On overcast days or for subjects in the shade, lighting contrast is very low. In these situations you simply use an exposure meter to determine exposure or follow the exposure table in the film instructions. With this very soft and shadowless type of lighting, you won't need to use fill-in flash or reflectors. This filter reduces the bluishness of color slides made with this type of lighting. No exposure compensation is necessary with the skylight filter.
Overcast days are ideal for taking pictures of people because the soft lighting flatters skin tones.
You're bound to encounter some unusual outdoor lighting situations, such as those found on foggy days or on the back porch during a rainstorm. In such situations, your best friend is an exposure meter or an automatic camera. On rainy days keep your camera dry by shooting from under shelter, using an umbrella, or placing your camera in a plastic bag with only the front of the lens poking out. Use a skylight filter too keep the front lens element dry.