This will be my first year taking my DSLR to the slopes, I want to get the best pics possible.
I have thought for a long time that the main thing I would need is a circular polariser to eliminate some of the glare from the snow but after watching a youtube video of a guy doing snow photography he suggested that it will make the sky look really dark because of something to do with altitude.
Is a polariser a good thing to use for snow sports?
Personally, I'd try it and see which works better. If the polarization kills too much of the light from the sky, then I'd remove it but if there is too much glare from the snow, I'd leave it. Both could be problems and the amount of each problem is going to depend on the composition and angle of your shots. If you don't have much sky, then the sky being a darker blue doesn't matter and if you are shooting with the sun to your back, there isn't going to be a whole lot of glare off the snow.
A polarizer might improve certain shots but it's by no means a must. I have done plenty of winter alpine and snowsports photography without a polarizer (mainly due to using ultrawide lenses).
Sometimes when shooting on the slopes you can't afford to waste time fiddling with a polarizer.
you might need some ND filters if you plan on doing any panning or shallow depth of field.
This question really comes down to what, if anything, in winter sport scenes will be polarized, and then eliminating some polarity will make the shot better?
The first obvious answer is the sky if it's clear blue. The deep blue is not polarized, but the whiter haze in front of that is, depending on angle to the sun. By adjusting the polarizer to minimize the haze, you make the sky appear deeper and darker blue. Most of the time I think that makes for a better picture, but that is subjective and up to you and what you want to show.
Reflections off of ice at certain angles will also be polarized. A polarizer can be used to either reduce or accentuate these reflections. Again, whether that's good or bad and which way is up to you and what you are trying to show.
The diffuse white light from snow will have some polarization at certain angles from the sun. This won't be as dramatic as reflections off of ice, but could sometimes be used to advantage to allow the texture of the snow to be seen better and making it appear less blown out.
So yes, a polarizer can be useful for some types of shots in some conditions. Whether that matters or whether that produces a desirable effect is only something you can answer.
Overall, when shooting with snow around, be careful not to overexpose it. You might want to set the automatic exposure to -1 or so, or set it to take many points into account, not just the center. All the snow blown out is a lot more crappy looking than the little bit a polarizer will add back - unless of course that's what you are trying to do for some reason.
Sky at right angles to the sun will be strongly polarized. E.g. if the sun is setting in the west then the band of sky from north to south will be polarized. Make an L with your thumb and forefinger. Point your finger at the sun. Any part of the sky you can point to with your thumb can be darkened dramatically with the polarizer.
This effect is stronger at high elevation for the same reason that the sky is bluer: less crud in the air. If you are above 8000 feet, you can get skies that look surreal they are so dark.
This can give you dark blue skies behind red parkas over white snow and dark green trees.
Snow will have fairly random polarization, due to the multiple angles -- exception: crusts and sheet ice. The filter will darken the snow some compared to the rest of the world. I expect this effect to be strongest at right angles to the sun too.
Take care with wide angles. They can get enough sky that you have different colours of blue in different parts of the image. You're pretty good up to the equivalent of 50 mm on a full frame. Wider than that, pay attention.
As in most things, practice makes perfect. If you have snow at your place before you go, take the time to play and learn before you have to try to do this and get you and your camera down the slope without becoming a garage sale.
Also: Take a spare battery. Keep the spare in a pocket. My Nikon gets unhappy at about -20 C in about half an hour.
By the end of hte day your camera is going to be cold. Bag it until it warms up. You don't need condensation on/in it. DO NOT OPERATE BELLOWS LENSES WITH A COLD CAMERA INSIDE A WARM BUILDING. (Bellows -- one that changes volume when focusing)