Technique
This is one of Turner's best-preserved pictures, probably because he used a relatively standard oil paint mix instead of the experimental materials he sometimes tried (these tend to be unstable and have deteriorated over time).
Turner used a technique known as scumbling to create his vision of sun-tinged clouds. First, he built up a background of sky with thin glazes of paint. Once this was dry, he lightly brushed thick, opaque paint (in vivid yellows and oranges) over the top, touching down at irregular intervals as he dragged his brush across.
It's a dramatic effect - the texture and intense colours mean that these areas stand out.
Next: Special effects