The vertical refraction in the atmosphere cannot be ignored for long-range observations. The exact value is difficult to predict as it requires a large number of temperature, pressure and humidity measurements all along the path of observation. However, the value can be measured using reciprocal zenith angles and other methods. This has been done and quantified many times at different locations under varying conditions.
List of refraction resources from Jesse Kozlowski: https://jessekozlowski.wordpress.com/2020/02/19/refraction-references/
- Monitoring of the refraction coefficient in the lower atmosphere using a controlled setup of simultaneous reciprocal vertical angle measurements
- Page 3
An essential finding of past research efforts is that the refraction coefficient k and vertical temperature gradient are directly related to each other [e.g., Brocks, 1939]. The refraction coefficient of a particular point, in the literature commonly referred to as the local refraction coefficient and denoted with c, is connected to the vertical temperature gradient ∂T/∂z (K/m) using [e.g., Bahnert, 1987; Joeckel et al., 2008]:where p is the pressure (HPa) and T is the temperature (K). The local refraction coefficient c is essentially a function of the temperature gradient ∂T/∂z, and depends only slightly on pressure p and temperature T [Wunderlich, 1985]. Temperature gradients allow modeling and reducing vertical refraction influences in surveying [e.g., Brocks, 1939; Heer and Niemeier, 1985; Kharaghani, 1987; Hennes, 2002; Ingensand, 2008].
- Has an overview of past studies into the effects of refraction in the atmosphere.
- “An essential finding of past research efforts is that the refraction coefficient k and vertical temperature gradient are directly related to each other [e.g., Brocks, 1939].”
- “…the thermal characteristics of the air strata of the lower atmosphere (lowest 20–30 m) are strongly subjected to the varying thermal properties of the surface [e.g., Angus‐ Leppan, 1969].”
- “In the evening, the Earth’s surface normally cools off faster than the overlaying air strata. Usually, this results in strong positive gradients ∂T/∂z [Angus‐Leppan, 1969]. Cloud cover attenuates the heat transfer from the Sun and, hence, from the ground, thus leading to smaller absolute values of the temperature gradients and refraction effects in the course of a day.”
- “Brocks[1950b] demonstrated that refraction effects in the lower atmosphere generally multiply with decreasing height. This is because heat transition from the Earth’s surface is the stronger, the less distant the atmospheric layers are. Therefore, the largest absolute values of temperature gradients are to be expected immediately above the ground.”
- “Stober  determined refraction coefficients between 0 and +2 from vertical angle measurements for ground clearances between 0.5 and 4 m over an ice field in Greenland.”
- “Kabashi obtained refraction coefficients from +1 to +18 from reciprocal vertical angles for a line of sight about 5 m over water.”
- Summary of Refraction Behavior
- “…refraction continuously increases to the positive range with peaks of about +12 to +16 equally present in the data around sunset.”
- “The refraction coefficients on cloudy days generally exhibit smaller variations.”
- “Our refraction experiments, carried out across homogeneously vegetated grassland with a ground clearance of about 1.8 m (i.e., an often used working height in surveying), showed a range of k between −4 and +16 for sunny summer days.”
- “…our empirical results show that the refraction coefficient k may reach magnitudes as large as +12 to +16 over grassland at 1.8 m.”
- Publication: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2010JD014067
- PDF: https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/2010JD014067
- Direct PDF Download
- Account of the Observations and Calculations, of the Principal Triangulation; and of the Figure, Dimensions and Mean Specific Gravity of the Earth as Derived Therefrom
- Alexander Ross Clarke, 1833
- Pages 542-550 as marked on the pages, or 571-579 of the PDF.
- “…the coefficient required is simply the ratio of the excess of the observed above the true zenith distance to the angle subtended by the two stations at the centre of the earth.”
- This means the observation appears above its true location.
- Coefficient of Refraction:
- +0.0809 for rays crossing the sea
- +0.0750 for rays not crossing the sea
- Note that over sea observations are raised more than not over the sea.
- Direct PDF Download
- Empirical Modelling of Refraction Error in Trigonometric Heighting Using Meteorological Parameters
- Gleichzeitig-gegenseitige Zenitwinkelmessung über größere Entfernungen mit automatischen Zielsystemen
Translated: “Simultaneous reciprocal Zenith Angle Measurement over Larger Distances with Automatic Target Systems“
- Ismail Kabashi 2004
- Measurements of the effects of refraction about 5 meters above the water at different times of the day.
- The coefficient of refraction was found to be between 0.5 and 17 on a clear day with a steeply increasing coefficient of refraction after sunset and peaking several hours after sunset.
- On a cloudy day, the coefficient of refraction was lower and less variable.
- Direct PDF Download
- “Refraktionsbestimmung auf dem grönländischen Inlandeis”
- Translated: “Refraction on the Greenland ice sheet”
- Prof. Dr.-Ing. Manfred Stober
- Translated: Shown are very different temperature gradients depending on solar radiation, wind and cloud cover. The resulting refraction coefficient takes values up to about at k = 1.7 (an order of magnitude higher than usual in Germany).
- Direct PDF Download: STO_Intergeo2004_Groenland-Refraktion
- Translated PDF: STO_Intergeo2004_Groenland-Refraktion.de.en
Online calculator for refraction using air temp at observer and target by Andrew T Young:
Overview of Young’s pages:
Other publications concerning refraction. I have reviewed some, but not all.
- An Accurate Method for Computing Atmospheric Refraction
- Astronomical refraction formulas for all zenith distances
- Atmospheric Refraction Predictions Based on Actual Atmospheric Pressure and Temperature Data
- Atmospheric Refraction. Part I
- Atmospheric Refraction. Part II
- Atmospheric Refractive Electromagnetic Wave Bending and Propagation Delay
- Errors in precise leveling
- INVESTIGATION OF REFRACTION IN THE LOW ATMOSPHERE
- Practical Formulas for the Refraction Coefficient
- PROPAGATION OF REFRACTION ERRORS IN TRIGONOMETRIC HEIGHT TRAVERSING AND GEODETIC LEVELLING
- Refraction by Earth’s Atmosphere near 12 Microns
- REFRACTION NEAR THE HORIZON
- Removal of Refraction Errors in Geodetic Levelling – 1979
- Results Of Leveling Refraction Tests by NGS TR NOS92 NGS22
- Earth Curvature and Atmospheric Refraction Effects on Radar Signal Propagation
- Atmospheric refraction: a history
- Refractive index of air:
new equations for the visible and near infrared
- Direct PDF Download: Refractive index of air: new equations for the visible and near infrared