Whilst Gerardo Furtado's answer covers what may be the coldest race, it is not the coldesta World Championship race, which I feel may be an important distinction to some people.
According to a list of race records on Wikipedia, the coldest World Championship race was the 1978 Canadian Grand Prix - which had a highest temperature of 5°C (41°F) during the race.
Temperature for Montreal on race-day
I believe the sudden drops are missing data points rather than it suddenly plummeting to 0°C