At and below 0 deg., reports with freezing rain and snow dominate. At -5 deg. and colder, precipitation is usually in the form of snow, and much of it is heavy. This category includes hours when two or more precipitation types, either frozen or liquid, occur when thunder is heard. No station reported thunder occurring with several types of precipitation. Heavy drizzle is the only liquid type not reported with thunder.
Fog occurs more often than any single precipitation type at all temperatures.
Additional high frequencies are found near the Great Lakes and along the east coast. In addition, several stations across northern Canada and Alaska report more than 10 hours. One station in the Aleutians has a large number of hours of thunder at all temperatures. These features in Canada and Alaska have not been identified previously. Thunderstorms at cold temperatures do not usually have many flashes, so thunder could be missed by observers due to such factors as noise level, office location, and work load.