A number of times I have been struggling to remove haze caused by atmospheric conditions. The De-Haze function in Camera Raw/Light rooms develop module makes accessible for everyone. One immediate issue I got with the function is that it is a picture wide setting. You are not able to limit the effect to an area of the picture.
Sometimes you want to limit the effect to an area of the photo, and then it is handy to know how the effect really works.
First step is to add an adjustment effect; notice when you choose levels it automatically adds a Mask. The mask is all white as default; remember the effect touches the picture where the Mask is white. This means we can also pain part of the Mask black later, should we want to leave an area unaffected. Atmospheric haze is in the blue RGB range. We can apply the images blue part to our Mask very easy by choosing Image>Apply Image.
In the Channel dropdown choose Blue.
Now double click on the Levels to bring up its dialog. Notice there is three sliders, there is no setting that fits all photos; you need to play with these sliders a little. I usually start with the two extreme outer sliders (Black and White). By bringing them slightly towards the center, I can change the contrast. Then I move on to the center slider to change the balance.