1999) assumes that daily temperature follows a simple sine curve with minimum and maximum 12h apart (06:00 and 18:00, respectively).
A sine curve with a minimum and maximum adjusted to match the temperature and time of day of the actual minimum and maximum was a good fit to the actual data for the period from minimum temperature to maximum temperature, although the actual temperatures led the sine curve slightly during this period.
Therefore, the rise from minimum to maximum is symmetrical and can be modelled by a single sine curve rather than requiring a pair of curves, and so Eqns 3a and b can be combined to the single expression in Eqn 6.
In signal measuring and manufacturing, a sine curve wave could be gotten with filtering methods.
Actually, any sine curve is corresponding to the intersection of a plane and a cylinder surface.
When the cylinder surface is folded up to a place, this ellipse become a sine curve .