Clouds can make challenging and wonderful subject matter, and towards the turn of the day, they can take on such subtle yet beautiful tones.  I often go out with my dogs, sketch kit and backpack chair in the late afternoon when the cumulus clouds are building and sit out somewhere lost in the country sketching clouds.  Here's a few examples of what I do, each being about 2 x 3 inches:

              

For me, the trick with clouds (and maybe for anything) is to paint them simply and still strive to maintain exquisite fidelity to their drawing and character.  We do want to get the feel without overworking, and towards that end I analyze patterns of shapes, prepare to distill areas with numerous colors and values down into one tone, simplify lines and masses and really leave out as much detail as I can get away with.  But at the same time, I am extremely careful to get this simplified drawing "right".  It's about letting go of trying to be literal, while at the same time still maintaining fidelity to the drawing and the character of what we are painting.  

To quote David Leffel, "rendering is longhand: painting is shorthand".  At its best, painting is really effective shorthand. 

That's a key for me: simplify and still strive for exquisite accuracy, so that it will ring true and keep its strength, undiluted by superfluous detail.