2 Structure of Atmosphere

Nilimesh Mridha

1.Learning Outcomes

2.ntroduction

3.Structure of atmosphere based on vertical temperature profile 3.1. Troposphere: structure and characteristics

3.1.1.Thickness and factors affecting it

3.1.2.Chemical composition

3.1.3.Temperature characteristics

3.1.4.Tropopause-Isothermal layer

3.1.5.Interesting facts

3.2.Stratosphere: structure and characteristics

3.2.1.Temperature stratification and ozone layer

3.2.2.Interesting facts

3.3.Mesosphere: structure and characteristics

3.4.Thermosphere: structure and characteristics

3.5.Exosphere: structure and characteristics

4.Structure of atmosphere based on gaseous composition

5.Layers of atmosphere based on electrical properties 5.1. Ionosphere and its sub layers

5.2. Some important phenomena of Ionosphere

6.Summary

1. Learning outcomes

• After studying this module, you shall be able to:
• Know about the vertical structure of the atmosphere
• Know temperature distribution, electrical properties and gaseous composition of layers Interesting phenomena happening in various layers

2. Introduction

The atmosphere can be generally defined as an invisible gaseous blanket around the earth held by earth’s gravitational force. In one way or another, it affects all the entity exists on the earth – it is closely connected to our lives. Without this blanket, it would be unimaginably cold at night and unbearably hot during the day. Unlike other planets or extraterrestrial bodies, the planet earth is very unique in a sense that it is covered with atmosphere that provides optimum condition to support life to flourish. The earth’s atmosphere are composed of two primary gases, viz., nitrogen and oxygen and water vapor that back up the life whereas carbon dioxide, hydrogen or helium gases are abundant with no water vapor in the atmospheres of other planets. Detailed study of the structure of earth’s atmosphere is extremely important to gather knowledge regarding its origin, atmospheric composition and processes, different phenomena related to weather and climate, aviation, communication system etc.

If we look into the earth’s atmosphere, it is seen that the atmosphere is not physically uniform throughout but has got considerable variations, especially in terms of pressure and temperature with height. An examination of vertical profile of the atmosphere reveals that it is comprised of a series of layers. These layers could be defined in several ways such as based onThe vertical temperature distribution Electrical properties of the layer, and The gaseous composition of it

These categorization of atmosphere is elaborated below:

3. Layers of atmosphere based on vertical temperature profile

Air temperature throughout atmosphere shows complicated vertical profile and may be divided into five main distinguished layers, namely troposphere, stratosphere, mesosphere, thermosphere and exosphere. They are layered one atop the other and each one is very much needed to support and protect the life on the earth.The atmospheric also contains transition layers between two successive main layers.The nomenclature of these transitional layers zones contains the term – pause at its end. These are tropopause (from the end of troposphere and till the beginning of stratosphere), stratopause (boundary separating stratosphere from mesosphere), mesopause (transition region between mesosphere and thermosphere) and thermopause (between thermosphere and exosphere).

Fig. 1. A schematic of the vertical profile of the atmosphere with relative position of its various layers. It also depicts the pattern of temperature changes with height from the ground and inside these layers. Approximate pressure levels at different altitude can also be noted.

(Source: www.teachertech.rice.edu)

3.1. Troposphere: structure and characteristics

The word ‘troposphere’ originates from the Greek word ‘tropein’ which means to change, circulate or mix. Troposphere is the lowest layer of the atmosphere which is close to the earth’s surface where almost all the weather phenomenon that we experience like convection, turbulence and most of the cloud formation occur.

3.1.1.   Thickness and factors affecting it

The thickness of this layer varies from 7-8 km at poles to 16-18 km over the equator. The depth of the troposphere is influenced by the latitude, season and time of the day. The reasons behind the non-uniformity in thickness of the troposphere are:

1..Equator gets high insolation and strong convective fluxes

As equator gets highest amount of insolation, it becomes warmer than other parts of the earth. According to Charles’ law, density reduces with increase in temperature at constant pressure which implies more the temperature over a place, more the air gets heated and rises upward due to decrease in density. Hence, due to differential heating of the earth’s surface, equatorial regions experience more amounts of convective fluxes. Simply, we can say that although both equator and poles have the same amount of atmospheric mass over it, the thermal expansion of air over equator is more than the poles and that makes the atmosphere thicker over equator than over the poles.

2.Higher gravitational pull on atmospheric gases at poles

Due to ellipsoid shape of the earth, the gravitational pull is not uniform throughout and it is more over poles. As a result atmospheric molecules are drawn with more force near the poles causing contraction of the atmosphere.

3.Equator has highest centrifugal force due to earth’s rotation

The atmosphere over equator tends to swell due to maximum speed of the rotating earth and the minimum Coriolis effect.

3.1.2.   Chemical composition

The chemical composition of this layer is basically uniform, with the important exception of water vapor. About 99% of the total water vapor in the atmosphere is present in the troposphere itself. The concentration of water vapor varies with latitudes. Equatorial region has maximum concentration of water vapor (up to 3%) and reduces towards polar region. At the same time water vapor is more at the surface and decreases with altitude.The troposphere primarily comprises of nitrogen (78%) and oxygen (21%) with very little amount of trace gases. Nearly 75% of the total mass of atmosphere is contained in troposphere and thus denser than the overlaying layers.

3.1.3.   Temperature characteristics

Lapse rate: We usually feel cooler on the top of a mountain than in the valley below and chances of encountering snow or ice is greater in a higher hill than a hill with low altitude. Such thing happen as, troposphere, in general, shows a decreasing trend of air temperature with elevation. Air temperature usually declines from the earth’s surface with increasing height up to about 11 km due to the fact that air near the surface gets heated up much more by earth’s emitted long wave radiation energy than the air which is away from it.The rate at which the air temperature reduces with altitude is called the temperature lapse rate and average valueof it is about 6.5 °C per km. This lapse rate is just an average value of day to day variations.

Inversion: Sometimes, an increase in the air temperature with height may actually happen. Such a condition is known as temperature inversion and this prevents the vertical mixing of air. Temperature inversion plays an important role in air pollution where pollutants released at ground level get confined in the lower layers of atmosphere thus affecting the human and animal lives.

3.1.4. Tropopause-Isothermal layer

It can be observed that just above a certain height of the troposphere, the lapse rate becomes zero which means the air temperature (about – 60°C) remains constant with altitude in that region i.e. an isothermal (equal temperature) layer. The bottom of this layer represents top of the troposphere and its upper limit as the starting point of the stratosphere. This transitional boundary differentiating the stratosphere from the troposphere is called the tropopause. It is typically higher over equatorial regions and lower in polar region. The height of tropopause generally increases in summer and decreases in winter at all latitudes. There are some ‘breaks’ in the tropopause where tropospheric air mixes with stratospheric air and vice versa. These breaks also indicate the position of high winds typically known as jet streams or ‘river of air’ having speeds exceeding 100 knots or 250 miles per hour.

3.1.5.   Interesting facts

• The troposphere harbors much of the weather which we experience on earth, beginning from wind to rain, thunderstorms and lightning, tornadoes, typhoons, hurricanes etc.
• Airplanes and jets flying at high altitudes must be adjusted with the lower pressure and density at that height.Maximum dust particles of the atmosphere are observed in the troposphere.Maximum air pressure is found at sea level and decreases as we go up. Similar is the trend for the amount of oxygen which is predominantly experienced by mountain climbers venturing to higher altitudes and needs artificial oxygen sources to survive.
• The troposphere facilitates temperature regulation and cloud formation. It has highest temperatures closer to its base; these warm temperatures help the troposphere retain water vapor, which later gets released in the form of precipitation.
• The exchange and movement of water between the earth and atmosphere is called the water cycle. The troposphere serves as the starting point for the earth’s water cycle.

3.2.   Stratosphere: structure and characteristics

The stratosphere extends up to an average height of 50 km from the earth surface. This layer is usually characterized by temperature inversion which keeps the vertical fluxes of the troposphere from dispersing into the stratosphere and thus generating a stratified layer.

3.2.1.   Temperature stratification and the ozone layer

Generally temperature rises in the stratosphere with height. The reason attributed to this is the presence of ozone gas. If ozone were absent, the air would become colder with height like in troposphere. Ozone is mainly confined in the lower stratosphere within 15–35 km and the thickness of ozone layer varies with seasons and latitudes with maximum at the equator and minimum at the poles. Stratospheric ozone constitutes about 90% of the total ozone present in our atmosphere with concentrations of about 2-8 ppm. The ozone (O3) here absorbs high energy UV light and gets split into atomic oxygen (O) and diatomic oxygen (O2). During formation of ozone by recombining these two forms of oxygen, heat is released into the stratosphere. Very low amount of UV light reaches to the  lower part of stratosphere, thus atomic oxygen is not observed here and heat as the byproduct of ozone formation is not produced. This vertical stratification, where above layer becomes warmer and lower layer becomes cooler, makes the stratosphere dynamically stable. No convection and associated turbulence is found in this layer. The level of maximum ozone concentration is found at an altitude about 25 km over the middle latitudes.However, the stratospheric air temperature is maximum near 50 km due to the fact that at this specific height the air is less dense than at 25 km and the intense solar energy heats up the fewer molecules at 50 km which increases the temperature to a much greater degree. At the same time, the downward movement of heat energy is extremely slow due to the low air density which results in lower temperature in the stratum below.

The transition boundary layer from the top of the stratosphere to the beginning of mesosphere i.e. the stratopause is isothermal in nature. The air temperature of stratopause remains constant around 0°C. Above stratopause, the temperature declines with height.

3.2.2.   Interesting facts

• Stratospheric ozone is vital for the survival of human beings and other life forms on earth, by absorbing UV radiation that would otherwise be fatal.
• Volcanic gases, dust, CFCs etc cause damage to the ozone layer of the stratosphere.
• Stratosphere has extremely low amount of water vapor and thus we don’t see any cloud there.
• Flying of airplanes in the stratosphere avoids turbulence, saves fuel burning and trip time.
• Sometimes research balloons with cameras are sent to record the weather information of the stratosphere.
• Stratosphere has no vertical winds. The temperature inversion in the stratosphere makes this layer very stable which does not allow significant vertical motion of winds. Thus vertical winds are nearly absent, in some cases may be up to a few centimeters per second. The incapability of the air to flow vertically is also the main reason why the Ozone depleting CFC takes so long to reach the height where the Sun’s energy is sufficient enough to break them apart. This also implies that some of the ozone depleting substances will still be there a centuries later from now.
• Aviation and jet Streams in Stratosphere- Unlike troposphere, stratosphere is free from the harsh weather changes and is preferred by commercial aircrafts usually flying at altitudes of 9– 12 km in the lower part of the stratosphere to adjust fuel burn. However, Jet liners face the problem of jet streams which are high velocity horizontal air currents and located near the tropopause. The location of the jet stream is particularly vital for aviation. Sometimes, jet streams plays important beneficial role in aviation by shorten the trip time and reducing the fuel consumption.

3.3.Mesosphere: structure and characteristics

The mesosphere, third layer in the atmosphere is situated 50 km from the earth’s surface right above the stratopause and goes up to 85 km height (from the surface of the earth) i.e. till below the thermosphere.

The literal meaning of mesosphere is ‘middle sphere‘ i.e. it exists in the middle of atmosphere. Mesosphere plays important role in protecting the planet from the meteors and asteroids falling to the earth by burning those with the gas particle present in the layer. The air is extremely thin here and the atmospheric pressure is very low, close to 1 mb indicating 99.9 percent of the atmospheric mass is found below this layer. The percentage of important gaseous components like nitrogen and oxygen in the mesosphere is similar which is seen at sea level. Due to very low density in this level, we cannot survive very long, as each breath will have far fewer oxygen molecules than at sea level.

I. Temperature

Air temperature usually drops when go up in the mesosphere as the air contains extremely low amount of ozone to absorb solar radiation. So, the upper part of the mesosphere loses more energy than it absorbs, which results in an energy deficit and cooling and the air temperature touches its lowest mean value, -90 ºC. This is at the highest region in mesosphere and also the coldest place on earth.

II. Formation of noctilucent clouds

Unlike in troposphere, greenhouse gases, like carbon dioxide and methane behave differently in the mesosphere. Here, the carbon dioxide releases heat into the space from the mesosphere and cools down. At the same time, solar radiation disintegrates the methane gas. Reactions take place among CO2, methane and ozone gas with water vapor present there under very low temperature of mesosphere, which leads to the formation of microscopic blue-white ice clouds popularly known as the noctilucent clouds. These beautiful clouds are clearly seen from the poles and during the sunset. A peculiar type of lightning is also observed in the mesosphere called ‘Elves’ or ‘Sprites’.

III. Nightglow

If we see the clear sky at night properly, we can see some glowing layer due to unbound non-ionized atoms of sodium having thickness of around 5 km. This sodium layer forms the nightglow which is the light emitted from the mesosphere and is located just below the mesopause.

IV.Strong winds

As temperature change is significant between the summer and winter poles, there are strong winds blowing from north to south in the lower mesosphere and in the upper mesosphere wind blows from east to west due to colder temperature there.

V.Interesting facts

• Studies showed that the mesosphere burns roughly 40 tons of meteors falling towards earth each day otherwise they could reach the earth and make severe damage to its surface. Burning of meteors is seen as shooting stars in the night sky.
• Satellites can’t stay in orbit in the mesosphere due to high atmospheric drag. The sounding rockets are used to study the mesosphere.
• The mesosphere is composed of oxygen, nitrogen and carbon dioxide and the air density is very low here.

3.4. Thermosphere: structure and characteristics

The “hot layer” above the mesosphere is the thermosphere. The transition boundary that divides the lower and colder mesosphere from the warmer thermosphere is the mesopause. The thermosphere derives its name from the Greek word ‘thermos’, meaning ‘heat’. Starting from the surface, it is the fourth layer of the Earth’s atmosphere. This also makes it the second farthest atmospheric layer from the Earth’s surface extending vertically for a distance of about 250 to 550 miles; the only layer farther than it being the exosphere. The thermosphere is mainly comprised with helium, atomic oxygen and atomic nitrogen.

Interesting facts

• The thermosphere can absorb the X-ray and the UV radiation comming from the sun.
• Solar activity increases the size of the thermosphere and makes it as one of the most dynamic layer. The only entity that can make it to this layer is a space shuttle. Excitingly, the orbit of the International Space Station (ISS) is placed in thermosphere.
• This layer exhibits some remarkable sky phenomena like the northern or southern lights.
• The charged particles of thermosphere greatly help in long distance radio communication.
• In the lower half of the thermosphere, the temperature increases very quickly with altitude. It is only after reaching a point that it becomes stable. In the upper thermosphere, temperature ranges from 500 oC to 2000 oC. The temperature here, is highly dependent on solar activity, and can reach 2000 oC or higher when solar activity is at its peak.
• Thin air also makes it difficult for the scientists to measure the temperature of this layer. Instead of measuring it directly, the scientists first measure the density of the air by measuring the drag on the satellites and use the same to determine the temperature. Other than density observations, scientists may also resort to direct satellite measurements to determine the temperature of this layer.

3.5.  Exosphere: structure and characteristics

Above the earth’s surface at the height of 500 km (top of the thermosphere), air is extremely thin with a mean free path (for gas molecules) of 10 km or more. Here, a number of fast-moving molecules or atoms have the capability of escaping the earth’s gravitational pull. This region where atoms and molecules shoot off into space is known as the exosphere, which signifies the upper limit of our atmosphere. Due to ambiguity of clear cut demarcation between the exosphere and the space, it is hard to determine the height of this layer.

4. Layers of atmosphere based on electrical properties

• 1. Ionosphere and its sub layers

The ionosphere is more appropriately described as an electrified region within the upper atmosphere rather than just a layer consisting of large concentrations of ions and free electrons. Ions are nothing but the charged atoms or molecules having deficient or extra electrons that are unable to absorb all of the transferred energy due to collision between colliding particles and radiation from the sun. Ionosphere stretches from 50 to 1,000 km and typically overlaps both the thermosphere and the exosphere. The lower levels of ionosphere have more number of ions compared to the upper levels. Hence, lower levels show the greater ionization and recombination as compared to the upper levels producing a highly unstable layer of the earth’s atmosphere.

We note that the ionization depends primarily on the amount of radiation received from the Sun and its activity. This is the reason that there are changes in the ionosphere on the diurnal and seasonal basis. The activity of the sun is associated with sunspot cycle; the higher the number of sunspots, more will be the solar output. Radiation received also varies with the position of earth in the revolutionary orbit and geographical location. There are some disturbances like solar flares or mechanisms which interrupt the ionosphere and decrease the ionization and also interact with its geomagnetic field. Accordingly, ionosphere has been divided into different sets of layers (Fig. 2) during day and night which are discussed below:

    I.D-Layer

The D layer explains why the AM Radio gets disturbed during day time, but quite smooth in night time. We can see in the above figure that the D layer is situated between 60 km to 90 km above the earth surface and is the innermost layer. This layer shows the low net ionization effect, but loses huge amount of wave energy due to frequent collisions of the electrons. Thus, the high-frequency (HF) radio waves are least attenuated by the D layer but suffer loss of energy therein. The absorption is maximum about midday and minimum at night which results in the disappearance of distant AM radio waves in the daytime.

II. E – Layer

The E layer is the intermediate layer, extending from 90 km to 120 km above the earth surface, with primary source of ionization being soft X-ray (1-10 nm) and far ultraviolet (UV) solar radiation causing ionization of molecular oxygen (O2). This layer disappears in the night because primary source of ionization is no longer present. The practical value of this layer is that it reflects long radio-waves in the 1-3 megahertz (MHz) back to earth, which enables them to be received at a distance, rather than disappear into space. It is also known as Heavyside-Kennely layer.

III. F-Layer

The F layer extends from about 200 km to more than 500 km above the surface of earth. The E-layer allows the penetration of short-radio waves, which continue until they reach the Appleton layer. Appleton layer reflects short-radio waves (which have penetrated the Heavyside-Kennely layer) back to earth. This is also supposed to be the region where polar auroras occur and where most of the meteors burn themselves out. Ionosphere is also known as thermosphere because of the high temperatures prevailing there. The temperature near the upper boundary of the thermosphere may become higher than 1000-1500°C. Along with temperature rise, sharp changes caused by the corpuscular and ultraviolet solar radiation are observed in it.

4.2.       Some important phenomena of Ionosphere

I.Ionospheric Scintillation

Scintillation influences the phase and power of the propagating radio signal. Ionospheric scintillation is the fast alteration of radio waves produced by small scale structures in the ionosphere. Severe scintillation can stops a GPS receiver from getting the accurate signal and produces huge errors in position calculation. Low and high latitudes exhibit more prominent scintillation over mid-latitudes (e.g. the United States).

II.Aurora

The aurora are illusive natural electro-magnetic phenomenon with luminous effect, which usually occurs near the northern (Aurora Borealis) and southern (Aurora Australis) poles in the night sky, best developed at a height of about 90 km (Fig. 3). Such effects are probably the result of magnetic storms and of electrical discharges from the sun during periods of sun-spot activity, causing ionization of gases, though this is still a matter of research. The origin of aurora can be traced to solar flares which are huge eruptions on the surface of the sun that occur periodically (every 11 years). Most of the solar radiation returns back to the sun, but some ionized particles are ejected into space at very high speeds. These solar flares travel at the speed of roughly 1.5 x 106 m s-1 and enter the earth’s atmosphere 30 hours after the occurrence of solar flares. Sine these particles are electrically charged, they are deflected towards the north and south poles by the magnetic fields of the earth. When these high-speed charged particles collide with the air molecules, they cause electrons to be freed. When the electrons recombine with air molecules, energy in the form of light is emitted. Occasionally the Aurora borealis is seen in England, but it is more common in northern Scotland. Figure 3 presents magnificent spectacles in northern Scandinavia and northern Canada.

5. Layers of atmosphere based on gaseous composition

Based on chemical composition, the atmosphere may be divided into the following two regions:

1. Homosphere

Below thermosphere, the composition of atmosphere is nearly uniform upto an altitude about 85 km (50 miles) due to turbulent mixing and hence, it is known as the homosphere. Though, the proportion of gases (nitrogen-78 %, oxygen-21 %, CO2-0.03% etc.) remains almost unchanging in this well mixed region, the air density as well as air pressure declines significantly with increasing height. The Three layers namely, the troposphere, the stratosphere and the mesosphere that have been described in the previous sections, conform the homosphere.

II.Heterosphere

The region starting from the base of the thermosphere and extending to the top of the atmosphere is often called the heterosphere. This means, the heterosphere is comprised of the thermosphere and the exosphere layers. Generally, heterosphere comprises less than 0.001% of the total mass of the earth’s atmosphere. In this region, gases are not well mixed or stirred due to very infrequent collisions between atoms and molecules which results in accumulation of heavier gases like nitrogen and oxygen at the bottom and lighter ones like helium and hydrogen at the top of the layer by the process of diffusion.

6. Summary

• Earth is having unique atmosphere to sustain life by providing two important gases such as nitrogen and oxygen unlike other planets. Vertical division of the atmosphere in terms of layers can be done according to its temperature profile, its electrical properties, or its gaseous composition.
• The rate at which the air temperature reduces with altitude is called the lapse rate and the opposite phenomenon is called an inversion.
• We live at the lowest part of the troposphere where all the weather phenomena that we are familiar with take place.
• The coldest atmospheric layer is the mesosphere; the warmest is the thermosphere. Maximum concentration of ozone gas is observed in the stratosphere.
• The ionosphere is known as an electrified region of the earth’s atmosphere extending from about 60 km to the top of the atmosphere and extremely helpful in long wave communication.

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