Perhaps the most widely sought after wine is the Icewine. By law, the grapes can not be harvest until the temperature dips to 17º F. This means that if the temperature falls to 20º F the grapes cannot be harvested. Grapes can go through several nights of below freezing temperatures just to warm up again during the day. This freezing and thawing gives the grapes character and perhaps the excellence of the regions Icewines.
News Source : Travel to Ontario wineries while visiting Niagara Falls