In the late 1980s, J. Pepi developed the most recent model for measuring the effects of high temperature and humidity. Pepi’s model is called the Summer Simmer Index and is given by

S = 1.98T - 1.09(1 - H)(T - 58) - 56.8

where T is the air temperature (in degrees Fahrenheit) and H is the relative humidity (as a decimal). If S remains constant, find the rate of change of humidity with respect to temperature when the temperature is 80°F.