All season tires perform well for 12 months of the year where there is no snow. However here in BC Canada other than a few areas on the coast the all season tire performs well in Spring Summer and Fall. (hence 3 seasons) However the all season tire was designed for wet and dry driving on pavement. NOT snow covered conditions. BC winters especially in the interior require snow tires that were especially designed for slippery conditions and very cold temperatures. We are not talking about just aggressive tread patterns either because snow tires are specifically designed with very soft compounds that help better grip the road during very cold weather.
Snow tire has added groove capacity helping dispel any of the excess snow as well the tire is made of a softer compound vs. an all season tire again giving it extra traction. Some tires are designed to actually grab the snow and throw it as the tire turns. Tread patterns are deigned to give extra traction. the snow symbol imprinted on the sidewall indicates in fact that it is a snow tire and will give superior performance in winter conditions vs. an all season tire.
Lets talk about a front wheel drive vehicle. If you only put 2 snow tires on the vehicle what you are doing is transferring the traction of the rear onto the front tires and that could lead to a possible over steer condition which could result in the car fish tailing or swinging out. To maintain optimum traction handling and stability we recommend 4 snow tires on all vehicles.
There is a flip side to snow tires. The same qualities that give them there improved performance during the winter months become far less appropriate when the weather turns warm. Snow tires are made of a softer compound, by using them in the summer months and warmer weather will create premature wear on the tire. Always remember to install your snows in the fall but take them off for the spring and summer. Its a good idea to buy an extra set of rims for your winter tires. This will make it much simpler to change the tires over between winter and summer.