The northern hemisphere is warmer in spring than in winter, because in spring

Both hemispheres are warmer in their Spring than they are
in their Winter, because . . .

A). the sun climbs higher in the sky during Spring than it does during
WInter ... shooting its rays more directly at the ground ...,

and

B).  the sun stays up in the sky longer in Spring than it does in
Winter, giving the ground more time to absorb its rays.