Follow-up on filters
I thought I would just put up a couple of shots to show the use of two different filters. The first shot was taken using a Lee PL-L polariser to darken the sky and help the clouds to stand out. Without this, the cloud formation was not very pronounced. Note that because the polarising effect on the sky is stronger at a right-angle to the direction of the sun, and I was using a wide angle lens the sky does not look evenly dark from one side of the image to the other. This is seen as a problem by some. Make your own decision on this.
The other similar framing of the scene was taken using a Lee 87 IR filter and infrared modified camera (Canon Rebel XT). The darkening of the sky caused by the IR filter is more even from left to right when compared to the polarised shot. The shots were taken in the evening, with the sun quite low in the sky (Norway 2005).