Here is some information about snow-covered vents.
When there is a heavy snowfall it's possible that the exhaust vents on the roof can become covered with snow. The heavier and wetter the snow, the more likely the chance the vents will be covered. All exhaust vents are susceptible to becoming covered, not just ridge vents. However, if one watched the snow clear from a house, the roof typically clears at the ridge first; therefore, the best type of vent to use would be a ridge vent. The heat escaping the attic will work its way through and melt its way out of one foot of snow. However, that depth does vary because the weight of the snow will make a difference, such as heavy wet snow.
In general, heavy wet snow is much more dense than powder snow because of this it will take longer to melt through and in some cases will actually form an ice tunnel under the snow where it will melt a cavity around the ventilation openings. These cavities can exist for other types of vents also, including roof louvers, wind turbines and power fans.
The amount of snow that will collect on a ridge vent with an external wind baffle and a ridge vent without an external wind baffle is the same but there are key differences. Snow falling without wind will fall straight down and cover both styles of ridge vents equally -- reducing performance equally. However, when there is snow falling with a wind the two styles of ridge vents collect snow very differently. The wind will pull snow out from behind the baffle in an externally baffled ridge vent, but the ridge vent without the external baffle does not have that advantage. Ridge vents without external baffles typically have a portion of the shingle overhanging the vent. This overhang creates an area that can trap snow. The wind blows the snow up against the unbaffled ridge vent and gets trapped under the shingle overhang. From there, the snow can melt as warm air tries to escape. It can then refreeze when the temperature drops. The cycle will continue until the unbaffled ridge vent is blocked by ice formed by the snow trapped against the ridge vent.
Generally, the higher the vent is off the roof deck the less likely the vent is to become covered by snow. Wind turbines will take longer to cover with snow, but they are so much less effective at ventilating in general. Power fans will blow snow away from the vent and prevent it from building up and eventually covering it, however, a power fan does not run constantly, and thus, do not provide continuous attic ventilation. In extreme snowfall areas, a gable louver system is often the best option. For example, in mountain regions where 2 feet or 3 feet of snow at one period is the norm, gable louvers provide the best means of venting an attic. The design should include as much intake ventilation as possible combined with an ample amount of gable ventilation in both ends of the house. The addition of a powered gable fan with a humidistat would be helpful also.
NOTE: If gable louvers are being installed to work in conjunction with a snow-covered vent (ridge vent, box vent, etc.), short-circuiting of the attic ventilation system is real possibility as soon as the ridge vents clear of the snow.