Attrition For Overland TravelThis system abstracts the rigors of travel by calculating an attrition value that is applied as hit point damage for characters traveling overland. The primary factor for determining attrition is climate data, namely, temperature, humidity, and wind speed. These numbers are used to derive a hit point value that roughly corresponds to the impact of weather on the human body (heat stress, core body temperature, etc).
Apparent Temperature is a calculated value derived from temperature, humidity, and wind speed that is used to determine attrition. The best way to get good data is to use a historical database of weather (like this one). Use a city that your party is located near and a establish a base date from which to draw the data. For example, if my game session is May 1, 1618, I would map that date to weather data from May 1, 2010. Specifically, you will need the high and the low, average humidity, and average wind speed for the day.
Generating A Value
Use the attached spreadsheet and fill in the values that are shaded light blue. The attrition value in HP is the value highlighted in yellow. Use successive lines from the spreadsheet for successive days since attrition is cumulative.
Using The System During Play
The best way to use this sheet during play is to generate a month's worth of values. Whenever your party travels, delete all the prior days in that month and start counting forward from there. For example, if you populated the entire month of May 2010 as May 1618 and then your party traveled starting May 4th, delete the lines for the 1st through the 3rd.
Attrition is calculated by using the Apparent Temperature in a correlation between temperature and mortality rate inspired by this reference. Apparent Temperature is a simplified Steadman equation. Two values are calculated, one based on the low temperature for the day and the other for the high temperature. They are then averaged as the day's attrition value.
Limitations And Common Sense
This system is only designed for creating a hit point effect for overland travel. It assumes people and draft animals are taking appropriate precautions against the weather, are fed and drinking, and traveling at a normal pace.
If these assumptions are not true, I don't change these mechanics. Rather, I increase travel time. For example, a heavy rain storm would slow the travel rate down by washing out roads, causing flash flooding, or washing out bridges or fords. So the damage calculation is the same, but the party is exposed to the elements for a longer period for time. If the party were eating only half rations, reduce their rate of travel as they can't keep a good pace up with an empty stomach. If they are under the effects of an Endure Weather spell, feel free to half the damage.
In the end, this calculation provides a realistic damage for travel; it is up to each DM to use common sense in integrating this into your game world. At least you have a strong foundation for adjudication.