Unity: CharacterController mid-air movement

using UnityEngine;
using System.Collections;

public class Mover : MonoBehaviour
{
    public float speed = 6.0F;
    public float jumpSpeed = 8.0F;
    public float gravity = 20.0F;
    private Vector3 moveDirection = Vector3.zero;
    private CharacterController controller;

    void Start()
    {
        controller = GetComponent<CharacterController>();
    }

    void Update()
    {
        moveDirection = new Vector3(Input.GetAxis("Horizontal"), moveDirection.y, Input.GetAxis("Vertical")); // make sure the old value of moveDirection.y won't get lost
        moveDirection = transform.TransformDirection(moveDirection);

        // don't apply speed to moveDirection.y. apply it only to x and z
        moveDirection.x *= speed;
        moveDirection.z *= speed;

        if (controller.isGrounded && Input.GetButton("Jump"))
        {
            moveDirection.y = jumpSpeed;
        }

        moveDirection.y -= gravity * Time.deltaTime;
        controller.Move(moveDirection * Time.deltaTime);
    }
}