How to get distance with raycast.distance

I have been using their Scripting API. But is makes the object move. This is the code

     public float moveForce = 1.0F;
     public float rotateTorque = 1.0F;
     public float hoverHeight = 4.0F;
     public float hoverForce = 5.0F;
     public float hoverDamp = 0.5F;
     public Rigidbody rb;
     void Start() {
         rb = GetComponent<Rigidbody>();
         rb.drag = 0.5F;
         rb.angularDrag = 0.5F;
     }
     void FixedUpdate() {
         rb.AddForce(Input.GetAxis("Vertical") * moveForce * transform.forward);
         rb.AddTorque(Input.GetAxis("Horizontal") * rotateTorque * Vector3.up);
         RaycastHit hit;
         Ray downRay = new Ray(transform.position, -Vector3.up);
         if (Physics.Raycast(downRay, out hit)) {
             float hoverError = hoverHeight - hit.distance;
             if (hoverError > 0) {
                 float upwardSpeed = rb.velocity.y;
                 float lift = hoverError * hoverForce - upwardSpeed * hoverDamp;
                 rb.AddForce(lift * Vector3.up);
             }
         }
     }

And it makes the object hover. I got rid of everything using the Rigidbody and the move floats.

     void FixedUpdate()
     {
         RaycastHit hit;
         Ray downRay = new Ray(transform.position, -Vector3.up);
         Debug.Log(hit.distance);
     }

But and error says "Use of unassigned local variable 'hit'". Can someone please explain how I can get the hit distance.