Refraction near the horizon-an empirical approach, case study 1: sunrise refractions observed at Edmonton, Alberta.

Normalizing near-horizontal (±) sunrise observations to a common logarithmic surface temperature gradient successfully reduces astronomical refraction variability. Using 108 observations across diverse altitudes, temperature gradients, and gradient heights, converting the data to median conditions achieved a root-sum-square (RSS) reduction of 9% for pressure/temperature and 36% for temperature gradients. The observed refractions, which initially scattered by ±4.7arcmin, were each converted into 24 ray-tracing simulations at different temperature gradient heights; below heights of ∼400m, the conversions agreed within ∼0.5±2.5 and ∼1±3arcmin. Additionally, accounting for wind as a partial pressure component reduced the RSS by 2.3%, whereas total pressure increased it.