Typhoon season is the yearly span when the Northwest Pacific Ocean basin experiences a significant increase in the formation and intensity of tropical cyclones. This period represents the climatological window when oceanic and atmospheric conditions align to fuel these storm systems. Understanding this season is not about fixed start and end dates but about recognizing the time of year when the threat of major storm impacts to East and Southeast Asia is highest. The heightened activity means countries in this region must maintain constant vigilance and preparedness for severe weather events.
Defining the Tropical Cyclone and Regional Terminology
All these intense weather systems are fundamentally the same phenomenon—a rotating, organized system of clouds and thunderstorms that develops over tropical or subtropical waters. The only difference between a typhoon, a hurricane, and a cyclone is simply the geographic area where the storm forms. They are all classifications of a tropical cyclone that has reached a sustained wind speed of at least 74 miles per hour (119 kilometers per hour).
Storms that develop over the North Atlantic Ocean and the Northeast Pacific Ocean are referred to as hurricanes. The term “typhoon” is used for those storms that form in the Northwest Pacific Ocean, west of the International Date Line. Conversely, storms of this strength in the South Pacific or Indian Ocean are generally called cyclones or severe tropical cyclones. This regional naming convention helps international weather agencies track and communicate warnings specific to their areas of responsibility.
Geographic Scope and Seasonal Timing
Typhoon season pertains specifically to the Northwest Pacific basin, which is the most active tropical cyclone basin. This vast area stretches from the coast of East Asia eastward to the 180th meridian and is where nearly one-third of the world’s tropical cyclones form. The storms frequently impact densely populated regions, including the Philippines, Taiwan, coastal China, Japan, and Vietnam.
While tropical cyclones can form year-round in this warm ocean basin, approximately 90% of the activity occurs between May and November. The season does not have the same rigid, officially defined boundaries as the Atlantic hurricane season, but this six-month span captures the overwhelming majority of named storms. Storm activity typically begins to ramp up in May and June as sea surface temperatures rise.
The peak of the typhoon season occurs from late August through early September, when the most numerous and intense storms tend to develop. Even as activity begins to decrease into November and December, powerful late-season typhoons can still pose a significant threat. The Philippines, in particular, experiences the highest frequency of landfalls, with an average of 20 tropical cyclones entering its area of responsibility annually.
Meteorological Conditions That Drive the Season
The seasonal window for typhoon activity is dictated by a specific set of atmospheric and oceanic conditions that must converge. One foundational requirement is sufficiently warm Sea Surface Temperatures (SSTs), which must be at least 26.5°C (80°F) and extend through a depth of about 50 meters. This warm water provides the moisture and heat energy necessary to fuel the storm’s engine through evaporation.
Another condition is low vertical wind shear, which is the change in wind speed or direction between the lower and upper levels of the atmosphere. When wind shear is low, the vertical structure of the storm remains intact, allowing heat to be released efficiently near the center and the storm to intensify. High wind shear, conversely, tears the storm apart, preventing organization.
A typhoon requires a pre-existing weather disturbance, such as a tropical wave, for the system to organize. This disturbance must also be far enough away from the equator—typically at least 300 miles (5 degrees of latitude)—for the Coriolis effect to provide the necessary rotational spin. Finally, high humidity in the middle levels of the atmosphere is needed to prevent dry air from infiltrating the storm’s core, which would otherwise weaken the deep convection.
Measuring Typhoon Intensity
Various meteorological agencies use different classification systems to measure a typhoon’s intensity. The Japan Meteorological Agency (JMA), which serves as the Regional Specialized Meteorological Center for the Western Pacific, uses a system based on 10-minute sustained wind speeds. This scale uses categories such as Tropical Depression, Tropical Storm, Severe Tropical Storm, and Typhoon.
The JMA subdivides the Typhoon classification into Strong, Very Strong, and Violent categories. The United States’ Joint Typhoon Warning Center (JTWC) uses a different scale that aligns more closely with the one used in the Atlantic. The JTWC uses 1-minute sustained wind speeds and applies the term “Super Typhoon” to any system with maximum sustained winds of 130 knots (150 mph) or greater, which is equivalent to a strong Category 4 or Category 5 on the Saffir-Simpson scale.