Gravity not working to right strength

I recently created a movement script that will only allow movement in the air; there are two problems: 1. Gravity isn't applying strongly enough, the object falls very slowly. 2. If you keep hitting the space bar you can continue to jump higher and higher.

Here is the code:

 using UnityEngine;
 using System.Collections;
 
 public class Example : MonoBehaviour {
     
     // Use this for initialization
     void Start () {
         
     }
     public float speed = 0.0F;
 public float jumpSpeed = 8.0F; 
 public float gravity = 20.0F;
     public float jumpMove = 6.0F;
 private Vector3 moveDirection = Vector3.zero;
 
 void Update() {
                 CharacterController controller = GetComponent<CharacterController> ();
                 // is the controller on the ground?
                 if (controller.isGrounded) {
                         //Feed moveDirection with input.
                         moveDirection = new Vector3 (Input.GetAxis ("Horizontal"), 0, Input.GetAxis ("Vertical"));
                         moveDirection = transform.TransformDirection (moveDirection);
                         //Multiply it by speed.
                         moveDirection *= speed;
                 } 
         else {
                         moveDirection = new Vector3 (Input.GetAxis ("Horizontal"), 0, Input.GetAxis ("Vertical"));
                         moveDirection = transform.TransformDirection (moveDirection);
                         //Multiply it by speed.
                         moveDirection *= jumpMove;
 }
 
         //Jumping
         if (Input.GetButton("Jump"))
             moveDirection.y = jumpSpeed;
         
     
     //Applying gravity to the controller
     moveDirection.y -= gravity * Time.deltaTime;
     //Making the character move
     controller.Move(moveDirection * Time.deltaTime);
 }
 }

Can someone please help me fix these problems?