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When you turn on your gas stove (or gas turbine if you
are in industry) each morning and you set the dial to "5",
you would like to get the same amount of heat from the burner
from one day to the next, even though the composition of the 
"natural" gas feeding your appliance may vary from day to day.
  If the Wobbe index of the gas remains constant from day to day
then the heat output of your appliance will remain the same from
day to day. The Wobbe index is a measure of the heat content
of the gas that flows through an orifice of constant size under
constant pressure per unit of time.
Based on elementary physics, Goffredo Wobbe in 1927 deduced 
that said heat content is a) proportional to the heat content of
a cubic foot (or meter) of gas at 
standard temperature and pressure (STP), and 
b) inversely proportional to the square root of the density
(or specific gravity) of the gas. 
I.e., at STP, lighter gas flows through the orifice
faster than heavier gas;