B&W photography

It would be possible to write a book about PSE and B&W. From the first PSE version, you had everything needed to convert to monochrome, including simulating color filters, working with channels, toning etc. You could find many sets of actions to help you in that task and achieve  any kind of effect if you did not understand working with gradient maps, tweaking channels via the levels command, using  unsharp mask or 'paint with light' actions... You'll still find hundreds of tutorials on the Web.

To be more up to date and go a little farther than the 'convert to black and white' mentioned by Greg, I suggest having a look at the numerous choices you have if you 'Open in Camera raw' your color images. Choose 'treatment' just under the 'basic' tab and 'black and white'. Click on the 'monochrome' menu button, and have fun!

Are the original photos in B&W?  If they are in color, you can use the Enhance>Convert to Black and White tool which allows for many adjustments to the image.

1. You can use one of the selection tools, e.g. lasso tool, selection brush, to select the cloud. Then, place the cloud on its own layer (CTRL+J). Next, open a Brightness/contrast adjustment layer above the layer with the cloud, and link the two layers (CTRL+G). The brightness & contrast can be adjusted with the sliders to suit.

2. Another approach is to apply the brighness/contrast adjustment  to the entire picture, The adjustment layer has a built in mask which can be used to "mask" the area where you don't wish the change to take effect.