Map: Tracking Typhoon Nanmadol

Typhoon Nanmadol is expected to surge through the Philippine Sea toward Japan this weekend, bringing heavy winds and rain.

As of Saturday morning in Japan, the Global Disaster Alert and Coordination System had warned that more than 39 million people could be exposed to a storm that is at a Category 1 level or higher. Read our coverage here.

The terms typhoon, hurricane and cyclone all refer to tropical cyclones; the term that is applied to a given storm depends on where it originates. Typhoons develop in the northwestern Pacific and usually affect Asia. Hurricanes form in the North Atlantic, the northeastern Pacific, the Caribbean Sea and the Gulf of Mexico.

In the Atlantic, major hurricanes are defined as tropical cyclones with maximum sustained winds of 111 m.p.h. or higher and are classed as either Category 3, 4 or 5 storms. But in the Asia Pacific region, there are variations in how individual countries grade typhoons.