Almost everyone enjoys sunny, warm days outside. Some will even take a sunny cool day if they enjoy winter sports, but almost everyone can agree that rainy, miserable days outside can be the worst for comfort and photography. Lying right in between are overcast days, those days when the sky is neither blue or black, clouds don't form nice, beautiful pillow like objects and the sun doesn't really shine anywhere specific. These overcast days can be a photographers best friend if you know what to shoot.
Commonly referred to as nature's softbox because of the lack of actual direct sunlight, rather it's filtered and diffused through layers of thick clouds. The direct result of this is a near shadowless day, absolutely perfect for moody and fun portraits. The lack of shadows means utilizing on or off camera flash on location, reflectors or light modifiers to help compensate for the normally appearing shadows that come out. The end result is a soft, feathered light that seems to near perfectly illuminate people.
Shooting portraits on overcast days does require some attention to detail though, slightly differently then shooting on sunny days. Because there is less light, and it's being heavily diffused through the clouds, paying attention to ISO becomes increasingly important. While the same time on a sunny day may be perfectly suited for 100 ISO, an overcast day may require 400, 640 or even as high as 1600 ISO depending how thick the cloud cover is.
Additionally, special attention needs to be paid to proper exposure. The lack of direct sun and shadows means that most subject matter (people) and the foreground and background can easily be lost to middle gray. In doing so, over or under exposing the shot and then correcting in post-production will result in heavy JPG artifacting for your final image, regardless of whether you shot in RAW or not.
To avoid this, meter specifically for your subject matter and use the flashing histogram on your DSLR to find blown out highlights (almost all models do this). It's also a good idea to bracket expose – that is take 1-2 photos under and over exposed, and utilize exposure compensations. Overcast portraits also make for some of the best on-location black and white conversions too!
In addition to portraits, moody landscape, seascape and other outdoorsy photos can be effective on overcast days. The soft sky, lack of harsh shadows and middle gray tones may look somewhat depressing as you shoot, but once you convert them to black and white the true magic starts to show through.
To shoot stunning photos like this you will almost certainly need a tripod and will want to shoot at the lowest possible ISO. Just like portraits, the middle gray through the image may throw off your light meter so it's best to bracket exposure, both over and under what your camera says.
Overcast days don't have to be about sad and depressing moments, they can be used to capture just about any photo that doesn't require harsh shadows. Also, it's a great time to focus the subject while leaving the background nice and neutral.