How easy is it to get snow blindness?

Along with most people in the country I have spent a fair amount of time out in the snow over the last week.

At the weekend, when things were at their worst in my area, I spend a good few hours clearing the snow, putting out food and water for the birds and walking in the white stuff. As it was snowing I opted for contact lenses and after a while I noticed that my vision was becoming unusual. Small objects in high contrast situations became difficult to focus on and I also noticed some visual disturbances in my periphery. Once indoors the effects slowly wore off.