Why is fading not smooth?

I am trying to create my own basic Day/night cycle script. I am having issues with the fading not being smooth. The Unity script reference says you fade from a to b by t, but I'm not sure what that really means t is doing What should t be and how does it work?

 #pragma strict
 var slider : float;
 var speed : int;
 var theTime : int;
 var theSun : GameObject;
 var morningLight : Color;
 var morningFog : Color;
 var midayLight : Color;
 var midayFog : Color;
 var duskLight : Color;
 var duskFog : Color;
 var nightLight : Color;
 var nightFog : Color;
 var Tod : int;
 
 
 function OnGUI () 
 {
     if(slider >= 360.0)
     {
         slider = 0;
     }
     slider = slider+Time.deltaTime;
     
     slider = GUI.HorizontalSlider(Rect(20,20,200,30), slider, 0,360);
     Tod = (slider/360) * 24;
     
     if(slider >= 0 && slider < 45)
     {
         RenderSettings.ambientLight = Color.Lerp (nightLight, morningLight, (Tod/2)+.5);
         RenderSettings.fogColor = Color.Lerp (nightFog, morningFog, (Tod/2));
     }
     if(slider >= 45 && slider < 135)
     {
         Debug.Log("Changetomiday");
         RenderSettings.ambientLight = Color.Lerp (morningLight, midayLight, (Tod/2)-(1.5));
         RenderSettings.fogColor = Color.Lerp (morningFog,midayFog, (Tod/2)-(2));
     }
     if(slider >= 135 && slider < 180)
     {
         RenderSettings.ambientLight = Color.Lerp (midayLight, duskLight, (slider/2)-(2));
         RenderSettings.fogColor = Color.Lerp (midayFog, duskFog, (slider/2)-(3));
     }
     if(slider >= 180 && slider <= 360)
     {
         RenderSettings.ambientLight = Color.Lerp (duskLight, nightLight, (slider/2)-(4));
         RenderSettings.fogColor = Color.Lerp (duskFog, nightFog, (slider/2)-(4));
     }
     theSun.transform.rotation = Quaternion.Euler(slider,0,0);
 }