Coefficient of lift question

[baleares]

Just doing test exams and came across this one. I have put the correct answer below. I don't understand what CL0 is but assume it is a state of the Coefficient of lift ? Would love an explanation. Thanks!

Q:With respect to angle of attack:

A: CL0 occurs with respect to the difference of the true wind and the chord.

This is how CLO is written:

http://www.themightyroars.com/ppl/cl0.png

[stella di mare]

Cl = Clo + (Cla * alpha)

where:

Cl = coefficient of lift
Clo = coefficient of lift at zero angle of attack
Cla = lift curve slope (the slope of Cl versus alpha)
alpha = angle of attack


From the lift formula Cl = L/(0.5*rho*V^2*S), the only two variables for a certain value of Cl at 0 alpha (producing a certain L) are V and S. Where S depends on the chord and V on the true wind over the airfoil.