I've realized this step by step, to show you my manner to work on photoshop, the realisation of "heat" has taken about 50 hours for a 3500 x 2550 pixels format. Here is the essentials steps to obtain the final result

1. Creation of one sketch , watercolors enriched for scanning

2.Put the first flat tint to delimit the work areas

3.Delimitation of the shadows zones

4.contrast and light effects accentuation

5.finish with all details to obtain the final work