My very favorite time to photograph horses is when they are in the snow. As long as I am bundled up so that I am not cold, I can stay out for hours photographing.
There are two different times I do snow photography – when it is snowing, and after it has snowed, and cleared up. Today I will discuss photographing when it is snowing. First, make sure you cover your camera up, especially if you are going to be out for a long time. You can buy those expensive camera covers but I will often just use a plastic bag, and have a towel handy to dry off my camera.
Now, when you approach your subject, keep in mind that your autofocus is going to try to focus on the falling snowflakes, not the horse. I usually put my focus on manual so I can focus on the horse, not the snowflakes.
Next, if it is snowing, it will be overcast, so make sure you increase your iso high enough – you may need 500, 640, or even 800 or 1000 if you are wanting to capture a moving horse.
Then, because you have a snowy mostly white scene you might need to increase your exposure compensation to as much as +1 or even +2 to get your exposure right. Your light meter is going to give you readings to get an average GREY scene, but you want a white scene, hence the need to use compensation. Check your camera’s manual to see how to do this. Try portraits when the horse is standing still, get the snow on his back, the icicles in his mane and whiskers if he has been outside for a while, and then get your subject moving, kicking up the snow.