I've monkeyed about trying to get the right look the past couple weekends
and have stumbled upon something I'm happy with and would like to share.
The solution i've found is 2 spheres, the skysphere, for the sky itself,
and another sphere properly textured for the clouds.

It's a fairly simple solution that doesn't rely on the built in pigments and
questionable color mapping and provides, I think, more realistic results
pretty quick and easy.

Now it's the inner cloud sphere that proved tricky, but my solution is this.
I found a number of cloud transparency maps on the internet for renders of
earth from space. I manip and use them for the transparency and then crop
and manip actual cloud photos for the color part of the texture. Then I can
rotate the cloud sphere and get any variety of sky I like.

Here are my skysphere settings for 4 distinct times of day. Of course you
can adjust the gradient to suit your own purposes, but the colors were
pulled and averaged from a number of photos, so they're pretty accurate:

// ***********************************
//  SKYSPHERE CODE
// ***********************************

//Sunrise
sky_sphere {
  pigment {
    gradient y
    color_map {
        [(1-cos(radians(20)))/2 color rgb <1,0.552941176,0.184313725>]
        [(1-cos(radians(60)))/2 color rgb
<0.474509804,0.525490196,0.62745098>]
        [(1-cos(radians(90)))/2 color rgb
<0.196078431,0.329411765,0.580392157>]
         }
  }
}


//Daysky
sky_sphere {
  pigment {
    gradient y
    color_map {
        [(1-cos(radians(50)))/2 color rgb <0.6,0.721568627,0.764705882>]
        [(1-cos(radians(60)))/2 color rgb
<0.482352941,0.674509804,0.839215686>]
        [(1-cos(radians(90)))/2 color rgb
<0.364705882,0.584313725,0.803921569>]
         }
  }
}

//Overcast
sky_sphere {
  pigment {
    gradient y
    color_map {
        [(1-cos(radians(50)))/2 color rgb
<0.835294118,0.831372549,0.811764706>]
        [(1-cos(radians(60)))/2 color rgb
<0.749019608,0.756862745,0.760784314>]
        [(1-cos(radians(90)))/2 color rgb
<0.62745098,0.623529412,0.631372549>]
         }
  }
}

//Nightsky
sky_sphere {
  pigment {
    gradient y
    color_map {
        [(1-cos(radians(30)))/2 color rgb
<0.254901961,0.290196078,0.48627451>]
        [(1-cos(radians(45)))/2 color rgb
<0.141176471,0.17254902,0.294117647>]
        [(1-cos(radians(90)))/2 color rgb
<0.133333333,0.137254902,0.207843137>]
         }
  }
}

// ***********************************
//  END SKYSPHERE CODE
// ***********************************

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