Brush edge for fog/cloud brush

How I have done it in the past is taken the hard edge brush, painted it on it's own layer and then using the eraser with one of the other cloud brushes without the hard edge, to erase that hard edge. Set up some brush dynamics to randomize the result. Once you've done that you can use the selection tool to grab the modified shape and create a new brush. Note: The new brush will be smaller than the original. Screenshots below.

Hard Edge Brush on its own layer

Hard edges erased

Select the new cloud shape by command/control + click on the layer thumbnail.

Go to Edit> Define Brush Preset

Name your brush.

Your new bruh will now show in your brush tip box, you may have to go look for it. If you hover over the thubnail the tool tip should show you the name.

It's also important to note that whatever size you create the brush is the size it will remain, so if you want a high resoluton bursh, make it at high resolution.

I hope this helps.