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?

Definition: Homosphere can be defined as the lowest part of the Earth’s atmosphere. It lies between the heterosphere and the surface of the earth. It is the earth’s atmosphere below the altitude of roughly 80 kms where there is an almost-homogenous composition of nitrogen (78%), oxygen (21%), argon (10%), carbon dioxide as well as traces of constituents like dust particles, aerosols and cloud droplets.

Description: The Earth’s atmosphere has two major zones or segments – homosphere and heterosphere. The homosphere is the lower segment of the two-part division of atmosphere and further consists of three regions namely troposphere, stratosphere and mesosphere. All the three regions have the same composition of air. However, the concentration of air keeps decreasing significantly as the altitude increases.

Table of Contents Show

The different layers of the atmosphere
The Troposphere
The Stratosphere
The Mesosphere
The Thermosphere and Ionosphere
The Exosphere
The Magnetosphere
Further information
Structure of the troposphere
Composition
Temperature
Atmospheric flow
Three Cell Model
Meridional flow
External links

Troposphere: The troposphere forms the bottom-most layer of the homosphere and thus closest to the earth’s surface. This is the layer where we live in. It is also known as the earth’s weather layer as it has all weather conditions. This layer exists in the ground to an altitude of about 11 kilometres. Commercial aircraft fly in troposphere. Data-gathering balloons also exist in this layer.

Stratosphere: The stratosphere forms the middle layer of the homosphere and lies directly above the troposphere. This layer lies between the altitude of roughly 15 kilometres and 50 kilometres. In the stratosphere, the temperature increases with the increase in altitude. The ozone layer lies in the stratosphere (above about 18-20 kms). Supersonic jets fly in this layer of the homosphere.

Mesosphere: The mesosphere forms the top-most layer of the homosphere. This layer exists above an altitude of roughly 50 kms and extends upto 80 kms. The temperature in the mesosphere decreases as the altitude increases. The meteors exist in this layer of the homosphere, which slows down their speed hurtling towards the atmosphere.

The atmosphere is comprised of layers based on temperature. These layers are the troposphere, stratosphere, mesosphere and thermosphere. A further region at about 500 km above the Earth's surface is called the exosphere.

The different layers of the atmosphere

The atmosphere can be divided into layers based on its temperature, as shown in the figure below. These layers are the troposphere, the stratosphere, the mesosphere and the thermosphere. A further region, beginning about 500 km above the Earth's surface, is called the exosphere.

The Troposphere

This is the lowest part of the atmosphere - the part we live in. It contains most of our weather - clouds, rain, snow. In this part of the atmosphere the temperature gets colder as the distance above the earth increases, by about 6.5°C per kilometre. The actual change of temperature with height varies from day to day, depending on the weather.

The troposphere contains about 75% of all of the air in the atmosphere, and almost all of the water vapour (which forms clouds and rain). The decrease in temperature with height is a result of the decreasing pressure. If a parcel of air moves upwards it expands (because of the lower pressure). When air expands it cools. So air higher up is cooler than air lower down.

The lowest part of the troposphere is called the boundary layer. This is where the air motion is determined by the properties of the Earth's surface. Turbulence is generated as the wind blows over the Earth's surface, and by thermals rising from the land as it is heated by the sun. This turbulence redistributes heat and moisture within the boundary layer, as well as pollutants and other constituents of the atmosphere.

The top of the troposphere is called the tropopause. This is lowest at the poles, where it is about 7 - 10 km above the Earth's surface. It is highest (about 17 - 18 km) near the equator.

The Stratosphere

This extends upwards from the tropopause to about 50 km. It contains much of the ozone in the atmosphere. The increase in temperature with height occurs because of absorption of ultraviolet (UV) radiation from the sun by this ozone. Temperatures in the stratosphere are highest over the summer pole, and lowest over the winter pole.

By absorbing dangerous UV radiation, the ozone in the stratosphere protects us from skin cancer and other health damage. However chemicals (called CFCs or freons, and halons) which were once used in refrigerators, spray cans and fire extinguishers have reduced the amount of ozone in the stratosphere, particularly at polar latitudes, leading to the so-called "Antarctic ozone hole".

Now humans have stopped making most of the harmful CFCs we expect the ozone hole will eventually recover over the 21st century, but this is a slow process.

The Mesosphere

The region above the stratosphere is called the mesosphere. Here the temperature again decreases with height, reaching a minimum of about -90°C at the "mesopause".

The Thermosphere and Ionosphere

The thermosphere lies above the mesopause, and is a region in which temperatures again increase with height. This temperature increase is caused by the absorption of energetic ultraviolet and X-Ray radiation from the sun.

The region of the atmosphere above about 80 km is also caused the "ionosphere", since the energetic solar radiation knocks electrons off molecules and atoms, turning them into "ions" with a positive charge. The temperature of the thermosphere varies between night and day and between the seasons, as do the numbers of ions and electrons which are present. The ionosphere reflects and absorbs radio waves, allowing us to receive shortwave radio broadcasts in New Zealand from other parts of the world.

The Exosphere

The region above about 500 km is called the exosphere. It contains mainly oxygen and hydrogen atoms, but there are so few of them that they rarely collide - they follow "ballistic" trajectories under the influence of gravity, and some of them escape right out into space.

The Magnetosphere

The earth behaves like a huge magnet. It traps electrons (negative charge) and protons (positive), concentrating them in two bands about 3,000 and 16,000 km above the globe - the Van Allen "radiation" belts. This outer region surrounding the earth, where charged particles spiral along the magnetic field lines, is called the magnetosphere.

Further information

Visit our Atmosphere National Science Centre

Read about our UV and ozone research

Lowest layer of Earth's atmosphere

Atmospheric Circulation: the Three Cell Model of the circulation of the planetary atmosphere of the Earth, of which the troposphere is the lowest layer.

The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of water vapour and aerosols, and is where most weather phenomena occur.[1] From the planetary surface of the Earth, the average height of the troposphere is 18 km (11 mi; 59,000 ft) in the tropics; 17 km (11 mi; 56,000 ft) in the middle latitudes; and 6 km (3.7 mi; 20,000 ft) in the high latitudes of the polar regions in winter; thus the average height of the troposphere is 13 km (8.1 mi; 43,000 ft).

The term troposphere derives from the Greek words tropos (rotating) and sphaira (sphere) indicating that rotational turbulence mixes the layers of air and so determines the structure and the phenomena of the troposphere.[2] The rotational friction of the troposphere against the planetary surface affects the flow of the air, and so forms the planetary boundary layer (PBL) that varies in height from hundreds of meters up to 2 km (1.2 mi; 6,600 ft). The measures of the PBL vary according to the latitude, the landform, and the time of day when the meteorological measurement is realized. Atop the troposphere is the tropopause, which is the functional atmospheric border that demarcates the troposphere from the stratosphere. As such, because the tropopause is an inversion layer in which air-temperature increases with altitude, the temperature of the troposphere remains constant.[2]The layer has the largest concentration of nitrogen.

The atmosphere of the Earth is in five layers:
(i) the exosphere at 600+ km;
(ii) the thermosphere at 600 km;
(iii) the mesosphere at 95–120 km;
(iv) the stratosphere at 50–60 km; and
(v) the troposphere at 8–15 km.
The distance from the planetary surface to the edge of the stratosphere is ±50 km, less than 1.0% of the radius of the Earth.

Structure of the troposphere

Composition

In the Earth’s planetary atmosphere, a volume of dry air is composed of 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.04% carbon dioxide, trace gases, and variable amounts of water vapor. The sources of atmospheric water vapor are the bodies of water (oceans, seas, lakes, rivers, swamps), and vegetation on the planetary surface, which humidify the troposphere through the processes of evaporation and transpiration respectively, and which influences the occurrence of weather phenomena; the greatest proportion of water vapor is in the atmosphere nearest the surface of the Earth. The temperature of the troposphere decreases at high altitude by way of the inversion layers that occur in the tropopause, which is the atmospheric boundary that demarcates the troposphere from the stratosphere. At higher altitudes, the low air-temperature consequently decreases the saturation vapor pressure, the amount of atmospheric water vapor in the upper troposphere.

Pressure

The maximum air pressure (weight of the atmosphere) is at sea level and decreases at high altitude because the atmosphere is in hydrostatic equilibrium, wherein the air pressure is equal to the weight of the air above a given point on the planetary surface. The relation between decreased air pressure and high altitude can be equated to the density of a fluid, by way of the following hydrostatic equation:

d P d z = − ρ g n = − m P g n R T {\displaystyle {\frac {dP}{dz}}=-\rho g_{n}=-{\frac {mPg_{n}}{RT}}}

where:

gn is the standard gravity
ρ is the density

z is the altitude
P is the pressure
R is the gas constant
T is the thermodynamic (absolute) temperature
m is the molar mass[3]

Temperature

The planetary surface of the Earth heats the troposphere by means of latent heat, thermal radiation, and sensible heat. The gas layers of the troposphere are less dense at the geographic poles and denser at the equator, where the average height of the tropical troposphere is 13 km, approximately 7.0 km greater than the 6.0 km average height of the polar troposphere at the geographic poles; therefore, surplus heating and vertical expansion of the troposphere occur in the tropical latitudes. At the middle latitudes, tropospheric temperatures decrease from an average temperature of 15°C (59°F) at sea level to approximately −55°C (−67°F) at the tropopause. At the equator, the tropospheric temperatures decrease from an average temperature of 20°C (68°F) at sea level to approximately −70°C to −75°C (−94 to −103°F) at the tropopause. At the geographical poles, the Arctic and the Antarctic regions, the tropospheric temperature decreases from an average temperature of 0°C (32°F) at sea level to approximately −45°C (−49°F) at the tropopause.[4]

Altitude

The temperature of the troposphere decreases with increased altitude, and the rate of decrease in air temperature is measured with the Environmental Lapse Rate ( $− d T / d z {\displaystyle -dT/dz}$

) which is the numeric difference between the temperature of the planetary surface and the temperature of the tropopause divided by the altitude. Functionally, the ELR equation assumes that the planetary atmosphere is static, that there is no mixing of the layers of air, either by vertical atmospheric convection or winds that could create turbulence.

The difference in temperature derives from the planetary surface absorbing most of the energy from the sun, which then radiates outwards and heats the troposphere (the first layer of the atmosphere of Earth) while the radiation of surface heat to the upper atmosphere results in the cooling of that layer of the atmosphere. The ELR equation also assumes that the atmosphere is static, but heated air becomes buoyant, expands, and rises. The dry adiabatic lapse rate (DALR) accounts for the effect of the expansion of dry air as it rises in the atmosphere, and the wet adiabatic lapse rate (WALR) includes the effect of the condensation-rate of water vapor upon the environmental lapse rate.

Environmental Lapse Rate (ELR)

Altitude Region	Lapse rate	Lapse Rate
(m)	(°C / km)	(°F / 1000 ft)
0.0 – 11,000	6.50	3.57
11,000 – 20,000	0.0	0.0
20,000 – 32,000	−1.0	−0.55
32,000 – 47,000	−2.8	−1.54
47,000 – 51,000	0.0	0.0
51,000 – 71,000	2.80	1.54
71,000 – 85,000	2.00	1.09

Compression and expansion

A parcel of air rises and expands because of the lower atmospheric pressure at high altitudes. The expansion of the air parcel pushes outwards against the surrounding air, and transfers energy (as work) from the parcel of air to the atmosphere. Transferring energy to a parcel of air by way of heat is a slow and inefficient exchange of energy with the environment, which is an adiabatic process (no energy transfer by way of heat). As the rising parcel of air loses energy while it acts upon the surrounding atmosphere, no heat energy is transferred from the atmosphere to the air parcel to compensate for the heat loss. The parcel of air loses energy as it reaches greater altitude, which is manifested as a decrease in the temperature of the air mass. Analogously, the reverse process occurs within a cold parcel of air that is being compressed and is sinking to the planetary surface.[2]

The compression and the expansion of an air parcel are reversible phenomena in which energy is not transferred into or out of the air parcel; atmospheric compression and expansion are measured as an Isentropic Process ( $d S = 0 {\displaystyle dS=0}$

) wherein there occurs no change in entropy as the air parcel rises or falls within the atmosphere. Because the heat exchanged (

d Q = 0 {\displaystyle dQ=0}

) is related to the change in entropy (

d S {\displaystyle dS}

d Q = T d S {\displaystyle dQ=TdS}

) the equation governing the air temperature as a function of altitude for a mixed atmosphere is:

d S d z = 0 {\displaystyle {\frac {\,dS\,}{dz}}=0}

where

S

is the entropy. The isentropic equation states that atmospheric entropy does not change with altitude; the adiabatic lapse rate measures the rate at which temperature decreases with altitude under such conditions.

Humidity

If the air contains water vapor, then cooling of the air can cause the water to condense, and the air no longer functions as an ideal gas. If the air is at the saturation vapor pressure, then the rate at which temperature decreases with altitude is called the saturated adiabatic lapse rate. The actual rate at which the temperature decreases with altitude is the environmental lapse rate. In the troposphere, the average environmental lapse rate is a decrease of about 6.5°C for every 1.0 km (1,000m) of increased altitude.[2] For dry air, an approximately ideal gas, the adiabatic equation is: $p ( z ) [ T ( z ) ] − γ γ − 1 = constant {\displaystyle p(z){\Bigl [}T(z){\Bigr ]}^{-{\frac {\gamma }{\,\gamma \,-\,1\,}}}={\text{constant}}}$

wherein

γ {\displaystyle \gamma }

is the heat capacity ratio (

γ ≈ {\displaystyle \gamma \approx \,}

7⁄5) for air. The combination of the equation for the air pressure yields the dry adiabatic lapse rate:

d T d z = − m g R γ − 1 γ = − 9.8 ∘ C / k m {\displaystyle {\frac {\,dT\,}{dz}}=-{\frac {\;mg\;}{R}}{\frac {\;\gamma \,-\,1\;}{\gamma }}=-9.8^{\circ }\mathrm {C/km} }

.[5][6]

Environment

The environmental lapse rate ( $d T / d z {\displaystyle dT/dz}$

), at which temperature decreases with altitude, usually is unequal to the adiabatic lapse rate (

d S / d z ≠ 0 {\displaystyle dS/dz\neq 0}

). If the upper air is warmer than predicted by the adiabatic lapse rate (

d S / d z > 0 {\displaystyle dS/dz>0}

), then a rising and expanding parcel of air will arrive at the new altitude at a lower temperature than the surrounding air. In which case, the air parcel is denser than the surrounding air, and so falls back to its original altitude as an air mass that is stable against being lifted. If the upper air is cooler than predicted by the adiabatic lapse rate, then, when the air parcel rises to a new altitude, the air mass will have a higher temperature and a lower density than the surrounding air and will continue to accelerate and rise.[2][3]

Tropopause

The tropopause is the atmospheric boundary layer between the troposphere and the stratosphere, and is located by measuring the changes in temperature relative to increased altitude in the troposphere and in the stratosphere. In the troposphere, the temperature of the air decreases at high altitude, however, in the stratosphere the air temperature initially is constant, and then increases with altitude. The increase of air temperature at stratospheric altitudes results from the Ozone layer’s absorption and retention of the ultraviolet (UV) radiation that Earth receives from the Sun.[7] The coldest layer of the atmosphere, where the temperature lapse rate changes from a positive rate (in the troposphere) to a negative rate (in the stratosphere) locates and identifies the tropopause as an inversion layer in which limited mixing of air layers occurs between the troposphere and the stratosphere.[2]

Atmospheric flow

The general flow of the atmosphere is from west to east, which, however, can be interrupted by polar flows, either north-to-south flow or a south-to-north flow, which meteorology describes as a zonal flow and as a meridional flow. The terms are used to describe localized areas of the atmosphere at a synoptic scale; the three-cell model more fully explains the zonal and meridional flows of the planetary atmosphere of the Earth.

Three Cell Model

Zonal Flow: a zonal flow regime indicates the dominant west-to-east flow of the atmosphere in the 500 hPa height pattern.

Meridional Flow: The meridional flow pattern of 23 October 2003 shows amplified troughs and ridges in the 500 hPa height pattern.

The three-cell model of the atmosphere of the Earth describes the actual flow of the atmosphere with the tropical-latitude Hadley cell, the mid-latitude Ferrel cell, and the polar cell to describe the flow of energy and the circulation of the planetary atmosphere. Balance is the fundamental principle of the model — that the solar energy absorbed by the Earth in a year is equal to the energy radiated (lost) into outer space. That Earth’s energy balance does not equally apply to each latitude because of the varying strength of the sunlight that strikes each of the three atmospheric cells, consequent to the inclination of the axis of planet Earth within its orbit of the Sun. The resultant atmospheric circulation transports warm tropical air to the geographic poles and cold polar air to the tropics. The effect of the three cells is the tendency to the equilibrium of heat and moisture in the planetary atmosphere of Earth.[8]

Zonal flow

A zonal flow regime is the meteorological term meaning that the general flow pattern is west to east along the Earth's latitude lines, with weak shortwaves embedded in the flow.[9] The use of the word "zone" refers to the flow being along the Earth's latitudinal "zones". This pattern can buckle and thus become a meridional flow.

Meridional flow

When the zonal flow buckles, the atmosphere can flow in a more longitudinal (or meridional) direction, and thus the term "meridional flow" arises. Meridional flow patterns feature strong, amplified troughs of low pressure and ridges of high pressure, with more north–south flow in the general pattern than west-to-east flow.[10]

References

^ "Troposphere". Concise Encyclopedia of Science & Technology. McGraw-Hill. 1984. It [the troposphere] contains about four-fifths of the mass of the whole atmosphere.
^ a b c d e f Danielson, Levin, and Abrams (2003). Meteorology. McGraw Hill.{{cite book}}: CS1 maint: uses authors parameter (link)
^ a b Landau and Lifshitz, Fluid Mechanics, Pergamon, 1979
^ Lydolph, Paul E. (1985). The Climate of the Earth. Rowman and Littlefield Publishers Inc. p. 12.
^ Kittel and Kroemer (1980). Thermal Physics. Freeman. chapter 6, problem 11.{{cite book}}: CS1 maint: uses authors parameter (link)
^ Landau and Lifshitz (1980). Statistical Physics. Vol. Part 1. Pergamon.{{cite book}}: CS1 maint: uses authors parameter (link)
^ "The Stratosphere — Overview". scied.ucar.edu. University Corporation for Atmospheric Research. Retrieved 25 July 2018.
^ "Meteorology – MSN Encarta, "Energy Flow and Global Circulation"". Encarta.Msn.com. Archived from the original on 2009-10-28. Retrieved 2006-10-13.
^ "American Meteorological Society Glossary – Zonal Flow". Allen Press Inc. June 2000. Archived from the original on 2007-03-13. Retrieved 2006-10-03.
^ "American Meteorological Society Glossary – Meridional Flow". Allen Press Inc. June 2000. Archived from the original on 2006-10-26. Retrieved 2006-10-03.