I'm curious to learn more about the research and thought that went into calculating the heat index. In particular I want to understand how Steadman or Rothfusz managed to fit a model (in terms of relative humidity and temperature) on data with variables related to human physiology.
After having a look at the references of a few papers about heat index (like this one). It looks like the original publications you are looking for are
- The Heat Index "Equation" (or, More Than You Ever Wanted to Know About Heat Index) by Lans P. Rothfusz available here, and
- The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science, by R. G. Steadman available here.
As a general suggestion, looking at the references in related papers is always a great way to find the original research articles on any topic.