Fortitude

Increasing
Each level point spent increases fortitude by 2. This in turn increases the hypothermal and hyperthermal insulation by 4.5. In the HUD you will see rounded down integers. Bear in mind to resist hypothermic effects at -5 °C(23 °F), hypothermal insulation of at least 200 is required - so you will not be able to resist all weather effects using fortitude alone without adding an obscene amount of points to the skill.

Utility
Each point spent on fortitude will increase your knock-out threshold by 1. This also reduces torpidity gain rate and increases torpidity drop rate.

A Human with 0 points in fortitude will be knocked-out once his torpidity reaches 50 points, a human with 1 point in fortitude requires 51 torpidity to fall unconscious and so on.

Torpidity also seems to reduce the chance of catching diseases like Mega Rabies and Swamp Fever. This is yet to be tested.