At the maximum height of a projectile, the velocity and acceleration are

At the highest point of a projectile the velocity and acceleration are perpendicular to each other $\left(90°\right)$

$\text{H}=\frac{{\text{u}}^{\text{2}}{\sin }^2\theta }{\text{2g}}$

$\text{H}\propto {\sin }^2\theta$