(Photoshop CS) White balance correction

When you take underexposure images, you can adjust image level to correct the image brightness. Please follow the easy steps.
1. Open an image. You can see that my image is underexposure

2. Select level as shown below
3. Adjust level by sliding a triangle at right side to the edge of level curve. You will get the correct exposure.
Or use white eyedropper to fix the problem.
Point to the area that should be white (For this case, I point to the cloud). You will get the correct image exposure too.
Let see the result :)