Character controller air control

So I wrote this movement script, using a character controller. I want to be able to move a tiny bit, while in the air. (like Minecraft, Counter strike etc.) But don't know how to change the movement control when in the air.

Here's the script:

 using UnityEngine;
 using System.Collections;
 
 public class FPSController : MonoBehaviour {
 
     //BOOLS
     public bool grounded = true;
 
     //FLOATS
     public float speed = 5;
     public float airAcceleration = 0.3f;
     public float jumpSpeed = 2;
     public float gravity = 35;
 
     //VECTORS
     public Vector3 moveDirection = Vector3.zero;
 
     //OTHERS
     CharacterController controller;
 
 
     void Start()
     {
         controller = GetComponent<CharacterController>();
     }
 
     // Update is called once per frame
     void Update () {
         controller.Move(moveDirection * Time.deltaTime);
 
        if (controller.isGrounded)
         {
             moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));
             moveDirection = transform.TransformDirection(moveDirection);
             moveDirection *= speed;
             if (Input.GetKey(KeyCode.Space))
             {
                 moveDirection.y = jumpSpeed;
             }
         }
         else
         {
             moveDirection.y -= gravity * Time.deltaTime;
         }
     }
 }