmosphere; the lithosphere, the hydrosphere, and the biosphere.

1. Atmosphere

The atmosphere is a mixture of different gases, particles and aerosols collectively known as air, which envelops the Earth. The atmosphere provides various functions, not least the ability to sustain life. The atmosphere protects us by filtering out deadly cosmic rays, powerful ultraviolet (UV) radiation from the Sun, and even meteorites on collision course

with Earth. Although traces of atmospheric gases have been detected well out into space, 99% of the mass of the atmosphere lies below about 25 to 30km altitude. The earth atmosphere extends approximately 2000kms above its surface. However 50% of the material lies within 5Km & 99% within 80Km from the surface.

The Atmosphere is divided into several parts according to temperature difference.

These parts are described bellow:

Troposphere:

The lowest layer of the atmosphere is called the troposphere. It ranges in thickness from 8km at the poles to 16km over the equator. The troposphere is bounded above by the tropopause, a boundary marked by stable temperatures. Above the troposphere is the stratosphere. Although variations do occur, temperature usually declines with increasing altitude in the troposphere. Hill walkers know that it will be several degrees cooler on the top of a mountain than in the valley below. The troposphere is denser than the layers of the atmosphere above it (because of the weight compressing it), and it contains up to 75% of the mass of the atmosphere. It is primarily composed of nitrogen (78%) and oxygen (21%) with only small concentrations of other trace gases. Nearly all atmospheric water vapour or moisture is found in the troposphere. The troposphere is the layer where most of the world's weather takes place. Since temperature, decreases with altitude in the troposphere, warm air near the surface of the Earth can readily rise, being less dense than the colder air above it. In fact, air molecules can travel to the top of the troposphere and back down again in a just a few days. Such vertical movement or convection of air generates clouds and ultimately rain from the moisture within the air, and gives rise to much of the weather, which we experience. The troposphere is capped by the tropopause, a region of stable temperature. Air temperature then begins to rise in the stratosphere. Such a temperature increase prevents much air convection beyond the tropopause, and consequently most weather phenomena, including towering cumulonimbus thunderclouds, are confined to the troposphere. Sometimes the temperature does not decrease with height in the troposphere, but increases. Such a situation is known as a temperature inversion. Temperature inversions limit or prevent the vertical mixing of air. Such atmospheric stability can lead to air pollution episodes with air pollutants emitted at ground level becoming trapped underneath the temperature inversion.

Stratosphere:

The stratosphere is the second major layer of the atmosphere. It lies above the troposphere and is separated from it by the tropopause. It occupies the region of atmosphere from about 12 to 50 km, although its lower boundary tends to be higher nearer the equator and lower nearer the poles. The stratosphere defines a layer in which temperatures rises with increasing altitude. At the top of the stratosphere, the thin air may attain temperatures close to 0°C. This rise in temperature is caused by the absorption of ultraviolet (UV) radiation from the Sun by the ozone layer. Such a temperature profile creates very stable atmospheric conditions, and the stratosphere lacks the air turbulence that is so prevalent in the troposphere. Consequently, the stratosphere is almost completely free of clouds or other forms of weather. The stratosphere provides some advantages for long-distant flight because it is above stormy weather and has strong, steady, horizontal winds. The stratosphere is separated from the mesosphere above by the stratopause.

Very few airplanes can fly as high as the stratosphere because the air is so thin that there is not enough lift to keep the aircraft supported. Some spy planes do fly in the lower stratosphere, however, such as the U-2 and the SR-71.

Mesosphere:

The mesosphere (literally middle sphere) is the third highest layer in our atmosphere, occupying the region 50 km to 80 km above the surface of the Earth, above the troposphere and stratosphere, and below the thermosphere. It is separated from the stratosphere by the stratopause and from the thermosphere by the mesopause. Temperatures in the mesosphere drop with increasing altitude to about -100°C. The mesosphere is the coldest of the atmospheric layers. In fact, it is colder then Antarctica's lowest recorded temperature. It is cold enough to freeze water vapor into ice clouds. You can see these clouds if sunlight hits them after sunset. They are called Noctilucent Clouds (NLC). NLCs are most readily visible when the Sun is from 4 to 16 degrees below the horizon. The mesosphere is also the layer in which a lot of meteors burn up while entering the Earth's atmosphere. From the Earth they are seen as shooting stars.

Thermosphere:

The thermosphere (literally "heat sphere") is the outer layer of the atmosphere, separated from the mesosphere by the mesopause. Within the thermosphere, temperatures rise continually to well beyond 1000°C. The few molecules that are present in the thermosphere receive extraordinary amounts of energy from the Sun, causing the layer to warm to such high temperatures. Air temperature, however, is a measure of the kinetic energy of air molecules, not of the total energy stored by the air. Therefore, since the air is so thin within the thermosphere, such temperature values are not comparable to those of the troposphere or stratosphere. Although the measured temperature is very hot, the thermosphere would actually feel very cold to us because the total energy of only a few air molecules residing there would not be enough to transfer any appreciable heat to our skin. The lower part of the thermosphere, from 80 to 550 km above the Earth's surface, contains the ionosphere. Beyond the ionosphere, extending out to perhaps 10,000 km is the exosphere or outer thermosphere, which gradually merges into space.

Exosphere

The exosphere is the highest layer of the atmosphere. Together with the ionosphere, it makes up the thermosphere. The exosphere extends to 10,000 km above the Earth's surface. This is the upper limit of our atmosphere. The atmosphere here merges into space in the extremely thin air. Air atoms and molecules are constantly escaping to space from the exosphere. In this region of the atmosphere, hydrogen and helium are the prime components and are only present at extremely low densities. This is the area where many satellites orbit the Earth.

Ionosphere

The ionosphere is a layer of ionized air in the atmosphere extending from almost 80 km above the Earth's surface altitudes of 600 km and more. Technically, the ionosphere is not another atmospheric layer. It occupies the same region of the upper atmosphere as the thermosphere. In this region of the atmosphere, the Sun’s energy is so strong that it breaks apart molecules and atoms of air, leaving ions (atoms with missing electrons) and free-floating electrons. The ionosphere is the region of the atmosphere where the auroras occur. Ionization of air molecules in the ionosphere is produced by ultraviolet radiation from the Sun, and to a lesser extent by high-energy particles from the Sun and from rays. The large number of free electrons in the ionosphere allows the propagation of electromagnetic waves. Radio signals - a form of electromagnetic radiation - can be "bounced" off the ionosphere allowing radio communication over long distances.

Pedosphere

The Pedosphere is the envelope of the Earth where soils occur& soil-forming factors are active. It only exists at the interface of the lithosphere, atmosphere, hydrosphere and biosphere.

2. Lithosphere (crust + upper mantle)

This outermost solid layer of the Earth provides a platform for most life forms. The solid bedrock bears a shallow layer of soil in which nutrient elements become available to organisms. The surface of the lithosphere is sculpted into landforms, which provide varied habitats for plants, animals, and humans.

3. Hydrosphere

The liquid realm of the Earth is principally the mass of water in the World’s oceans. It also includes solid ice in mountain and continental glaciers. Water Occurs as a gaseous vapor, liquid droplets, and solid ice crystals. In the lithosphere, Water is found in the uppermost layers in soils and in ground water reservoirs. 97% seawater, 2% frozen, <1%>

4. Biosphere

Most of the biosphere is contained in the shallow surface zone called the life layer. It includes the surface of the lands and the upper 100 meters of the ocean. On land, the life layer is the zone of interactions among the biosphere, lithosphere, and atmosphere.

Earth's Atmospheric Composition:

Species	Abundance (%)	Activity
N2	78.1	inert
O2	20.9	active (oxidizing)
Ar	0.93	inert
CO2	0.034	greenhouse, increasing
Ne	0.0028	inert
CH4	0.0005	greenhouse (reducing), increasing
H2O	0.1 - 3	variable
Ozone	Trace	Important to absorb UV from sun