God Rays: Why do they radiate?

[Peter McLennan]

There must be a simple answer to this, but it's eluding me.

In this image, typical of "god rays", the beams radiate.  But why? 
The sun is essentially a point source at an infinite distance.  Why aren't the rays parallel?

[pcgpcg]

Why aren't the rays parallel?

They are parallel, just like train tracks are parallel. They don't appear parallel because of vanishing-point perspective.
Imagine if you had a string in each hand and the end of each string went to the sun. The strings are parallel.
Spread your arms wide and the two strings appear to be diverging in two different directions, just like God rays.

[Otto Phocus]

You supplied the answer in your post.  The Sun is "essentially" a point source, but is not a point source of light... just pretty close to a point source. Close does not count, especially when it is so far away. 

So what you have is a source of light that is not quite a point source, being projected through an aperture (aka hole in the cloud) that is around 94 million miles away from the source. This combined with the aperture being much much smaller than the size of the sun produces diffraction. George Airy wrote a bit about this. This diffracted light rays reflect off of particles in the atmosphere and becomes visible. 

This is why these rays are not always apparent.

[Peter McLennan]

Thanks, guys for both answers.  The railway tracks analogy sorta makes sense, even though the viewpoint may not be from right on the tracks. 

"Why do they radiate?"  Was imprecisely written.  I meant  radiate in the angular sense, not the luminance sense.

Thanks for both replies.
 

[Alan Klein]

Maybe the light is diffused above the clouds and the beams are not the suns rays directly.  Then the rays beneath the clouds would align with the holes in the clouds.

[EricV]

The answers mentioning perspective and diffusion and secondary scattering are correct.

Light rays coming directly from the sun are parallel to within 1/2 degree (the angular width of the sun as seen from the earth).  Diffraction depends on the size of the aperture (compared to the wavelength of light), but not the size or distance of the source.  Diffraction through a hole in a cloud is completely negligible.

[Alan Klein]

Eric what's the difference between diffusion and secondary scattering?

[EricV]

Eric what's the difference between diffusion and secondary scattering?

Diffusion is not the right technical term here.  I just meant what I think you meant -- light coming through the clouds might not come directly from the sun, but rather from higher clouds which scatter the light, allowing larger angles to make it through.