What is the weight of 1 litre of Helium at STP (or density)? And 1 litre of Hydrogen? And 1 litre of Air?

To being able to calculate how much weight a given balloon can lift  (the lifting power) based on the volume of the balloon,  it is first necessary to know what is the weight of the gas inside a balloon with a given volume. Or more technically speaking, what is the density of the gas. This post aims to answer to this last question, by considering the gas to be Helium, Hydrogen and Air, and by considering the balloon as being subject to Standard conditions for temperature and pressure (STP).

We shall now apply what we have seen in my previous post to calculate the weight of X litres of Gas.

1. Determine the volume (in litres) occupied by 1 mole of the Gas;
2. With the result in 1. determine to how much moles correspond X litres of Gas;
3. Using the molar mass of the Gas ,which is expressed in g/mol , and the result in 2, get the weight in grams.

Realizing what we have just said, lets determine the weight of 1 litre of Helium:

Step 1. Determine the volume (in litres) occupied by 1 mole of the Helium:

At STP we have:

T (Temperature)= 273.15 K
P (Absolute pressure)= 0.986 atm

Additionally we have the ideal gas constant R= 0.08205746(14) L atm K^-1 mol^-1 which we can approximate to R= 0.082 L atm K^-1 mol^-1

And n= 1 since we want to determine the volume of 1 mole of helium.

Now, by using the ideal gas law we just have to replace the variables by their correspondent values:

PV = nRT
V= (nRT) / P
V= (1 x 0.082 x 273.15) / 0.986
V= 22.398 / 0.986 = 22.7 litres

Notice that above in the ideal gas law we have user as units for n, T and P, the same units in which the R constant is expressed: mol, K, and atm respectively.

So we have just determined that 1 mole of Helium occupies 22.7 litres.

Step 2. With the result in 1. determine to how much moles correspond 1 litres of Helium:

Now we have:

1 mol of Helium ____(corresponds)_____ 22.7 litres
X mol of Helium____(corresponds)_____ 1 litres

And by applying the rule of three, which is a particular form of cross-multiplication:

X = (1 * 1) / 22.7 = 0.044 mol

So we have just determined that 1 litre of Helium contains 0.044 moles.

Step 3. Using the molar mass of Helium ,which is expressed in g/mol, and the result in 2, get the weight in grams:

And now we have reached to the last step. As seen in a previous post, the molar mass (molecular weight) of Helium is equal to its atomic weight (since Helium has only 1 atom). In this way, and knowing that the atomic weight of Helium A_r(Helium)= 4.003 g/mol, it is easy to calculate the molar mass of Helium, M(Helium):

M(Helium)= A_r(Helium)= 4.003 g/mol

In this way, we can now use once again the rule of three:

4.003 g         ____(corresponds)_____ 1 mol
X g of Helium____(corresponds)_____ 0.044 mol

X = (0.044 * 4.003) / 1 = 0.176g

Which means that the weight of 1 litre of Helium is 0.176g, which is equivalent to:
Helium density (ρ) = 0.176g/l = 0.176 Kg/m³

If we wanted to determine the weight of 1 litre of hydrogen instead of Helium, we would just have to replace the molar mass of Helium by the molar mass of Hydrogen - please be aware that hydrogen in diatomic (H2). See bellow the calculations:

Step 1:

It is the exact same calculations that were made above for Helium, meaning that:

1 mole of Hydrogen occupies 22.7 litres.

Step 2:

It is the exact same calculations that were made above for Helium, meaning that:

1 litre of Hydrogen contains 0.044 moles.

Step 3:

The molar mass (molecular weight) of Hydrogen is equal to twice its atomic weight (since  Hydrogen  has 2 atoms - diatomic). In this way, and knowing that the atomic weight of Helium A_r(Helium)= 1.008 g/mol, it is easy to calculate the molar mass of  Hydrogen , M( Hydrogen ):

M( Hydrogen )= A_r( Hydrogen ) * 2= 2.016 g/mol

In this way, we can now use once the rule of three:

2.016 g                ____(corresponds)_____ 1 mol
X g of  Hydrogen ____(corresponds)_____ 0.044 mol

X = (0.044 * 2.016) / 1 = 0.089 g

Which means that the weight of 1 litre of Hydrogen is 0.089g, which is almost half the weight of Helium, and which is equivalent to:
Hydrogen density (ρ) = 0.089g/l = 0.089 Kg/m³

If we wanted to determine the weight of 1 litre of Air, we would just have to use the molar mass of Air. See bellow the calculations:

Step 1:

It is the exact same calculations that were made before, meaning that:

1 mole of Air occupies 22.7 litres.

Step 2:

It is the exact same calculations that were made before, meaning that:

1 litre of Air contains 0.044 moles.

Step 3:

The determination of the molar mass (molecular weight) of Air is complicated  because the chemical composition of air varies. The most notable variation is related with the presence of water vapour (H₂O), which is the gas phase of water. Following what is stated in Wikipedia, the water vapour represents 0.40% of the atmosphere and at the Earth's surface may represent 1% to 4%. Here, you can read a very interesting article on the air compositions and properties, including information regarding the water vapour and air density. But since the water vapour in the Air varies, let's simplify things and do the calculation for dry Air.
And as stated in the IUPAC Gold Book, "the composition of the major components in dry air is relatively constant (percent by volume given): nitrogen, 78.084; oxygen, 20.946; argon, 0.934; carbon dioxide, 0.033; neon, 0.0018; helium, 0.000524; methane, 0.00016; krypton, 0.000114; hydrogen 0.00005; nitrous oxide, 0.00003; xenon, 0.0000087". See bellow the table that presents and overview of these values and after the calculations to determine the molar mass of dry air.

Gas	Chemical symbol	% by volume	Atomic Weight	Molecular Weight
Nitrogen	N2	78.084	N: 14.007	28.014
Oxygen	O2	20.946	O: 15.999	31.998
Argon	Ar	0.934	Ar: 39.95	39.95
Carbon dioxide	CO2	0.033	C: 12.011 O: 15.999	63.996
Neon	Ne	0.0018	Ne: 20.18	20.18
Helium	He	0.000524	He: 4.003	4.003
Methane	CH4	0.00016	C: 12.011 H: 1.008	16.043
Krypton	Kr	0.000114	Kr: 83.80	83.80
Hydrogen	H2	0.00005	H: 1.008	2.016
Nitrus Oxide	N2O	0.00003	N: 14.007 O: 15.999	44.013
Xenon	Xe	0.0000087	Xe: 131.3	131.3

The atomic weights of the table above were retrieved from table 4 of the IUPAC Recommended Atomic Weights of 2009, where the atomic weights are abridged to four significant digits, and which apply to elements of natural terrestrial origin (as the gases in the air do). The values that are provided in an interval, like hydrogen, were retrieved from table 6 instead.

So now to determine the molecular weight of Air it is just a question of performing a weighted multiplication as shown bellow:

M(Air)= 0.78084 * 28.014 + 0.20946 * 31.998 + 0.00934 * 39.95 + 0.00033 * 63.996 + 0.000018 * 20.18 + 0.00000524 * 4.003 + 0.0000016 * 16.043 + 0.00000114 * 83.80 + 0.0000005 * 2.016 + 0.0000003 * 44.013 + 0.000000087 * 131.3 = 28,9715355715 = 28.972 g/mol

In this way, we can now use once the rule of three:

28.972 g             ____(corresponds)_____ 1 mol
X g of  Air           ____(corresponds)_____ 0.044 mol

X = (0.044 * 28.9715) / 1 = 1.275 g

Which means that the weight of 1 litre of Air is 1.275g, which is equivalent to:
Air density (ρ) = 1.275g/l = 1.275 Kg/m³


Quicker way to reach a value almost as accurate as the above of 1.275g:


If for the determination of the molecular weight of Air we used only the three elements most existing in Air, Nitrogen, Oxygen, and Argon, the final result would be very similar: 

M(Air)= 0.78084 * 28.014 + 0.20946 * 31.998 + 0.00934 * 39.95 = 28.950 g/mol

In this way, we can now use once again the rule of three:

28.950 g              ____(corresponds)_____ 1 mol
X g of  Air           ____(corresponds)_____ 0.044 mol

X = (0.044 * 28.950) / 1 = 1.274 g

A difference of just 0.001g

Of course, and before concluding, there is an even easier way to know the densities of helium and hydrogen - by consulting the many existing tables and charts, like this one in Wikipedia. For air, less sources exist.

To provide with a quick overview, the table bellow shows some of the values we have just determined for Helium, Hydrogen and Air.

Gas / Mixture	Molecular Weight / Molar Mass (g/mol)	Weight of 1 litre (g) / Density(ρ)
Helium	4.003	0.176
Hydrogen	2.016	0,089
Air	28,972	1,275

Please notice that for all the above calculations we have the following approximation:

- We are considering standard conditions for temperature and pressure (STP).

Fundamental (thermodynamics) concepts of gas (helium) balloons

This post details some aspects of the physical, chemical and mathematical behavior around high-altitude ballooning which after understood will allow us to derive important characteristics of gas balloons important to correctly dimension the system. We will focus on helium balloons but whenever applicable we shall explain the differences to hydrogen balloons.

In this post we shall introduce some important concepts that we will use in later posts.

Amount of substance

The amount of substance, also known as chemical amount, is a quantity that measures the size of an ensemble of elementary entities, such as atoms, molecules, electrons, and other particles. The SI unit for amount of substance is the mole, and its symbol is: mol. 

Atomic weight

Let's first clarify that the weight of the elements has different names which are all synonymous: standard atomic weight (as it appears in the entry for each chemical element in Wikipedia, for example for helium), relative atomic mass and atomic weight (of the element) - read here, and here, the controversy around these names. Hereafter I will use mostly the name atomic weight because its is the name used by IUPAC, which is the institution responsible for publishing the atomic weights of the elements. The atomic weights are published by the IUPAC and revised each 2-years. Here we have their revision of 2009, and here their periodic table. Other source could be the Wikipedia periodic table with atomic weights, but from there it is not clear which revision of IUPAC they are using. Other source, from the Stanford university, presents not the atomic weights but the molecular weight (relative molar mass), using the deprecated IUPAC Recommended Atomic Weights of 1997. The molecular weight for elements like Helium is the same than the atomic weight because Helium has only 1 atom. But for elements like Atmospheric Nitrogen (N2) that is diatomic, the molecular weight is 2x the molecular weight (in this case = to atomic weight) of the monatomic N.

The atomic weight of element E is represented by the symbol: A_r(E).

The atomic weight is presented as a dimensionless physical quantity, and even in the existing tables no units are provided. See also the IUPAC article "ATOMIC WEIGHT -THE NAME, ITS HISTORY, DEFINITION, AND UNITS" where the dimensionless nature of th eatomic weight is referred. But then in every place where the atomic weight is used for some calculation, it is expressed as a quantity of grams per mole. For instance in this well explained course of Carnegie Mellon University. So for all the calculation hereafter, and until we get some better explanation of the above, we shall consider the atomic weight has being a quantity of grams per mole.

To give an an example, by consulting table 4 of the IUPAC Recommended Atomic Weights of 2009, where the atomic weights are abridged to four significant digits, and which apply to elements of natural terrestrial origin (as our balloon helium does), we can see that the Helium has an atomic weight of approximately 4.003, which as explained above, we will interpret as Helium weighting approximately 4.003 grams per mole.

Standard conditions for temperature and pressure

The Standard conditions for temperature and pressure, better known as STP, are "standard sets of conditions for experimental measurements established to allow comparisons to be made between different sets of data". Different institutions may follow different values for the STP, but we shall follow the values recommend by the IUPAC for chemistry:

• Temperature of 273.15 K (0 °C, 32 °F) and
• an absolute pressure of 100 kPa (14.504 psi, 0.986 atm, 1 bar)

Latter on well shall use the STP for making some calculations that depend on the temperature and pressure: as the temperature and pressure depend on the altitude, and as a balloon may be in different altitudes, sometimes is good to make a simplification and just consider the STP.

Gas constant

The gas constant (also known as ideal, universal or molar gas constant) is a "physical constant which is featured in many fundamental equations in the physical sciences, like bellow in the ideal gas law. It is denoted by the symbol R. Physically, the gas constant is the constant of proportionality that happens to relate the energy scale to the temperature scale, when a mole of particles at the stated temperature is being considered. The value for this constant is recommended by the Committee on Data for Science and Technology (CODATA) for international use. The value we shall use comes from the "CODATA Recommended Values of the Fundamental Physical Constants: 2010" which are in-line with the values presented in the gas constant entry in Wikipedia:

R= 8.3144621(75) J mol^-1 K^-1

The gas constant can be presented in a multitude of different units. In the gas constant entry in Wikipedia we can count the constant R as being available in 23 different units. We should choose the unit of the R constant in accordance to the unit(s) that we then want then the result of our calculations. In our case, and for the calculations that shall be presented in future posts, we shall use the R constant as:

R= 0.08205746(14) L atm K^-1 mol^-1

Ideal gas law

The ideal gas law "is the equation of state of a hypothetical ideal gas" - it approximates the behaviour of many gases under many circumstances. But be aware that is just an approximation - the reality shows that different gases behave differently. This equation relates pressure, volume and temperature to determine the state of a gas. We shall use this equation in its common form:

PV = nRT

where:

P = pressure of the gas;

V = volume of the gas;

n = amount of substance of gas (see above);

R = gas constant (see above);

T = temperature

Intuition behind the reason why gas balloons rise

In this post I will try to explain the intuition that allows us to understand why gas balloons rise into the atmosphere. This post will prepare the readers to better understand the physics and mathematics of gas balloons that I will be presenting in my next post.

Water tank analogy
Imagine that you have a tank full of water. Imagine also that you have a one litter plastic bottle filled-in with air and with a string attached to the neck of the bottle. Now imagine that there is a magical hand (occupying no volume) that picks up the wire and dives the bottle in the water tank. The bottle will only stay suspended under the water if the magical hand keeps holding the string. If the hand let the string go, the plastic bottle will emerge. This is common knowledge, but lets try to gain an intuition on what is happening. When the one litter bottle is submerged in the tank, it has to displace one litter of water so that there is room for the bottle to occupy its space in the tank. This means also that the water level in the tank will increase in one litter (if the magical hand had volume we would have to consider it also to know the increase of the water level in the tank). Now lets think the following. The air is lighter than water, this means that the one litter air bottle is lighter than one litter of water. This means that, when the magical hand releases the bottle string, the one litter of water that was displaced to make room for the bottle, as it is heavier than the bottle, will want to claim its place back, and when it does, the bottle will have to occupy the space where that water was, and this process will be repeated as long has the bottle is surrounded by water. When the bottle emerges it will have water bellow and air above. As the bottle is lighter than the water it will not submerge again. And as it is heavier than the air (because of the weight of the bottle itself) it will not raise up into the atmosphere. This is the law of buoyancy. 

Fig. firstly the water tank is empty; then a magical hand dives an air plastic bottle; afterwards the magical hand releases the air bottle string and the water surrounding the bottle, which is heavier, claims it space where the bottle is located; finally the bottle is pushed up into the surface because whenever the bottle is surrounded by the heavier water it will claim the space it occupies and as the contents of the tank can only expand to above, it is to the above that the bottle moves until is not surrounded any more by water but also by the lighter air (@Paulo Carmo)

So, to summarise:

- Lets have two things. Thing 'A' and thing 'B'; A and B can be objects, gas, etc.;
- 'A' is lighter than 'B';
- If 'A' is surrounded by 'B', the space occupied by A will claimed by B;
-  Now we have 'A' occupying the space of 'B' and vice versa;
- If 'B' continues being surrounded by heavier things, then the space it occupies shall be claimed again;

- 'B' moves in the direction of where it can claim space

With the water tank analogy clear on our mind let's now mode deep into the atmosphere.

Gas balloons in the atmosphere

While the behaviour of objects in the water is very well know since we practice with it on a daily basis, the interaction of objects with the atmosphere is a bit more esoteric to most people. So, equipped with the previous water tank analogy, lets now imagine that all this air above our heads, the atmosphere, is in contained inside a air tank (only open on the top at the end of the atmosphere, 10000Km above the earth's sea level). Now imagine that we fill in a balloon with an element lighter than air like hydrogen (the lightest existing element) or with helium (the second lightest existing element) and string it to an heavy rock. If we do that we get an object which is lighter than the air that surrounds it (when released from the rock), and if we make it big enough we can even attach objects to the balloon that its total weight is still less than the air surrounding it. Now the mechanics of the raise of the balloon is exactly the same of when we had a water tank. When the gas balloon string is released, the air that was displaced to make room for the gas balloon, as it is heavier than the balloon, will want to claim its place back, and when it does, the balloon will have to occupy the space where that air was, and this process will be repeated as long has the balloon is surrounded by air and no other event (explained bellow) occur, which will make the balloon continuously rise deep into the atmosphere. But now the question is, and does it stop rising? Well, theoretically, as long as the gas balloon is surrounded by elements lighter that the balloon itself, it could keep rising until the "end of days", but here a different phenomena occurs - the atmospheric pressure - that does not allow the balloon to rise forever. So in the atmosphere the same law of buoyancy applies.

Atmospheric pressure and its impact in the gas balloons

The atmospheric pressure, or air pressure, is the force applied into a surface by the weight of air existing in the atmosphere above that surface. This is applicable to the Planet Earth or any other planet with an atmosphere. The referred force is applied to any object in the planet including yourself, or in what concerns this post, to a balloon.
The air in the atmosphere is constituted by small particles called air molecules, and depending on the altitude, the concentration of air molecules in the atmosphere is bigger (at sea level) or smaller (in the higher altitudes of the atmosphere). The figure bellow, which was built on data taken from the following site, shows the force which is applied by the atmospheric pressure to a man at different altitudes.  

Fig. Atmospheric pressure at different altitudes - not in scale (@Paulo Carmo)

For instance, at sea level, 0 meters of altitude, the atmospheric pressure applies a force of 1.03 Kg/cm2 to a man. This is due to the weight of all air existing in the atmosphere above the man. If the man were at an altitude of 12192 meters, the atmospheric pressure would be almost 9 times less, particularly of 0.191 Kg/cm2. This is due to the fact that the man at 12192 meters has much less air (molecules) above is head. Other related phenomena is that the air density (in the right side of the figure) decreases with the altitude (as does the air pressure). The air density is the mass per unit volume of Earth's atmosphere, and it changes also with changes in temperature or humidity. The air density is related with the fact that the air can be compressed to fit in a smaller volume (volume is the space that contains an object. In the case of a gas, or air, the volume of the container will tell you the volume of the gas. Volume is typically measured in litres or milliliters. For example, an open 1-litre bottle at sea-level contains 1-liter of air).When the air is compressed it is said to be under high pressure, which is what happens, for instance, at sea level.
A question that can be formulated is the following: why don't we fell, at sea level, the force of 1.03 Kg/cm2, that is applied to us by the air existing in the atmosphere above us? Well the reason is that we have air inside our body too, and that air balances out the pressure outside so that we do not feel the air pressure.

But all this information is to explain what is the impact of the air pressure in a gas balloon as it rises into the atmosphere. Well, as the balloon rises, and as we have seen, the atmospheric pressure drops, and so the force which is applied to the exterior of the balloon decreases. Since the gas inside the balloon stays the same (the balloon is sealed), also the force that it exerts outward stays the same while the balloon rises.
So, when you are at the Earth surface and fill-in the balloon with gas, the equilibrium of internal and external forces tin the balloon will result in a given size of the balloon. As the balloon rises, the air pressure external to the balloon will drop and since the internal gas pressure is maintained, what happens is that the balloon will increase in size - because the pressure that the gas inside the balloon applies encounter less resistance from the external air pressure. When the internal gas pressure is bigger than the sum of the external air pressure and the pressure (resistance) of the balloon fabric, the balloon bursts. And this is the event that prevents the balloon to rise forever - at a given altitude the balloon bursts.

High-altitude balloon stack

In this post it will be described what is the typical high-altitude balloon stack. In this and in the next posts the terms high-altitude balloon and near space photography will be used interchangeably.

Fig. High-altitude balloon stack (@Paulo Carmo)

The figure on the left shows a typical stack for high-ballooning. With stack we mean a pile or heap of sub-systems that together compose the high-balloon system:

Balloon (sometimes called envelope): the balloon is the device that provides the lifting force that allows the whole system to reach an high-altitude in the Earth's atmosphere. As the system rises the balloon expands until eventually it bursts and the whole system starts falling back into Earth's surface. It is filled-in with a gas lighter than the air, usually helium or hydrogen. Today helium is more used because it is less explosive than the hydrogen even though is more expensive.

Parachute: the parachute is activated after the balloon bursts and the whole system starts falling back into Earth's surface. Before this, the parachute is in tension between the balloon above and the radar reflector and payload bellow. When the balloon bursts, it stops providing the tension force for the parachute bellow and so the parachute naturally opens providing a drag force that decreases the falling velocity of the whole system.

Radar reflector: the radar reflector is a safety apparatus that is placed between the parachute and the payload and it serves the purpose of presenting an high radar reflectivity and in this way increasing the chances of the whole system being detected by a flying asset (air plane, ultra-light, etc) and consequently reducing the probability of an air collision.

Payload: the payload contains the whole purpose of the system, which is the set of equipment that will accomplish the mission. An example payload could be constituted by the following modules:

• Communications: this module is responsible for the communication with ground. Its major task is to communicate back the position of the system so that it can be tracked and found after landing; usually radio-frequency communication is used, but GSM communications is also employed;
• Sensors: this module contains any device that detects a physical condition in the world. It contains several sensors depending on the mission. Its primary sensor is the GPS receiver, but other sensors can be aboard like camera, temperature, pressure and others;
• Actuators: this module contains any device such as switches, that perform actions such as turning things on or off or making adjustments in the system; one practical example is an actuator that is able to move a camera to control the angle in which a photograph is taken;
• Computing: computing power is necessary to all sorts of things like:
• acquiring the sensor readings and storing and/or communicating them (through the communications module);
• controlling the electronics and other equipment;
• other.
• Power: this module will be responsible for the power to feed the equipments in the payload; it is wise to have separate and independent power lines in order that problems in one line does not affect other line.

It must be noticed that all components of the high-altitude stack must be tied up very securely. Depending on the components your are tying, different types of strings are used, like nylon string and others.

The balloon, the parachute and the radar reflector are sometimes named flight system. This means that the high-altitude balloon stack is in fact constituted by two major subsystems, the flight system (with the balloon, the parachute and the radar reflector) and the payload (with all its subsystems).

Near space photography, high-altitude balloons and the Earth's atmosphere

I have recently decided to get involved in a hobby called Near Space Photography, which is very closely related to High-altitude ballooning. This hobby consists in being able to take photographs of the Earth as seen from the Near Space, usually (if not always) using an high-altitude balloon. When the balloon reaches its maximum altitude, it is possible to take pictures of the black canopy of space, and viewing clearly a big swath of the earth with a curved horizon out to several hundred Km's. Before explaining where it is located the Near Space, let me briefly present the Earth's Atmosphere which encompasses a set of main layers as follows (the description bellow regarding the atmosphere and its layers was adapted from the 'Atmosphere of Earth' entry in Wikipedia, together with other sources):

Atmosphere layers
(@Public Domain,
not to scale)

Atmosphere

The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. An altitude of 120 km is where atmospheric effects become noticeable during atmospheric re-entry of spacecrafts. The Kármán line, at 100 km, also is often regarded as the boundary between atmosphere and outer space. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect), and reducing temperature extremes between day and night (the diurnal temperature variation).

Atmospheric stratification describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. 

Air is the name given to atmosphere used in breathing and photosynthesis.

Exosphere

The exosphere is named from the ancient Greek "ἔξω éxō" meaning outside, external, beyond. The outermost layer of Earth's atmosphere extends from the exobase upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the magnetosphere or the solar wind.

Thermosphere

Temperature increases with height in the thermosphere from the mesopause up to the thermopause, then is constant with height. This is why this layer  is named from the Greek "θερμός" (thermos) meaning heat. Unlike in the stratosphere, where the temperature rise is caused by absorption of radiation by ozone, in the thermosphere the temperature rise is a result of the extremely low density of molecules. The temperature of this layer can rise to 1,500 °C (2,700 °F), though the gas molecules are so far apart that temperature in the usual sense is not well defined. The International Space Station has a stable orbit within the middle of the thermosphere, between 320 and 380 kilometres. Auroras also occur in the  thermosphere . The top of the thermosphere, called the exobase, varies in height with solar activity and ranges from about 350–800 km.

Mesosphere

The mesosphere is named from the Greek word mesos (middle). This is related with the fact that together the stratosphere, mesosphere and lowest part of the thermosphere are collectively referred to as the "middle atmosphere", which spans heights from approximately 10 to 100 km. The mesosphere extends from the stratopause to 80–85 km. It is the layer where most meteors burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The upper boundary of the mesosphere is the mesopause, which can be the coldest naturally occurring place on Earth with temperatures of 190 K (−83 °C).

Stratosphere

The stratosphere extends from the tropopause to about 51 km. Temperature increases with height due to increased absorption of ultraviolet radiation by the ozone layer, which restricts turbulence and mixing. The ozone layer is mainly located in the lower portion of the stratosphere from approximately 20 to 30 kilometres (12 to 19 mi) above Earth, though the thickness varies seasonally and geographically. The ozone layer absorbs 97–99% of the Sun's medium-frequency ultraviolet light (from about 200 nm to 315 nm wavelength), which potentially damages exposed life forms on Earth. While the temperature may be −60 °C at the tropopause, the top of the stratosphere is much warmer, and may be near freezing, in fact, the top of the stratosphere has a temperature of about −3°C (270 K), just slightly below the freezing point of water. The stratopause, which is the boundary between the stratosphere and mesosphere, typically is at 50 to 55 km. The pressure here is 1/1000 of the pressure at sea level.

Troposphere

The troposphere begins at the surface and extends to between 7 km at the poles and 20 km at the tropics. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols. The troposphere is mostly heated by transfer of energy from the surface, so on average the lowest part of the troposphere is warmest and temperature decreases with altitude. This promotes vertical mixing (hence the origin of its name in the Greek word "τροπή", trope, meaning turn or overturn. 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 deep depending on the landform and time of day. The tropopause is the boundary between the troposphere and stratosphere and is a temperature inversion. In the tropopause it can be found the main jet streams which are fast flowing, narrow air currents with wind velocities that can go from 92 km/h to 398 km/h, typically from West to the East. The northern hemisphere polar jet flows over the middle to northern latitudes of North America, Europe, and Asia and their intervening oceans, while the southern hemisphere polar jet mostly circles Antarctica all year round. This phenomena is not so typical over the Portugal latitude, so it should not affect a balloon launched from there. While air content and atmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants and terrestrial animals is currently only known to be found in Earth's troposphere and artificial atmospheres. Most of the phenomena we associate with day-to-day weather occur in the troposphere.

In order to better illustrate the differences between the atmosphere layers and also because this will be useful for a next Post where I will present the physics and maths applied to a weather balloon, the figure that follows show how the air density, pressure and temperature change as the altitude increases and the several atmosphere layers are traversed.

Comparison of the 1962 US Standard Atmosphere graph of
geometric altitude against air density, pressure, the speed of
sound and temperature with approximate altitudes of
various objects (@GNU Free Documentation License)

Regarding this figure I would like to highlight the following. From the Earth surface, the air density and pressure drops consistently up-to between 30 to 40km of altitude when both reach 0. Regarding the temperature things are more complex. The temperature in the Troposphere drops a lot from 290 kelvin to a bit less than 220 kelvin. Then in the stratosphere the temperature initially, and during almost 10 km, maintains constant, but then it raises from 220 kelvin to 270 kelvin. Then in the Mesosphere it drops again. An analysis of the temperature profile in the Atmosphere reveal that the temperature is in fact a good metric to distinguish among atmospheric layers, because:

• We know that the general pattern of the temperature profile with altitude is constant;
• The temperature profile pattern changes coincide with changes in the atmospheric layer- the temperature stops dropping when we reach the end of the Troposphere (called the Tropopause) and starts increasing in the Stratosphere up-to its limit (the Statopause) when it is maintained constant for a while until we enter in the Mesosphere where a drop of the temperature occurs.

In any case the discussion regarding the air density, pressure and temperature patterns with altitude is only included in this Post because this characterisation is relevant for the next post where I explain intuitively how a weather (gas) balloon works and is able to go up into the atmosphere.

Getting back to the Near Space, different (not many) definitions exist about it, but to be honest I do not know to which point the Near Space is a formal, standard concept, or if it is just a more informal concept created to designate a given region in the atmosphere. The typical references state that the Near Space refer to an area of the Earth's atmosphere between 20 to 100 km above the sea level, encompassing the stratosphere, mesosphere, and thermosphere. The Near Space is an area of Earth's atmosphere where there is very little air, but where the remaining amount generates far too much drag for satellites to remain in orbit. A vehicle designed to operate in the Near Space is sometimes called nearcraft, and there are two types of vehicles that usually operate in the Near Space, these include sub-orbital rockets, which make quick jumps into and out of near space, and high-altitude balloons. Regarding the high-altitude balloons, which are those of interest to us in what respects Near Space photography, the most common type is the weather balloon (or sounding balloon) which may reach altitudes of 40 km (25 miles) or more, limited by diminishing pressures causing the balloon to expand to such a degree that it disintegrates. So  these balloons are released into de Near Space and in particular into the Stratosphere.
There are references to other types of high-altitude balloons, Scientific balloons, that can remain at high altitudes for several days, but usually an high-altitude balloon is released form Earth's surface and then rises up to an altitude where it bursts and falls back into the surface, all this in some hours time. But in 2012 I think it is already possible to speak about a third type of high-altitude balloon, because in the last years there have been some many launches of this type in different parts of the world, which could be referred by hobby balloon, Near Space photography balloon or amateur balloon. These category of high-altitude balloons differ from the weather or scientific balloons in its mission which in this case is hobby-related, taking pictures, personal technological achievement, or just for fun, but in fact all  have in common (weather and scientific balloon included) that the balloon payload is carried in both cases by a weather balloon.

Now to recap, the Near Space photography hobby aims to send to the Near Space, in particular, to the Stratosphere,  a payload carried by a weather (gas) balloon which ultimately aims to take photographs of the Earth at different altitudes. Of course taking pictures is many times just one of the features of the payload, because many other things are, and can be done, in addition to taking photos, namely measuring atmospheric parameters like temperature, pressure and wind, and many other possible experiments and measures (ozone, CO2, other).

Before finalising this post it is important to refer that high-altitude balloons are usually filled either with helium or hydrogen. The helium is more expensive than hydrogen, but is also less explosive, and so, due to the latter, today the majority of balloons, especially the hobby-related amateur ones are filled in with helium. This will certainly be my choice, but caution must be taken because also with helium things can go wrong.