This video by photoshopuniverse shows you how to simulate lightning in a night time scenic photo. For this a night time scene is required. Open the image in Photoshop and create a new layer and with the gradient tool create a gradient as shown from the top left to the bottom right. Apply the filter, difference clouds on the layer and inverse the layer as shown. Exaggerate the lightening by using the adjust levels command. Change the layer mode to stream and tweak the hue and saturation so that the lightning blends in with the image.