Atmosphere.. layer of the atmosphere

The atmosphere of the Earth may be divided into several distinct layers, as the following figure indicates.

Layers of the Earth's atmosphere

The Troposphere

The troposphere is where all weather takes place; it is the region of rising and falling packets of air. The air pressure at the top of the troposphere is only 10% of that at sea level (0.1 atmospheres). There is a thin buffer zone between the troposphere and the next layer called the tropopause.

The Stratosphere and Ozone Layer

Above the troposphere is the stratosphere, where air flow is mostly horizontal. The thin ozone layer in the upper stratosphere has a high concentration of ozone, a particularly reactive form of oxygen. This layer is primarily responsible for absorbing the ultraviolet radiation from the Sun. The formation of this layer is a delicate matter, since only when oxygen is produced in the atmosphere can an ozone layer form and prevent an intense flux of ultraviolet radiation from reaching the surface, where it is quite hazardous to the evolution of life. There is considerable recent concern that manmade flourocarbon compounds may be depleting the ozone layer, with dire future consequences for life on the Earth.

The Mesosphere and Ionosphere

Above the stratosphere is the mesosphere and above that is the ionosphere (or thermosphere), where many atoms are ionized (have gained or lost electrons so they have a net electrical charge). The ionosphere is very thin, but it is where aurora take place, and is also responsible for absorbing the most energetic photons from the Sun, and for reflecting radio waves, thereby making long-distance radio communication possible.The structure of the ionosphere is strongly influenced by the charged particle wind from the Sun (solar wind), which is in turn governed by the level of Solar activity. One measure of the structure of the ionosphere is the free electron density, which is an indicator of the degree of ionization. Here are electron density contour maps of the ionosphere for months in 1957 to the present. Compare these simulations of the variation by month of the ionosphere for the year 1990 (a period of high solar activity with many sunspots) and 1996 (a period of low solar activity with few sunspots):

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atmospheric pressure

Atmospheric Pressure force exerted by the weight of the air Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface. In the diagram below, the pressure at point "X" increases as the weight of the air above it increases. The same can be said about decreasing pressure, where the pressure at point "X" decreases if the weight of the air above it also decreases.
Thinking in terms of air molecules, if the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases. The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure. Atmospheric pressure is measured with an instrument called a "barometer", which is why atmospheric pressure is also referred to as barometric pressure.

In aviation and television weather reports, pressure is given in inches of mercury ("Hg), while meteorologists use millibars (mb), the unit of pressure found on weather maps.

As an example, consider a "unit area" of 1 square inch. At sea level, the weight of the air above this unit area would (on average) weigh 14.7 pounds! That means pressure applied by this air on the unit area would be 14.7 pounds per square inch. Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars.

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The atmosphere

The atmosphere of Earth is a layer of gasessurrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperatureextremes between day and night (the diurnal temperature variation).

The common name given to the atmospheric gases used in breathing and photosynthesis isair. By volume, dry air contains 78.09% nitrogen, 20.95% oxygen,^[1] 0.93% argon, 0.039% carbon dioxide, and small amounts of other gases. Air also contains a variable amount of water vapor, on average around 1%. Although air content andatmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants andterrestrial animals currently is only known to be found in Earth's troposphere and artificial atmospheres.

The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. The Kármán line, at 100 km (62 mi), or 1.57% of the Earth's radius, is often used as the border between the atmosphere and outer space. Atmospheric effects become noticeable during atmospheric reenty of spacecraft at an altitude of around 120 km (75 mi). Several  layerscan be distinguished in the atmosphere, based on characteristics such as temperature and composition 

 Composition

Air is mainly composed of nitrogen, oxygen, andargon, which together constitute the major gases of the atmosphere. Water vapor accounts for roughly 0.25% of the atmosphere by mass. The concentration of water vapor (a greenhouse gas) varies significantly from around 10 ppmv in the coldest portions of the atmosphere to as much as 5% by volume in hot, humid air masses, and concentrations of other atmospheric gases are typically provided for dry air without any water vapor.^[3] The remaining gases are often referred to as trace gases,^[4] among which are thegreenhouse gases such as carbon dioxide, methane, nitrous oxide, and ozone. Filtered air includes trace amounts of many other chemical compounds. Many substances of natural origin may be present in locally and seasonally variable small amounts as aerosols in an unfiltered air sample, including dust of mineral and organic composition, pollen and spores, sea spray, andvolcanic ash. Various industrial pollutants also may be present as gases or aerosols, such aschlorine (elemental or in compounds), fluorinecompounds and elemental mercury vapor. Sulfur compounds such as hydrogen sulfide and sulfur dioxide (SO₂) may be derived from natural sources or from industrial air pollution.

Composition of dry atmosphere, by volume^[5]

ppmv: parts per million by volume (note: volume fractionis equal to mole fraction for ideal gas only, see volume (thermodynamics))
Gas	Volume
Nitrogen(N2)	780,840 ppmv (78.084%)
Oxygen (O2)	209,460 ppmv (20.946%)
Argon (Ar)	9,340 ppmv (0.9340%)
Carbon dioxide(CO2)	397 ppmv (0.0397%)
Neon (Ne)	18.18 ppmv (0.001818%)
Helium (He)	5.24 ppmv (0.000524%)
Methane(CH4)	1.79 ppmv (0.000179%)
Krypton (Kr)	1.14 ppmv (0.000114%)
Hydrogen(H2)	0.55 ppmv (0.000055%)
Nitrous oxide (N2O)	0.325 ppmv (0.0000325%)
Carbon monoxide(CO)	0.1 ppmv (0.00001%)
Xenon (Xe)	0.09 ppmv (9×10−6%) (0.000009%)
Ozone (O3)	0.0 to 0.07 ppmv (0 to 7×10−6%)
Nitrogen dioxide(NO2)	0.02 ppmv (2×10−6%) (0.000002%)
Iodine (I2)	0.01 ppmv (1×10−6%) (0.000001%)
Ammonia(NH3)	trace
Not included in above dry atmosphere:
Water vapor(H2O)	~0.25% by mass over full atmosphere, locally 0.001%–5%[3]

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MORSE CODES

Morse Code

.-	A	Alpha
-...	B	Bravo
-.-.	C	Charlie
-..	D	Delta
.	E	Echo
..-.	F	Foxtrot
--.	G	Golf
....	H	Hotel
..	I	India
.---	J	Juliet
-.-	K	Kilo
.-..	L	Lima
--	M	Mike
-.	N	November
---	O	Oscar
.--.	P	Papa
--.-	Q	Quebec
.-.	R	Romeo
...	S	Sierra
-	T	Tango
..-	U	Uniform
...-	V	Victor
.--	W	Whiskey
-..-	X	X-Ray
-.--	Y	Yankee
--..	Z	Zulu
.----	1	Wun
..---	2	Too
...--	3	Tree
....-	4	Fow-er
.....	5	Fife
-....	6	Six
--...	7	Seven
---..	8	Ait
----.	9	Nin-er
-----	0	Ze-ro

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