Inastronomy,air mass orairmass is a measure of the amount of air along the line of sight when observing a star or other celestial source from belowEarth's atmosphere (Green 1992). It is formulated as the integral ofair density along thelight ray.

As it penetrates theatmosphere, light is attenuated byscattering andabsorption; the thicker atmosphere through which it passes, the greater theattenuation. Consequently,celestial bodies when nearer thehorizon appear less bright than when nearer thezenith. This attenuation, known asatmospheric extinction, is described quantitatively by theBeer–Lambert law.

"Air mass" normally indicatesrelative air mass, the ratio of absolute air masses (as defined above) at oblique incidence relative to that atzenith. So, by definition, the relative air mass at the zenith is 1. Air mass increases as theangle between the source and the zenith increases, reaching a value of approximately 38 at the horizon. Air mass can be less than one at anelevation greater thansea level; however, mostclosed-form expressions for air mass do not include the effects of the observer's elevation, so adjustment must usually be accomplished by other means.

Tables of air mass have been published by numerous authors, includingBemporad (1904),Allen (1973),^[1] andKasten & Young (1989).

Theabsolute air mass is defined as:$${\displaystyle \sigma =\int \rho \mathrm{d}s.}$$where${\displaystyle \rho }$ isvolumetric density ofair. Thus${\displaystyle \sigma }$ is a type ofoblique column density.

In thevertical direction, theabsolute air mass at zenith is:$${\displaystyle {\sigma }_{\mathrm{z}\mathrm{e}\mathrm{n}}=\int \rho \mathrm{d}z}$$

So${\displaystyle {\sigma }_{\mathrm{z}\mathrm{e}\mathrm{n}}}$ is a type ofvertical column density.

Finally, therelative air mass is:$${\displaystyle X=\frac{\sigma }{{\sigma }_{\mathrm{z}\mathrm{e}\mathrm{n}}}}$$

Assuming air density to be uniform allows removing it from the integrals. The absolute air mass then simplifies to a product:$${\displaystyle \sigma =\overline{\rho }s}$$where${\displaystyle \overline{\rho }=\mathrm{c}\mathrm{o}\mathrm{n}\mathrm{s}\mathrm{t}.}$ is the average density and thearc length${\displaystyle s}$ of the oblique and zenith light paths are:

In the corresponding simplified relative air mass, the average density cancels out in the fraction, leading to the ratio of path lengths:$${\displaystyle X=\frac{s}{s_{\mathrm{z}\mathrm{e}\mathrm{n}}}.}$$

Further simplifications are often made, assuming straight-line propagation (neglecting ray bending), as discussed below.

The angle of a celestial body with the zenith is thezenith angle (in astronomy, commonly referred to as thezenith distance). A body's angular position can also be given in terms ofaltitude, the angle above the geometric horizon; the altitude${\displaystyle h}$ and the zenith angle${\displaystyle z}$ are thus related by$${\displaystyle h={90}^{\circ }-z.}$$

Atmospheric refraction causes light entering the atmosphere to follow an approximately circular path that is slightly longer than the geometric path. Air mass must take into account the longer path (Young 1994). Additionally, refraction causes a celestial body to appear higher above the horizon than it actually is; at the horizon, the difference between the true zenith angle and the apparent zenith angle is approximately 34 minutes of arc. Most air mass formulas are based on the apparent zenith angle, but some are based on the true zenith angle, so it is important to ensure that the correct value is used, especially near the horizon.^[2]

When the zenith angle is small to moderate, a good approximation is given by assuming a homogeneous plane-parallel atmosphere (i.e., one in which density is constant and Earth's curvature is ignored). The air mass${\displaystyle X}$ then is simply thesecant of the zenith angle${\displaystyle z}$:$${\displaystyle X=\sec z.}$$

At a zenith angle of 60°, the air mass is approximately 2. However, because theEarth is not flat, this formula is only usable for zenith angles up to about 60° to 75°, depending on accuracy requirements. At greater zenith angles, the accuracy degrades rapidly, with${\displaystyle X=\sec z}$ becoming infinite at the horizon; the horizon air mass in the more realistic spherical atmosphere is usually less than 40.

Many formulas have been developed to fit tabular values of air mass; one byYoung & Irvine (1967) included a simple corrective term:$${\displaystyle X=\sec z_{\mathrm{t}}\left[1-0.0012({\sec }^2{z}_{\mathrm{t}}-1)\right],}$$where${\displaystyle z_{\mathrm{t}}}$ is the true zenith angle. This gives usable results up to approximately 80°, but the accuracy degrades rapidly at greater zenith angles. The calculated air mass reaches a maximum of 11.13 at 86.6°, becomes zero at 88°, and approaches negative infinity at the horizon. The plot of this formula on the accompanying graph includes a correction for atmospheric refraction so that the calculated air mass is for apparent rather than true zenith angle.

Hardie (1962) introduced a polynomial in${\displaystyle \sec z-1}$:$${\displaystyle X=\sec z-0.0018167(\sec z-1)-0.002875(\sec z-1{)}^2-0.0008083(\sec z-1{)}^3}$$which gives usable results for zenith angles of up to perhaps 85°. As with the previous formula, the calculated air mass reaches a maximum, and then approaches negative infinity at the horizon.

Rozenberg (1966) suggested$${\displaystyle X={\left(\cos z+0.025e^{-11\cos z}\right)}^{-1},}$$which gives reasonable results for high zenith angles, with a horizon air mass of 40.

Kasten & Young (1989) developed^[3]

$${\displaystyle X=\frac{1}{\cos z+0.50572({6.07995}^{\circ }+{90}^{\circ }-z{)}^{-1.6364}},}$$which gives reasonable results for zenith angles of up to 90°, with an air mass of approximately 38 at the horizon. Here the second${\displaystyle z}$ term is indegrees.

Young (1994) developed$${\displaystyle X=\frac{1.002432{\cos }^2{z}_{\mathrm{t}}+0.148386\cos z_{\mathrm{t}}+0.0096467}{{\cos }^3{z}_{\mathrm{t}}+0.149864{\cos }^2{z}_{\mathrm{t}}+0.0102963\cos z_{\mathrm{t}}+0.000303978}}$$in terms of the true zenith angle${\displaystyle z_{\mathrm{t}}}$, for which he claimed a maximum error (at the horizon) of 0.0037 air mass.

Pickering (2002) developed$${\displaystyle X=\frac{1}{\sin (h+244/(165+47h^{1.1}))},}$$where${\displaystyle h}$ is apparent altitude${\displaystyle ({90}^{\circ }-z)}$ in degrees. Pickering claimed his equation to have a tenth the error ofSchaefer (1998) near the horizon.^[4]

Interpolative formulas attempt to provide a good fit to tabular values of air mass using minimal computational overhead. The tabular values, however, must be determined from measurements or atmospheric models that derive from geometrical and physical considerations of Earth and its atmosphere.

Ifatmospheric refraction is ignored, it can be shown from simple geometrical considerations (Schoenberg 1929, 173) that the path${\displaystyle s}$ of a light ray at zenith angle${\displaystyle z}$ through a radially symmetrical atmosphere of height${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$ above the Earth is given by$${\displaystyle s=\sqrt{R_{\mathrm{E}}^2{\cos }^{2}z+2R_{\mathrm{E}}{y}_{\mathrm{a}\mathrm{t}\mathrm{m}}+y_{\mathrm{a}\mathrm{t}\mathrm{m}}^2}-R_{\mathrm{E}}\cos z}$$or alternatively,$${\displaystyle s=\sqrt{{\left(R_{\mathrm{E}}+y_{\mathrm{a}\mathrm{t}\mathrm{m}}\right)}^2-R_{\mathrm{E}}^2{\sin }^{2}z}-R_{\mathrm{E}}\cos z}$$where${\displaystyle R_{\mathrm{E}}}$ is the radius of the Earth.

The relative air mass is then:$${\displaystyle X=\frac{s}{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}=\frac{R_{\mathrm{E}}}{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}\sqrt{{\cos }^{2}z+2\frac{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}{R_{\mathrm{E}}}+{\left(\frac{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}{R_{\mathrm{E}}}\right)}^2}-\frac{R_{\mathrm{E}}}{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}\cos z.}$$

If the atmosphere ishomogeneous (i.e.,density is constant), the atmospheric height${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$ follows fromhydrostatic considerations as:^{[citation needed]}$${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}=\frac{k{T}_0}{mg},}$$where${\displaystyle k}$ is theBoltzmann constant,${\displaystyle T_0}$ is the sea-level temperature,${\displaystyle m}$ is the molecular mass of air, and${\displaystyle g}$ is the acceleration due to gravity. Although this is the same as the pressurescale height of anisothermal atmosphere, the implication is slightly different. In an isothermal atmosphere, 37% (1/e) of the atmosphere is above the pressure scale height; in a homogeneous atmosphere, there is no atmosphere above the atmospheric height.

Taking${\displaystyle T_0=288.15\mathrm{K}}$,${\displaystyle m=28.9644\times 1.6605\times {10}^{-27}\mathrm{k}\mathrm{g}}$, and${\displaystyle g=9.80665\mathrm{m}/{\mathrm{s}}^2}$ gives${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}\approx 8435\mathrm{m}}$. Using Earth's mean radius of 6371 km, the sea-level air mass at the horizon is$${\displaystyle X_{\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}}=\sqrt{1+2\frac{R_{\mathrm{E}}}{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}}\approx 38.87.}$$

The homogeneous spherical model slightly underestimates the rate of increase in air mass near the horizon; a reasonable overall fit to values determined from more rigorous models can be had by setting the air mass to match a value at a zenith angle less than 90°. The air mass equation can be rearranged to give$${\displaystyle \frac{R_{\mathrm{E}}}{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}=\frac{X^2-1}{2\left(1-X\cos z\right)};}$$matching Bemporad's value of 19.787 at${\displaystyle z}$ = 88°gives${\displaystyle R_{\mathrm{E}}/y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$ ≈ 631.01 and${\displaystyle X_{\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}}}$ ≈ 35.54. With the same value for${\displaystyle R_{\mathrm{E}}}$ as above,${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$ ≈ 10,096 m.

While a homogeneous atmosphere is not a physically realistic model, the approximation is reasonable as long as the scale height of the atmosphere is small compared to the radius of the planet. The model is usable (i.e., it does not diverge or go to zero) at all zenith angles, including those greater than 90° (see§ Homogeneous spherical atmosphere with elevated observer). The model requires comparatively little computational overhead, and if high accuracy is not required, it gives reasonable results.^[5]However, for zenith angles less than 90°, a better fit to accepted values of air mass can be had with severalof the interpolative formulas.

In a real atmosphere, density is not constant (it decreases with elevation abovemean sea level. The absolute air mass for the geometrical light path discussed above, becomes, for a sea-level observer,$${\displaystyle \sigma ={\int }_0^{y_{\mathrm{a}\mathrm{t}\mathrm{m}}}\frac{\rho \left(R_{\mathrm{E}}+y\right)\mathrm{d}y}{\sqrt{R_{\mathrm{E}}^2{\cos }^{2}z+2R_{\mathrm{E}}y+y^2}}.}$$

Several basic models for density variation with elevation are commonly used. The simplest, anisothermal atmosphere, gives$${\displaystyle \rho ={\rho }_0{e}^{-y/H},}$$where${\displaystyle {\rho }_0}$ is the sea-level density and${\displaystyle H}$ is the densityscale height. When the limits of integration are zero and infinity, the result is known asChapman function. An approximate result is obtained if some high-order terms are dropped, yielding (Young 1974, p. 147),$${\displaystyle X\approx \sqrt{\frac{\pi R}{2H}}\exp \left(\frac{R{\cos }^{2}z}{2H}\right)\mathrm{e}\mathrm{r}\mathrm{f}\mathrm{c}\left(\sqrt{\frac{R{\cos }^{2}z}{2H}}\right).}$$

An approximate correction for refraction can be made by taking (Young 1974, p. 147)$${\displaystyle R=7/6R_{\mathrm{E}},}$$where${\displaystyle R_{\mathrm{E}}}$ is the physical radius of the Earth. At the horizon, the approximate equation becomes$${\displaystyle X_{\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}}\approx \sqrt{\frac{\pi R}{2H}}.}$$

Using a scale height of 8435 m, Earth's mean radius of 6371 km, and including the correction for refraction,$${\displaystyle X_{\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{i}\mathrm{z}}\approx 37.20.}$$

The assumption of constant temperature is simplistic; a more realistic model is thepolytropic atmosphere, for which$${\displaystyle T=T_0-\alpha y,}$$where${\displaystyle T_0}$ is the sea-level temperature and${\displaystyle \alpha }$ is the temperaturelapse rate. The density as a function of elevation is$${\displaystyle \rho ={\rho }_0{\left(1-{\frac{\alpha }{T}}_{0}y\right)}^{1/(\kappa -1)},}$$where${\displaystyle \kappa }$ is the polytropic exponent (or polytropic index). The air mass integral for the polytropic model does not lend itself to aclosed-form solution except at the zenith, so the integration usually is performed numerically.

Earth's atmosphere consists of multiple layers with different temperature and density characteristics; commonatmospheric models include theInternational Standard Atmosphere and theUS Standard Atmosphere. A good approximation for many purposes is a polytropictroposphere of 11 km height with a lapse rate of 6.5 K/km and an isothermalstratosphere of infinite height (Garfinkel 1967), which corresponds very closely to the first two layers of the International Standard Atmosphere. More layers can be used if greater accuracy is required.^[6]

When atmospheric refraction is considered,ray tracing becomes necessary (Kivalov 2007), and the absolute air mass integral becomes^[7]$${\displaystyle \sigma ={\int }_{r_{\mathrm{o}\mathrm{b}\mathrm{s}}}^{r_{\mathrm{a}\mathrm{t}\mathrm{m}}}\frac{\rho \mathrm{d}r}{\sqrt{1-{\left(\frac{n_{\mathrm{o}\mathrm{b}\mathrm{s}}}{n}\frac{r_{\mathrm{o}\mathrm{b}\mathrm{s}}}{r}\right)}^2{\sin }^{2}z}}}$$where${\displaystyle n_{\mathrm{o}\mathrm{b}\mathrm{s}}}$ is the index of refraction of air at the observer's elevation${\displaystyle y_{\mathrm{o}\mathrm{b}\mathrm{s}}}$ above sea level,${\displaystyle n}$ is the index of refraction at elevation${\displaystyle y}$ above sea level,${\displaystyle r_{\mathrm{o}\mathrm{b}\mathrm{s}}=R_{\mathrm{E}}+y_{\mathrm{o}\mathrm{b}\mathrm{s}}}$,${\displaystyle r=R_{\mathrm{E}}+y}$ is the distance from the center of the Earth to a point at elevation${\displaystyle y}$, and${\displaystyle r_{\mathrm{a}\mathrm{t}\mathrm{m}}=R_{\mathrm{E}}+y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$ is distance to the upper limit of the atmosphere at elevation${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$. The index of refraction in terms of density is usually given to sufficient accuracy (Garfinkel 1967) by theGladstone–Dale relation$${\displaystyle \frac{n-1}{n_{\mathrm{o}\mathrm{b}\mathrm{s}}-1}=\frac{\rho }{{\rho }_{\mathrm{o}\mathrm{b}\mathrm{s}}}.}$$

Rearrangement and substitution into the absolute air mass integral gives$${\displaystyle \sigma ={\int }_{r_{\mathrm{o}\mathrm{b}\mathrm{s}}}^{r_{\mathrm{a}\mathrm{t}\mathrm{m}}}\frac{\rho \mathrm{d}r}{\sqrt{1-{\left(\frac{n_{\mathrm{o}\mathrm{b}\mathrm{s}}}{1+(n_{\mathrm{o}\mathrm{b}\mathrm{s}}-1)\rho /{\rho }_{\mathrm{o}\mathrm{b}\mathrm{s}}}\right)}^2{\left(\frac{r_{\mathrm{o}\mathrm{b}\mathrm{s}}}{r}\right)}^2{\sin }^{2}z}}.}$$

The quantity${\displaystyle n_{\mathrm{o}\mathrm{b}\mathrm{s}}-1}$ is quite small; expanding the first term in parentheses, rearranging several times, and ignoring terms in${\displaystyle (n_{\mathrm{o}\mathrm{b}\mathrm{s}}-1{)}^2}$ after each rearrangement, gives (Kasten & Young 1989)$${\displaystyle \sigma ={\int }_{r_{\mathrm{o}\mathrm{b}\mathrm{s}}}^{r_{\mathrm{a}\mathrm{t}\mathrm{m}}}\frac{\rho \mathrm{d}r}{\sqrt{1-\left[1+2(n_{\mathrm{o}\mathrm{b}\mathrm{s}}-1)(1-\frac{\rho }{{\rho }_{\mathrm{o}\mathrm{b}\mathrm{s}}})\right]{\left(\frac{r_{\mathrm{o}\mathrm{b}\mathrm{s}}}{r}\right)}^2{\sin }^{2}z}}.}$$

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In the figure at right, an observer at O is at an elevation${\displaystyle y_{\text{obs}}}$ above sea level in a uniform radially symmetrical atmosphere of height${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$. The path length of a light ray at zenith angle${\displaystyle z}$ is${\displaystyle s}$;${\displaystyle R_{\mathrm{E}}}$ is the radius of the Earth. Applying thelaw of cosines to triangle OAC,expanding the left- and right-hand sides, eliminating the common terms, and rearranging gives$${\displaystyle s^2+2\left(R_{\text{E}}+y_{\text{obs}}\right)s\cos z-2R_{\text{E}}{y}_{\text{atm}}-y_{\text{atm}}^2+2R_{\text{E}}{y}_{\text{obs}}+y_{\text{obs}}^2=0.}$$

Solving the quadratic for the path lengths, factoring, and rearranging,$${\displaystyle s=\pm \sqrt{{\left(R_{\text{E}}+y_{\text{obs}}\right)}^2{\cos }^{2}z+2R_{\text{E}}\left(y_{\text{atm }}-y_{\text{obs}}\right)+y_{\text{atm}}^2-y_{\text{obs}}^2}-(R_{\text{E}}+y_{\text{obs}})\cos z.}$$

The negative sign of the radical gives a negative result, which is not physically meaningful. Using the positive sign, dividing by${\displaystyle y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$, and cancelling common terms and rearranging gives the relative air mass:$${\displaystyle X=\sqrt{{\left(\frac{R_{\text{E}}+y_{\text{obs}}}{y_{\text{atm}}}\right)}^2{\cos }^{2}z+\frac{2R_{\text{E}}}{y_{\text{atm}}^2}\left(y_{\text{atm}}-y_{\text{obs}}\right)-{\left(\frac{y_{\text{obs}}}{y_{\text{atm}}}\right)}^2+1}-\frac{R_{\text{E}}+y_{\text{obs}}}{y_{\text{atm}}}\cos z.}$$

With the substitutions${\displaystyle \hat{r}=R_{\mathrm{E}}/y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$ and${\displaystyle \hat{y}=y_{\mathrm{o}\mathrm{b}\mathrm{s}}/y_{\mathrm{a}\mathrm{t}\mathrm{m}}}$, this can be given as$${\displaystyle X=\sqrt{{(\hat{r}+\hat{y})}^2{\cos }^{2}z+2\hat{r}(1-\hat{y})-{\hat{y}}^2+1}-(\hat{r}+\hat{y})\cos z.}$$

When the observer's elevation is zero, the air mass equation simplifies to$${\displaystyle X=\sqrt{{\left(\frac{R_{\text{E}}}{y_{\text{atm}}}\right)}^2{\cos }^{2}z+\frac{2R_{\text{E}}}{y_{\text{atm}}}+1}-\frac{R_{\text{E}}}{y_{\text{atm}}}\cos z.}$$

In the limit of grazing incidence, the absolute airmass equals thedistance to the horizon. Furthermore, if the observer is elevated, thehorizon zenith angle can be greater than 90°.

Atmospheric models that derive from hydrostatic considerations assume an atmosphere of constant composition and a single mechanism of extinction, which isn't quite correct. There are three main sources of attenuation (Hayes & Latham 1975):Rayleigh scattering by air molecules,Mie scattering byaerosols, and molecular absorption (primarily byozone). The relative contribution of each source varies with elevation above sea level, and the concentrations of aerosols and ozone cannot be derived simply from hydrostatic considerations.

Rigorously, when theextinction coefficient depends on elevation, it must be determined as part of the air mass integral, as described byThomason, Herman & Reagan (1983). A compromise approach often is possible, however. Methods for separately calculating the extinction from each species usingclosed-form expressions are described inSchaefer (1993) andSchaefer (1998). The latter reference includessource code for aBASIC program to perform the calculations. Reasonably accurate calculation of extinction can sometimes be done by using one of the simple air mass formulas and separately determining extinction coefficients for each of the attenuating species (Green 1992,Pickering 2002).

Inoptical astronomy, the air mass provides an indication of the deterioration of the observed image, not only as regards direct effects of spectral absorption, scattering and reduced brightness, but also an aggregation ofvisual aberrations, e.g. resulting from atmosphericturbulence, collectively referred to as the quality of the "seeing".^[8] On bigger telescopes, such as theWHT (Wynne & Worswick 1988) andVLT (Avila, Rupprecht & Beckers 1997), the atmospheric dispersion can be so severe that it affects the pointing of the telescope to the target. In such cases an atmospheric dispersion compensator is used, which usually consists of two prisms.

TheGreenwood frequency andFried parameter, both relevant foradaptive optics, depend on the air mass above them (or more specifically, on thezenith angle).

Inradio astronomy the air mass (which influences the optical path length) is not relevant. The lower layers of the atmosphere, modeled by the air mass, do not significantly impede radio waves, which are of much lower frequency than optical waves. Instead, some radio waves are affected by theionosphere in the upper atmosphere. Neweraperture synthesis radio telescopes are especially affected by this as they “see” a much larger portion of the sky and thus the ionosphere. In fact,LOFAR needs to explicitly calibrate for these distorting effects (van der Tol & van der Veen 2007;de Vos, Gunst & Nijboer 2009), but on the other hand can also study the ionosphere by instead measuring these distortions (Thidé 2007).

In some fields, such assolar energy andphotovoltaics, air mass is indicated by the acronym AM; additionally, the value of the air mass is often given by appending its value to AM, so that AM1 indicates an air mass of 1, AM2 indicates an air mass of 2, and so on. The region above Earth's atmosphere, where there is no atmospheric attenuation ofsolar radiation, is considered to have "air mass zero" (AM0).

Atmospheric attenuation of solar radiation is not the same for all wavelengths; consequently, passage through the atmosphere not only reduces intensity but also alters thespectral irradiance.Photovoltaic modules are commonly rated using spectral irradiance for an air mass of 1.5 (AM1.5); tables of these standard spectra are given inASTM G 173-03. The extraterrestrial spectral irradiance (i.e., that for AM0) is given inASTM E 490-00a.^[9]

For many solar energy applications when high accuracy near the horizon is not required, air mass is commonly determined using the simple secant formula described in§ Plane-parallel atmosphere.

• Allen, C. W. (1973).Astrophysical Quantities (3rd ed.). London: Athlone Press.ISBN 0-485-11150-0.OCLC 952445.
• ASTM E 490-00a (R2006). 2000. Standard Solar Constant and Zero Air Mass Solar Spectral Irradiance Tables. West Conshohocken, PA: ASTM. Available for purchase fromASTM.Optical Telescopes of Today and Tomorrow
• ASTM G 173-03. 2003. Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37° Tilted Surface. West Conshohocken, PA: ASTM. Available for purchase fromASTM.
• Avila, Gerardo; Rupprecht, Gero; Beckers, J. M. (1997). Arne L. Ardeberg (ed.). "Atmospheric dispersion correction for the FORS Focal Reducers at the ESO VLT".Optical Telescopes of Today and Tomorrow. Proceedings of SPIE. 2871 Optical Telescopes of Today and Tomorrow:1135–1143.Bibcode:1997SPIE.2871.1135A.doi:10.1117/12.269000.S2CID 120965966.
• Bemporad, A. 1904. Zur Theorie der Extinktion des Lichtes in der Erdatmosphäre.Mitteilungen der Grossh. Sternwarte zu Heidelberg Nr. 4, 1–78.
• Garfinkel, Boris (1967)."Astronomical Refraction in a Polytropic Atmosphere".The Astronomical Journal.72:235–254.Bibcode:1967AJ.....72..235G.doi:10.1086/110225.
• Green, Daniel W. E. 1992.Magnitude Corrections for Atmospheric Extinction.International Comet Quarterly 14, July 1992, 55–59.
• Hardie, R. H. 1962. InAstronomical Techniques. Hiltner, W. A., ed. Chicago: University of Chicago Press, 184–. LCCN 62009113.Bibcode1962aste.book.....H.
• Kivalov, Sergey N. (2007). "Improved ray tracing air mass numbers model".Applied Optics.46 (29):7091–8.Bibcode:2007ApOpt..46.7091K.doi:10.1364/AO.46.007091.ISSN 0003-6935.PMID 17932515.
• Hayes, D. S.; Latham, D. W. (1975)."A Rediscussion of the Atmospheric Extinction and the Absolute Spectral-Energy Distribution of Vega".The Astrophysical Journal.197:593–601.Bibcode:1975ApJ...197..593H.doi:10.1086/153548.ISSN 0004-637X.
• Janiczek, P. M., and J. A. DeYoung. 1987.Computer Programs for Sun and Moon Illuminance with Contingent Tables and Diagrams, United States Naval Observatory Circular No. 171. Washington, D.C.: United States Naval Observatory.Bibcode1987USNOC.171.....J.
• Kasten, F.; Young, A. T. (1989). "Revised optical air mass tables and approximation formula".Applied Optics.28 (22):4735–4738.Bibcode:1989ApOpt..28.4735K.doi:10.1364/AO.28.004735.PMID 20555942.
• Pickering, K. A. (2002)."The Southern Limits of the Ancient Star Catalog"(PDF).DIO.12 (1):20–39.
• Rozenberg, Grzegorz V. (1966).Twilight: A Study in Atmospheric Optics. New York: Plenum Press.ISBN 978-1-4899-6353-6.LCCN 65011345.OCLC 1066196615.
• Schaefer, Bradley E. (1993)."Astronomy and the Limits of Vision".Vistas in Astronomy.36 (4):311–361.Bibcode:1993VA.....36..311S.doi:10.1016/0083-6656(93)90113-X.
• Schaefer, B. E. 1998. To the Visual Limits: How deep can you see?.Sky & Telescope, May 1998, 57–60.
• Schoenberg, E. 1929. Theoretische Photometrie, Über die Extinktion des Lichtes in der Erdatmosphäre. InHandbuch der Astrophysik. Band II, erste Hälfte. Berlin: Springer.
• Thidé, Bo (2007-12-01). "Nonlinear physics of the ionosphere and LOIS/LOFAR".Plasma Physics and Controlled Fusion.49 (12B):B103 –B107.arXiv:0707.4506.Bibcode:2007PPCF...49..103T.doi:10.1088/0741-3335/49/12B/S09.ISSN 0741-3335.S2CID 18502182.
• Thomason, L. W.; Herman, B. M.; Reagan, J. A. (1983-07-01)."The Effect of Atmospheric Attenuators with Structured Vertical Distributions on Air Mass Determinations and Langley Plot Analyses".Journal of the Atmospheric Sciences.40 (7):1851–1854.Bibcode:1983JAtS...40.1851T.doi:10.1175/1520-0469(1983)040<1851:TEOAAW>2.0.CO;2.ISSN 0022-4928.
• van der Tol, Sebastiaan; van der Veen, Alle-Jan (2007)."Ionospheric Calibration for the LOFAR Radio Telescope".2007 International Symposium on Signals, Circuits and Systems. Vol. 2. pp. 1–4.doi:10.1109/ISSCS.2007.4292761.ISBN 978-1-4244-0968-6.
• de Vos, M.; Gunst, A. W.; Nijboer, R. (2009)."The LOFAR Telescope: System Architecture and Signal Processing"(PDF).Proceedings of the IEEE.97 (8):1431–1437.Bibcode:2009IEEEP..97.1431D.doi:10.1109/JPROC.2009.2020509.ISSN 0018-9219.S2CID 4411160.
• Wynne, C. G.; Worswick, S. P. (1988-02-01)."Atmospheric dispersion at prime focus".Monthly Notices of the Royal Astronomical Society.230 (3):457–471.Bibcode:1988MNRAS.230..457W.doi:10.1093/mnras/230.3.457.ISSN 0035-8711.
• Young, A. T. 1974. Atmospheric Extinction. Ch. 3.1 inMethods of Experimental Physics, Vol. 12Astrophysics, Part A:Optical and Infrared. ed. N. Carleton. New York: Academic Press.ISBN 0-12-474912-1.
• Young, Andrew T. (1994-02-20)."Air Mass and Refraction".Applied Optics.33 (6):1108–1110.Bibcode:1994ApOpt..33.1108Y.doi:10.1364/AO.33.001108.ISSN 0003-6935.PMID 20862124.
• Young, Andrew T.; Irvine, William M. (1967)."Multicolor photoelectric photometry of the brighter planets. I. Program and Procedure".The Astronomical Journal.72:945–950.Bibcode:1967AJ.....72..945Y.doi:10.1086/110366.

• Reed Meyer'sdownloadable airmass calculator, written in C (notes in the source code describe the theory in detail)
• NASA Astrophysics Data System A source for electronic copies of some of the references.

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• Atmospheric optical phenomena
• Optical quantities

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