Filters for clouds

I've always been a bit disappointed with the sky on my black and white landscape photos. I want the clouds to stand out but often they are almost invisible. To try and remedy this problem I thought I would try and photograph the same sky with different filters so see if they made much difference. I loaded up my Nikon F with Fuji Neopan and gave it a go. First with no filter at all, then a yellow filter and finally a red 25A filter. The 25A is really quite dark and takes about 3 stops off the exposure.

The photo on the left is with no filter, in the middle is with the yellow filter and on the right with the red 25A. The difference between them is quite subtle, with the clouds getting a little more defined and the contrast increasing slightly. I don't think there is much difference between the yellow and the red filter. Certainly the loss of 3 stops isn't really worth it!

I took the Nikon out to Canary Wharf in London recently and tried some building shots with the yellow filter. At the start of the outing there was a good amount of sunshine with some cloud. This is one of the first photos I took that day. I quite pleased with the cloud contrast.

Later in the day the cloud cover increased and it wasn't possible to get the type of sky/could photos I was after

Having done some research I am planning to try out some Rollei Retro 80 film. I believe that might give the high contrast super sharp look I am after.