i'm starting to realise that ppl just dont like gum trees ... i've just had a look thru a few pages of the silhouette challenge so i'd be able to comment on this one and i must say you had some really stiff competition ... but i still reckon this should have done a lot better ... i really like the weird angle of the tree against the horizon and those beautiful clouds ...
I've always thought it was 'Cumulo-Nimbus' - which these clouds are not. Try Google with your spelling and you might be in for a surprise! (Maybe you meant it to be spelled like that?)
The silhouette effect is good here but the composition lets it down, unfortunately I can't see a focal point... in other words, I can't easily tell what this photo is of.