Learn how to transform sky images with rays of light in this free Photoshop tutorial
Set the Foreground colour to black and the Background to white (press D for the shortcut). Open up your cloud image and create a new layer.
Go to the menu Filter and to Render>Clouds. This will apply a smothering of black and white clouds to this new layer.
Head to Filter>Blur>Radial Blur, select the Zoom option and then set Amount to 70. Hit OK, and the clouds should blur outwards. Set the layer’s blend mode to Screen to reveal the blue sky.
On the clouds layer, click the Add Layer Mask button. With the Brush tool set to a large size with a soft edge (0% Hardness), erase parts of the effect where you want the original sky to show through to complete the look of rays of light.