Mesosphere & Thermosphere

The mesosphere (/ˈmɛsoʊsfɪər/; from Greek mesos "middle" and sphaira "balls") is the layer of the Earth's atmosphere that is directly above the stratosphere and directly below the mesopause. In the mesosphere, temperature decreases as the altitude increases. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures below 130 K (−226 °F; −143 °C). The exact upper and lower boundaries of the mesosphere vary with latitude and with season, but the lower boundary of the mesosphere is usually located at heights of about 50 kilometres (160,000 ft; 31 mi) above the Earth's surface and the mesopause is usually at heights near 100 kilometres (62 mi), except at middle and high latitudes in summer where it descends to heights of about 85 kilometres (53 mi).

The stratosphere, mesosphere and lowest part of the thermosphere are collectively referred to as the "middle atmosphere", which spans heights from approximately 10 kilometres (33,000 ft) to 100 kilometres (62 mi). The mesopause, at an altitude of 80–90 km (50–56 mi), separates the mesosphere from the thermosphere—the second-outermost layer of the Earth's atmosphere. This is also around the same altitude as the turbopause, below which different chemical species are well mixed due to turbulent eddies. Above this level the atmosphere becomes non-uniform; the scale heights of different chemical species differ by their molecular masses.

The thermosphere is the layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer of the atmosphere, ultraviolet 

radiation causes photoionization/photodissociation of molecules, creating ions in the ionosphere. Taking its name from the Greekθερμός (pronounced thermos) meaning heat, the thermosphere begins about 85 kilometres (53 mi) above the Earth. At these high altitudes, the residual atmospheric gases sort into strata according to molecular mass (see turbosphere). Thermospheric temperatures increase with altitude due to absorption of highly energetic solar radiation. Temperatures are highly dependent on solar activity, and can rise to 2,000 °C (3,630 °F). Radiation causes the atmosphere particles in this layer to become electrically charged (see ionosphere), enabling radio waves to be refracted and thus be received beyond the horizon. In the exosphere, beginning at 500 to 1,000 kilometres (310 to 620 mi) above the Earth's surface, the atmosphere turns into space.

The highly diluted gas in this layer can reach 2,500 °C (4,530 °F) during the day. Even though the temperature is so high, one would not feel warm in the thermosphere, because it is so near vacuum that there is not enough contact with the few atoms of gas to transfer much heat. A normal thermometer might be significantly below 0 °C (32 °F), at least at night, because the energy lost by thermal radiation would exceed the energy acquired from the atmospheric gas by direct contact. In the anacoustic zone above 160 kilometres (99 mi), the density is so low that molecular interactions are too infrequent to permit the transmission of sound.

The dynamics of the thermosphere are dominated by atmospheric tides, which are driven by the very significant diurnal heating. Atmospheric waves dissipate above this level because of collisions between the neutral gas and the ionospheric plasma.

The International Space Station orbits the Earth within the middle of the thermosphere, between 330 and 435 kilometres (205 and 270 mi) (decaying by 2 km/month and raised by periodic reboosts), whereas the Gravity Field and Steady-State Ocean Circulation Explorer satellite at 260 kilometres (160 mi) utilized winglets and an innovative ion engine to maintain a stable orientation and orbit.

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Stratosphere & Troposphere

The stratosphere (/ˈstrætəˌsfɪər, -toʊ-) is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. About 20% of the atmosphere's mass is contained in the stratosphere. The stratosphere is stratified in temperature, with warmer layers higher and cooler layers closer to the Earth. The increase of temperature with altitude, is a result of the absorption of the Sun's ultraviolet radiation by ozone. This is in contrast to the troposphere, near the Earth's surface, where temperatures decreases with altitude. The border between the troposphere and stratosphere, the tropopause, marks where this temperature inversion begins. Near the equator, the stratosphere starts at 18 km (59,000 ft; 11 mi); at mid latitudes, it starts at 10–13 km (33,000–43,000 ft; 6.2–8.1 mi) and ends at 50 km (160,000 ft; 31 mi); at the poles, it starts at about 8 km (26,000 ft; 5.0 mi). Temperatures vary within the stratosphere with the seasons, in particular with the polar night (winter). The greatest variation of temperature, takes place over the poles in the lower stratosphere; those variations are largely steady at lower latitudes and higher altitudes.

The troposphere is the lowest portion of Earth's atmosphere, and is also where all weather takes place. It contains approximately 75% of the atmosphere's mass and 99% of it is water vapor and aerosols. The average depths of the troposphere are 20 km (12 mi) in the tropics, 17 km (11 mi) in the mid latitudes, and 7 km (4.3 mi) in the polar regions in winter. The lowest part of the troposphere, where friction with the Earth's surface influences air flow, is the planetary boundary layer. This layer is typically a few hundred meters to 2 km (1.2 mi) deep depending on the landform and time of day. Atop the troposphere is the tropopause, which is the border between the troposphere and stratosphere. The tropopause is an inversion layer, where the air temperature ceases to decrease with height and remains constant through its thickness.

The word troposphere derives from the Greek: tropos for "turn, turn toward, trope" and "-sphere" (as in, the Earth), reflecting the fact that rotational turbulent mixing plays an important role in the troposphere's structure and behaviour. Most of the phenomena we associate with day-to-day weather occur in the troposphere.

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What is tropospheric ozone?

Ozone (O₃) is a constituent of the troposphere (it is also an important constituent of some regions of the stratosphere commonly known as the ozone layer). The troposphere extends from the Earth's surface to between 12 and 20 kilometers above sea level and consists of many layers. Ozone is more concentrated above the mixing layer, or ground layer. Ground-level ozone, though less concentrated ozone aloft, is more of a problem because of its health effects.

Photochemical and chemical reactions involving it drive many of the chemical processes that occur in the atmosphere by day and night. At abnormally high concentrations brought about by human activities (largely incomplete combustion of fossil fuels, such as gasoline, diesel, etc.), it is a pollutant, and a constituent of smog. Many highly energetic reactions produce it, ranging from combustion to photocopying. Often laser printers will have a smell of ozone, which in high concentrations is toxic. Ozone is a powerful oxidizing agent readily reacting with other chemical compounds to make many possibly toxic oxides.

Tropospheric ozone is a greenhouse gas and initiates the chemical removal of methane and other hydrocarbons from the atmosphere. Thus, its concentration affects how long there compounds remain in the air. 

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What is sunlight?

Sunlight is a portion of the electromagnetic radiation given off by the Sun, in particular infrared, visible, and ultraviolet light. On Earth, sunlight is filtered through Earth's atmosphere,and is obvious as daylight when the Sun is above the horizon. When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and radiant heat. When it is blocked by the clouds or reflects off other objects, it is experienced as diffused light. The World Meteorological Organization uses the term "sunshine duration" to mean the cumulative time during which an area receives direct irradiance from the Sun of at least 120 watts persquare meter.

The ultraviolet radiation in sunlight has both positive and negative health effects, as it is both a principal source of vitamin D3 and amutagen. 

Sunlight takes about 8.3 minutes to reach Earth from the surface of the Sun. A photon starting at the centre of the Sun and changing direction every time it encounters a charged particle would take between 10,000 and 170,000 years to get to the surface.

Sunlight is a key factor in photosynthesis, the process used by plants and other autotrophic organisms to convert light energy, normally from the Sun, into chemical energy that can be used to fuel the organisms' activities .

Although the solar corona is a source of extreme ultraviolet and X-ray radiation, these rays make up only a very small amount of the power output of the Sun. The spectrum of nearly all solar electromagnetic radiation striking the Earth's atmosphere spans a range of 100 nm to about 1 mm  (1,000,000 nm). This band of significant radiation power can be divided into five regions in increasing order of wavelengths:

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence, also invisible to the human eye). Due to absorption by the atmosphere very little reaches Earth's surface. This spectrum of radiation has germicidal properties, and is used in germicidal lamps.

• Ultraviolet B or (UVB) range spans 280 to 314 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for thephotochemical reaction leading to the production of the ozone layer. It directly damages DNA and causes Sunburn, but is also required for vitamin D synthesis in the skin and fur of mammals. 

• Ultraviolet A or (UVA) spans 315 to 400 nm. This band was once held to be less damaging to DNA, and hence is used in cosmetic artificial sun tanning (tanning booths andtanning beds) and PUVA therapy for psoriasis. However, UVA is now known to cause significant damage to DNA via indirect routes (formation of free radicals and reactive oxygen species), and is able to cause cancer.

• Visible range or light spans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked eye. It is also the strongest output range of the Sun's total irradiance spectrum.

• Infrared range that spans 700 nm to 1,000,000 nm (1 mm) . It is responsible for an important part of the electromagnetic radiation that reaches Earth. It is also divided into three types on the basis of wavelength:

• Infrared-A: 700 nm to 1,400nm
• Infrared-B: 1,400 nm to 3,000 nm
• Infrared-C: 3,000 nm to 1 mn.

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What is solar radiation?

What is solar radiation?

Bad thoughts usually come to mind when you think of the word 'radiation.' But solar radiation is actually a very beneficial thing-it's sunlight! Every living thing on Earth depends on sunlight for survival. It warms the planet, provides food for plants, and in general, just makes us feel pretty darn good. I don't know about you, but I think being outside on a sunny day soaking up all that energy is a really nice feeling!

Solar radiation is all of the light and energy that comes from the sun, and there are many different forms. The electromagnetic spectrum explains the different types of light waves that are emitted from the sun. Light waves are similar to waves you see on the ocean-they move up and down and travel from one place to another. The difference is that instead of the water vibrating up and down, light waves are vibrations of electromagnetic fields, hence the name the electromagnetic spectrum.

You can think of the spectrum like a piano keyboard. One end of the keyboard has low notes while the other has high notes. The same is true for the electromagnetic spectrum. One end has low frequencies and the other high frequencies. Low frequency waves are low-energy wave with a long wavelength. The length of the wave itself is very long for a given period of time. These are things like radar, TV and radio waves. High frequency waves are high-energy waves with a short wavelength. This means that the length of the wave itself is very short for a given period of time. These are thing like gamma rays, X-rays and ultraviolet rays.

You can think of it like this: Low frequency waves are like going up a hill that slowly rises in elevation, while high frequency waves are like going up a steep hill very quickly. The height of each hill is the same, but the elevation either slopes gently over a longer incline or slopes up quickly over a shorter incline. Visible light, which is the sunlight we and other animals can see with our eyes, falls in almost the middle of the spectrum.

We can't see any other waves on the spectrum (which are all just different forms of light), but that doesn't mean they aren't there. In face, insects see ultraviolet light but not our visible light. Flowers look very different to them than they do to us, and this helps them know which plants to visit and which ones to stay away from.

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Effects of global warming

The effects of global warming are the environmental and social changes caused (directly or indirectly) by human emissions of greenhouse gases. There is a scientific consensus that climate change is occurring, and that human activities are the primary driver. Many impacts of climate change have already been observed, including glacier retreat, changes in the timing of seasonal events (e.g., earlier flowering of plants), and changes in agricultural productivity.

Future effects of climate change will vary depending on climate change policies and social development. The two main policies to address climate change are reducing human greenhouse gas emissions (climate change mitigation) and adapting to the impacts of climate change. Geoengineering is another policy option.

Near-term climate change policies could significantly affect along-term climate change impacts. Stringent mitigation policies might be able to limit global warming (in 2100) to around 2 °C or below, relative to pre-industrial levels. Without mitigation, increased energy demand and extensive use of fossil fuels might lead to global warming of around 4 °C. Higher magnitudes of global warming would be more difficult to adapt to, and would increase that risk of negative impacts.

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Ozone depletion

Ozone depletion describes two distinct but relate phenomena observed since the late 1970s: a steday decline of about four percent in the total amount of ozone in Earth's stratosphere  (the ozone layer), and a much larger springtime decrease in stratospheric ozone around Earth's polar regions. The latter phenomenon is referred to as the ozone hole. In addition to these well-known stratospheric phenomena, there are also springtime polar tropospheric ozone depletion events.

The details of polar ozone hole formation differ from that of mid-latitude thinning but the most important process in both iscatalytic destruction of ozone by atomic halogens. The main source of these halogen atoms in the stratosphere is photodissociation of man-made halocarbon refrigerants, solvents, propellants, and foam-blowing agents  (chlorofluorocarbon(CFCs), HCFCs, freons, halons). These compounds are transported into the stratosphere by winds after being emitted at the surface. Both types of ozone depletion were observed to increase as emissions of halocarbons increased. 

CFCs and other contributory substances are referred to as ozone-depleting substances (ODS). Since the ozone layer prevents most harmful UVB wavelengths (280–315 nm) of  ultraviolet light (UV light) from passing through the Earth's atmosphere, observed and projected decreases in ozone generated worldwide concern, leading to adoption of the Montreal Protocol that bans the production of CFCs, halons, and other ozone-depleting chemicals such as carbon tetrachloride and trichloroethane. It is suspected that a variety of biological consequences such as increases in sunburn, skin cancer, cataracts, damage to plants, and reduction of plankton populations in the ocean's photic zone may result from the increased UV exposure due to ozone depletion. 

Ozone layer depletion

The most-pronounced decrease in ozone has been in the lower stratosphere. However, the ozone hole 

is most usually measured not in terms of ozone concentrations at these levels (which are typically a few parts per million) but by reduction in the total column ozone above a point on the Earth's surface, which is normally expressed in Dobson units, abbreviated as "DU". Marked decreases in column ozone in the Antarctic spring and early summer compared to the early 1970s and before have been observed using instruments such as the Total Ozone Mapping Spectrometer.

Reductions of up to 70 percent in the ozone column observed in the austral (southern hemispheric ) spring over Antarctica and firs reported in 1985 (Farman et al.) are continuing. Since the 1990s, Antarctic total column ozone in September and October continued to be 40-50 percent lower than pre-ozone-hole values. A gradual trend toward "healing" was reported in 2016.

In the Arctic, the amount lost is more variable year-to-year than in the Antarctic. The greatest Arctic declines, up to 30 percent, are in the winter and spring, when the stratosphere is coldest.

Reactions that take place on polar stratospheric clouds (PSCs) play an important role in enhancing ozone depletion. PSCs form more readily in the extreme cold of the Arctic and Antarctic stratosphere. This is why ozone holes first formed, and are deeper, over Antarctica. Early models failed to take PSCs into account and predicted a gradual global depletion, which is why the sudden Antarctic ozone hole was such a surprise to may scientists.

In middle latitudes, it is more accurate to speak of ozone depletion rather than holes. Total column ozone declined to about six percent below pre-1980 values between 1980 and 1996 for the mid-latitudes of 35–60°N and 35–60°S. In the northern mid-latitudes, it then increased from the minimum value by about two percent from 1996 to 2009 as regulations took effect and the amount of chlorine in the stratosphere decreased. In the Southern Hemisphere's mid-latitudes, total ozone remained constant over that time period. In the tropics, there are no significant trends, largely because halogen-containing compounds have not yet had time to break down and release chlorine and bromine atoms at tropical latitudes.

Large volcanic eruptions have been sown to have substantial albeit uneven ozone-depleting effects, as observed (for example) with the 1991 eruption of Mt. Pinotubo in the Philippines 

Ozone depletion also explains much of the observed reduction in stratospheric and upper tropospheric temperatures. The source of the warmth of the stratosphere is the absorption of UV radiation by ozone, hence reduced ozone leads to cooling. Some stratospheric cooling is also predicted from increase in greenhouse gases such as CO2 and CFCs themselves; however he ozone-induced cooling appears to be dominant.

Predictions of ozone levels remain difficult, but the precision of models' predictions of observed values and the agreement among different modeling techniques have increased steadily. The World Meteorological Organization Global Ozone Research and Monitoring Project-Report No. 44 comes out strongly in favor of the Montreal Protocol, but notes that a UNEP 1994 Assessment overestimated ozone loss for hte 1994-1997 period. 

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