So “16.4 mph north” is sufficient.
No, it’s not. “North” is a two dimensional direction on the surface of the earth. But we live, and can throw babies, in three dimension. Therefore you need to specify the inclination as well. “56 furlongs per minute, north, 0.3 radians” would suffice.





The problem is that there are an infinite number of directions that are going north. Going north parallell to the ground is not the same direction as an upwards or downwards path. You can have any inclination between (-pi/2, pi/2) and still be going north. If you don’t specify the inclination, it is likely to be assumed to be zero, i.e. the baby is moving parallell to the earths normal plane, but it’s it is uncertain. That’s why you need two vectors in 3d space.