What Is a Low Air Pressure Reading
Air Density Calculator
Created by Dominik Czernia , PhD candidate and Bogna Szyk
Reviewed by
Małgorzata Koperska , Medico and Jack Bowater
Final updated:
Feb 15, 2022
- What is the density of air? - density of air at sea level
- How to calculate the air density?
- Air density definition - what is the density of air formula?
- Air density tabular array - the density of dry air
- Density of air in kg / 1000³ and density of air in English units
- Standard air density
- What is air pressure?
- What is relative humidity?
- What is the dew point?
Use this air density computer to instantly find how tightly packed an object's molecules are, allowing you to estimate the ρ (air density) parameter basing on the local temperature and pressure conditions. This value is vital for many farther calculations, such as determining the aerodynamic elevate forces or the performance of wind turbines. Continue reading to go a amend understanding of the human relationship between the local weather and ρ, and learn what air density levels you can expect in diverse regions.
The density of air depends on many factors and can vary in different places. It mainly changes with temperature, relative humidity, pressure and hence with altitude (have a wait on the air density tabular array beneath). The air pressure tin can be related to the weight of the air over a given location. It is piece of cake to imagine that the higher yous stand up, the less air is above you and the pressure is lower (check out our definition of pressure!). Therefore, air pressure decreases with increasing altitude. In the following text, you lot will find out what is the air density at sea level and the standard air density.
You might also want to check out our speed of sound calculator which is a tool that helps y'all calculate the speed of audio in dry air and water at any temperature.
What is the density of air? - density of air at sea level
The density of air is usually denoted past the Greek alphabetic character ρ, and information technology measures the mass of air per unit volume (due east.chiliad. thou / 10003). Dry out air mostly consists of nitrogen (~78 %) and oxygen (~21 %). The remaining i % contains many different gases, among others, argon, carbon dioxide, neon or helium. Notwithstanding, the air volition terminate to be dry out air when water vapor appears.
As a mixture of gases, air doesn't have a constant density; this value depends largely on air composition. Most components have similar densities and don't influence the overall density in a substantial fashion. One exceptions is water vapor; the more h2o vapor in the air, the lower its density.
For dry air, its density at sea level at 59 °F (15 °C) and 14.7 psi (1013.25 hPa) (mean sea-level pressure), is approximately 0.0765 lb/cu ft (i.225 kg/m³). If you alter the air temperature, humidity, or the altitude (and hence the pressure level), the air density volition change, too. As a rule of thumb, you can await a drib of 0.0022-0.0023 lb/cu ft (0.035-0.036 kg/m³) per thousand ft of altitude modify.
How to calculate the air density?
To discover the air density at whatsoever given location, you will need some bones weather parameters. You can usually find them on your local atmospheric condition stations website.
- Air pressure: the barometric pressure expressed in hPa. If the analyzed location is at a high distance, you tin can apply our air pressure at altitude calculator to institute a more accurate value for this parameter.
- Air temperature: only, the exterior temperature in °C.
- Relative humidity or dew signal: our air density estimator is able to use ane of these values to compute the other or you can also use our dew point calculator. Dew point is the temperature below which the h2o vapor starts to condensate, in °C.
The method of finding the air density is quite simple. You lot have to separate the pressure level exerted by the air into two fractional pressures: of the dry air and of the water vapor. Combining these two values gives you lot the desired parameter.
- Calculate the saturation vapor pressure level at given temperature
Tusing the formulap₁ = six.1078 * ten^[7.five*T /(T + 237.3)], whereTis measured in degrees Celsius. Saturation vapor pressure is the vapor pressure at 100% relative humidity (our estimator uses a more than authentic, but besides fashion more complicated that we didn't want to show here. If you are curious, check "Function ESW(T)" in the provided link). - Find the bodily vapor pressure, multiplying the saturation vapor pressure by the relative humidity:
pv = p₁ * RH. - Subtract the vapor pressure from the full air pressure to observe the pressure of dry out air:
pd = p - pv. - Input the calculated values into the post-obit formula:
ρ = (pd / (Rd * T)) + (pv / (Rv * T))
where:
-
pdis the force per unit area of dry air in Pa, -
pvis the h2o vapor pressure in Pa, -
Tis the air temperature in Kelvins, -
Rdis the specific gas abiding for dry air equal to 287.058 J/(kg·M), and -
Rvis the specific gas abiding for water vapor equal to 461.495 J/(kg·K).
Air density definition - what is the density of air formula?
The bones definition of air density is very like to the general definition of density. It tells us how much does a certain volume of air weigh. We can express it with the following density of air formula:
ρ = mass of air / volume
From the in a higher place equation, y'all may suspect that the density of air is a constant value that describes a certain gas holding. Yet, the density of every matter (solids, liquids, gases) depends, stronger or weaker, not only on the chemical composition of the substance only also on the external conditions like pressure and temperature.
Because of these dependencies and the fact that the Earth'due south temper contains various gases (mostly nitrogen, oxygen, argon and water vapor) the air density definition needs to exist further expanded. A proper modification has been made in our air density computer with the density of air formula shown in the department called "How to calculate the air density?".
By the fashion, nosotros would similar to bring up an interesting betoken. What do yous think? Is moist air heavier or lighter than dry air? The right respond may not be as intuitive as you lot can think at starting time. In fact, the more water vapor we add to the air, the less dense it becomes! You may discover this hard to believe, but we volition try to convince with some few logical argumentations.
Showtime of all, we need to refer to the Avogadro'due south law which states that
equal volumes of all gases, at the aforementioned temperature and pressure, have the same number of molecules.
Imagine that you put dry air into a container of stock-still volume, temperature and force per unit area. The perfectly dry out air is equanimous of:
- 78% molecules of nitrogen Due north₂ that has two N atoms with atomic weight 14 u (total weight is 28 u),
- 21% molecules of oxygen O₂ that has two O atoms with diminutive weight xvi u (total weight is 32 u), and
- ane% molecules of argon Ar (Ar has one atom with atomic weight 39.viii u).
Note that every molecule listed is heavier that or equal to xviii u. Now, let's add some water vapor molecules to the gas with the total atomic weight of 18 u (H₂O - two atoms of hydrogen 1 u and one oxygen 16 u). Co-ordinate to Avogadro'south law, the total number of molecules remains the same in the container under the aforementioned conditions (volume, pressure, temperature). It means that water vapor molecules have to supervene upon nitrogen, oxygen or argon. Because molecules of H₂O are lighter than the other gases, the total mass of the gas decreases, decreasing the density of the air besides.
Air density table - the density of dry out air
In the previous sections, we used the term dry air a couple of times. Withal, what does it actually mean? There are ii definitions:
- The about common definition is air without any h2o vapor inside it. The air in the atmosphere is never perfectly dry since information technology always contains some h2o, though.
- Another, more than realistic definition says that dry out air is air with depression relative humidity and thus with a depression dew point.
A well-known approximation of dew point is a logarithmic role of relative humidity. As yous may know, when the function of a logarithm approaches zero, its value goes to minus infinity. Therefore, a dew point doesn't exist for the zero relative humidity. Withal, yous tin can still calculate what is the density of dry out air with our air density calculator! Just select dry out air in the "air type" field, where nosotros take ignored dew point/relative humidity in the computations.
For a better understanding of how temperature and pressure influence air density, let's focus on a example of dry air. It contains mostly molecules of nitrogen and oxygen that are moving around at incredible speeds. Apply our particles velocity calculator to encounter how fast they can motion! For example, the boilerplate speed of a nitrogen molecule with a mass of 14 u (u - unified atomic mass unit) at room temperature is most 670 one thousand/s - two times faster than the speed of sound! Moreover, at higher temperatures, gas molecules farther advance. Equally a consequence, they push button harder confronting their surroundings, expanding the volume of the gas (it is described by the ideal gas law formula). And the higher the volume with the same corporeality of particles, the lower the density. Therefore, air's density decreases equally the air is heated.
The opposite effect is achieved with pressure. Imagine that you have a gas cylinder with a abiding volume. The increased pressure of the cylinder translates into the increased number of molecules inside - the density of air becomes higher.
Altitude has a significant influence on air density because every bit you get college, the greater the pressure level and temperature drops. At high altitudes, the amount of oxygen in the air per unit volume is lower, because at that place is less air in total. Therefore, if climbers decide to achieve the tops of the highest mountains, they usually need an oxygen cylinder with a mask to be able to exhale. This trouble doesn't announced in airplanes since cabins are pressurized to continue the air density inside similar to footing level. To become a feeling of how air properties changes with the altitude, have a look at the following air density table for dry out air (information from the NASA Technical Study Server, ). Information technology follows that the density of dry air at 16 000 ft (~ 5 km) is about two times lower than the density at the ocean level.
| Distance [ft (grand)] | Temperature [°F (°C)] | Pressure [psi (hPa)] | Air density [lb / cu ft (kg / m³)] |
|---|---|---|---|
| sea level | 59 (fifteen) | xiv.7 (1013.25) | 0.077 (1.23) |
| 2 000 (610) | 51.9 (11.1) | thirteen.vii (941.seven) | 0.072 (1.xvi) |
| 4 000 (1219) | 44.7 (7.1) | 12.7 (873.3) | 0.068 (1.09) |
| half dozen 000 (1829) | 37.6 (3.one) | xi.7 (808.2) | 0.064 (1.02) |
| 8 000 (2438) | 30.5 (-0.eight) | ten.viii (746.2) | 0.06 (0.95) |
| 10 000 (3048) | 23.3 (-iv.eight) | 10 (687.3) | 0.056 (0.9) |
| 12 000 (3658) | 16.2 (-viii.8) | nine.two (631.6) | 0.052 (0.84) |
| 14 000 (4267) | ix.1 (-12.viii) | 8.4 (579) | 0.048 (0.77) |
| 16 000 (4877) | 1.nine (-16.7) | vii.7 (530.9) | 0.045 (0.72) |
Density of air in kg / m³ and density of air in English language units
The SI unit for density is kilogram per cubic meter (kg / m³). Nevertheless, in some cases it is more than convenient to use:
- gram per cubic centimeter (1000 /c m³), 1 k/cm³ = 0.001 kg / m³;
- kilogram per liter (kg / 50), 1 kg/L = thou kg / m³;
- gram per milliliter (k / mL), one g/mL = k kg / thousand³.
The choice of units depends on the situation. Sometimes you know the volume of the air container in liters, some other time yous need to measure its dimensions to obtain book in cubic meters. Experience costless to alter the units in our air density estimator!
The same applies to the density of air in Purple units. Example of these units include:
- pound per cubic anxiety (lb / cu ft);
- pound per cubic yard (lb / cu yd), 1 lb / cu yd ≈ 0.037 lb / cu ft;
- ounce per cubic inch (oz / cu in), i oz / cu in = 108 lb / cu ft;
- pound per gallon (United states) (lb / US gal), 1 lb / U.s.a. gal ≈ 7.48 lb / cu ft.
If you need to convert mass density units of any solids, liquids or gases, our density converter may exist exactly the tool yous were looking for!
In physics, in that location are also other types of density. Check out the energy density of fields calculator and the number density figurer to learn more almost them! The main divergence is in the units. In the former, in that location are joules instead of kilograms, and in the latter, kilograms are replaced by a dimensionless number.
Standard air density
Because temperature and air pressure level vary from place to place, we need to define reference air atmospheric condition. Recently, there has been a variety of alternative definitions for the standard conditions (for example in technical or scientific calculations). If y'all study or piece of work in the technology, technology or chemical industries, you should always check what standards were used by the writer of the publication, article or book. Y'all need to know what they meant by "standard" conditions. Non only exercise the standards modify on a regular basis, but they are also set by various organizations (some take fifty-fifty more than than one definition of standard reference conditions). In the list below, you tin notice several standard reference pressures p₀ and temperatures T₀ in current use (remember that there are many more of them):
- International Union of Pure and Applied Chemical science(IUPAC): Standard Temperature and Pressure (STP),
p₀ = ten⁵ Pa,T = 0 °C; - Establish of Standards and Engineering science (NIST): ISO 10780,
p₀ = ane atm,T = 0 °C; - International Civil Aviation Organization (ICAO): International Standard Atmosphere (ISA),
p₀ = one atm,T = 15 °C; - United States Environmental Protection Agency (EPA): Normal Temperature and Pressure (NTP),
p₀ = i atm,T = 20 °C; - International Marriage of Pure and Practical Chemical science: Standard Ambient Temperature and Pressure (SATP),
p₀ = ten⁵ Pa,T = 25 °C;
So, if you want to respond the question what is the standard air density, yous should choose the appropriate standard weather condition. You tin compute them with our air density calculator with an assumption that relative humidity is relatively small (dry air). For example, the standard air density for STP is ρ₀ = 1.2754 kg / m³, for NIST is ρ₀ = ane.2923 kg / one thousand³ and for SATP is ρ₀ = ane.1684 kg / yard³.
What is air pressure?
Air pressure is a physical property of a gas that tells us with how much forcefulness it has when acting on the environment. Let's consider a cubic container (run into effigy beneath) with some air airtight inside. Co-ordinate to the kinetic theory of gases, the molecules of the gas are in abiding movement with a velocity that depends on the thermal energy. Particles collide with each other and with the walls of a container, exerting a tiny force on them. However, since the number of the enclosed molecules reaches most ~ten²³ (order of magnitude of Avogadro's constant), the total force becomes pregnant and measurable - this is pressure.
There are also other types of force per unit area which we have described in our other calculators. Now when you lot know what is air pressure level, you should certainly bank check out hydrostatic pressure calculator (force is exerted by liquids) and radiations force per unit area calculator (force is exerted by lite) and larn more about them!
What is relative humidity?
The relative humidity RH is defined every bit the ratio of the partial pressure of the water vapor to the equilibrium vapor pressure level of water at a given temperature. Partial pressure is the pressure of one component of the air if taken individually, at the aforementioned volume and temperature of the whole. When you sum the partial pressures of all the gases in the air, you lot will obtain its total pressure which we can directly measure:
p_total = p_N₂ + p_O₂ + p_Ar + p_H₂O + ...
The equilibrium vapor pressure of water is the pressure exerted by a vapor that is in thermodynamic equilibrium with its liquid phase at a given temperature. It is a measure of the tendency of molecules or atoms to escape from a surface of a liquid and go a gas. As the temperature increases, the equilibrium vapor pressure increases also.
Relative humidity RH ranges between 0% and 100%, where 0% means the dry air, and 100% is air that is completely saturated with water vapor. You lot should be aware that relative humidity of 100% doesn't mean that air only consists of water. When relative humidity is of 100%, the cooling of the air gives rise to condensation of water vapor.
What is the dew signal?
Dew point is a concrete quantity that is strictly related to the humidity of air. It is the temperature at which the water vapor independent in air reaches its saturation state. When the air is further cooled, the h2o vapor volition condense to form liquid water - dew. There are several ways to guess the dew point. Our air density calculator uses the following formula:
DP = 243.12 * α / (17.62 - α),
where α is a parameter that depends on the relative humidity RH and temperature T:
α = ln(RH/100) + 17.62 * T / (243.12 + T).
Relative humidity is expressed in percents and temperature in Celsius degrees. Because dew point is direct linked to relative humidity you need to enter only one of those parameters in the air density calculator.
Human body uses the evaporation of sweat to cool down during a hot day. The evaporation speed of the perspiration depends on how much wet is in the air. If the air is already saturated (relative humidity = 100%), perspiration will not evaporate, and you will exist coated with sweat. Thankfully, when air is moved from your body by a wind, sweat will evaporate faster, making y'all feel a pleasant coolness. Discomfort besides arises when the dew bespeak is depression (dry out air) causing the skin to fissure and become irritated more hands.
In the table below, you will discover dew point and (related to information technology) relative humidity at the temperature of 68°F (20°C). The dew point can't be higher than the air temperature because relative humidity can't exceed 100%. On the other paw, when relative humidity equals 0%, we say that the air is a dry out air (dew point loses its purpose considering in that location is no water).
| Dew point [°F (°C)] | Relative humidity at 61.6°F (xx°C) [%] |
|---|---|
| over 60 (xvi.4) | over 80 |
| 57.8 (14.iii) | 70 |
| 53.6 (12) | 60 |
| 48.7 (9.3) | fifty |
| 42.8 (6) | 40 |
| 35.4 (1.nine) | 30 |
| below 25.4 (-3.7) | below than xx |
Dominik Czernia , PhD candidate and Bogna Szyk
Source: https://www.omnicalculator.com/physics/air-density
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