thanks for your comments!

ok,here you go with the scalable version:
Code:

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material mtl_himmel
{
	effect = "himmel.fx";
}

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somewhere else you set or change the variables (or fade those values in functions and create various fog):
Code:

---

mtl_himmel.skill1 = float(50); //vertical offset
mtl_himmel.skill2 = float(1500); //fading scale
mtl_himmel.skill3 = float(0.25); //density

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and here is the new fx-file:
Code:

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float4x4 matWorldViewProj;

float4 vecFog;
float4 vecSkill1;

struct VS_IN
{
	float4 position : POSITION;
	float2 texCoord : TEXCOORD0;
};

struct VS_OUT
{
	float4 position : POSITION;
	float2 texCoord : TEXCOORD0;
	float Fog : FOG;
};

VS_OUT mainVS (VS_IN In)
{
	VS_OUT Out;
	Out.position = mul(In.position,matWorldViewProj);
	Out.Fog = ((In.position.y-vecSkill1.x)/vecSkill1.y)-vecSkill1.z;
	Out.texCoord = In.texCoord;
	return Out;
}


texture entSkin1;

sampler basemap = sampler_state
{
	Texture = <entSkin1>;
	MipFilter = LINEAR;
	MinFilter = LINEAR;
	MagFilter = LINEAR;
	AddressU = Wrap;
	Addressv = Wrap;
};

float4 mainPS (VS_OUT In) : COLOR0
{
	return tex2D(basemap,In.texCoord);
}

technique heightFog
{
	pass p0
	{
		VertexShader = compile vs_1_1 mainVS();
		PixelShader = compile ps_1_1 mainPS();
	}
}

---