A typhoon is a mature tropical cyclone that originates over warm ocean waters, representing a powerful, rotating storm system characterized by a low-pressure center, strong winds, and heavy rainfall. The volatility of these systems means that understanding when they typically occur is paramount for preparation and safety in the regions they affect. The timing of this activity is governed by specific physical conditions in the atmosphere and ocean, which define the annual period known as the typhoon season.
Defining the Typhoon Basins and Terminology
The term “typhoon” is a regional name for a tropical cyclone, used specifically in the Northwest Pacific Ocean basin, extending from the International Date Line westward to the Asian mainland. The same storm system is called a “hurricane” if it develops in the Atlantic Ocean or the Northeast Pacific Ocean. In the South Pacific or Indian Ocean, it is generally referred to as a “cyclone.” The difference between these names is based solely on the storm’s formation location, not its physical structure. The Northwest Pacific basin is the most active tropical cyclone basin in the world, generating an average of about 26 named storms each year.
Official Start and End Dates of the Western Pacific Season
The Western North Pacific (WNP) basin technically allows for year-round tropical cyclone formation, but the official “season” refers to the period of reliable and frequent development. The main season generally spans from May through November, representing the months of highest activity. While the season is not marked by rigid, pre-defined dates like the Atlantic hurricane season, the period from May to October sees the vast majority of storm formations. Some meteorological centers define the season based on when tropical cyclones are most likely to affect specific countries, which may shift the boundaries slightly.
Meteorological Factors Driving Seasonality
The timing of the typhoon season is dictated by the annual cycle of three primary meteorological factors necessary for tropical cyclogenesis.
Warm Water and Low Shear
The first is warm sea surface temperatures (SSTs), which must be at least 26.5 degrees Celsius down to a depth of approximately 50 meters to provide the necessary heat and moisture. As summer progresses, the ocean absorbs solar energy, causing SSTs to sustain this thermal threshold from late spring into the autumn. Another element is low vertical wind shear, which is the change in wind speed or direction with height. High wind shear disrupts storm formation, but the WNP environment typically features weaker shear during the summer.
Pre-existing Disturbances
The third factor involves pre-existing weather disturbances, such as the monsoon trough, which provide the initial spin, or vorticity, necessary for consolidation. These disturbances are more common during the peak summer and early autumn months, aligning with the warmest ocean temperatures.
Seasonal Variation in Frequency and Intensity
Storm activity is not uniform throughout the defined typhoon season, instead following a distinct progression in both frequency and intensity. The absolute peak of the season, when the highest number of storms form, is typically concentrated between August and October. This window represents the culmination of favorable conditions, as ocean temperatures peak and atmospheric circulation patterns are most conducive to storm development. Early-season storms (May or June) are generally weaker and tend to form closer to the equator. The most intense typhoons are statistically more likely to occur during the autumn months of September and October, due to the deepest layer of warm water having built up over the summer.