Typhoons and tropical depressions are terms well-known to the public when unusual natural phenomena occur. However, not everyone understands the correct distinctions between these two natural occurrences.
What is a Tropical Depression?
Scientifically, typhoons and tropical depressions are collectively referred to as tropical cyclones, which are areas of rotating winds that can span hundreds of kilometers, forming over tropical seas. In the Northern Hemisphere, winds spiral into the center in a counterclockwise direction.
The atmospheric pressure in a typhoon is significantly lower than in the surrounding areas, often dropping below 1000 mb. The difference in air pressure between the center of the storm and its surroundings is what causes the extremely high wind speeds.
A typhoon or tropical depression can be likened to a giant cake; when you cut the cake in half, you see that the center is the “eye” of the storm, surrounded by the “walls” of the storm.
Thus, a typhoon can be viewed as a swirling area of wind from all directions blowing into the center of the storm. The closer you get to the center, the stronger the winds can become, sometimes reaching several hundred kilometers per hour. However, directly at the center lies a relatively calm area known as the “eye” of the storm. The surrounding air spirals inward, not in straight lines, but in a spiral pattern.
During a storm, air near the surface is drawn inward from all sides toward the center. As the air reaches the center, it rises upward and spreads out in all directions. Typhoons originate from tropical seas and move a massive amount of moist air. As this moist air rises, the water vapor it contains condenses into clouds, leading to heavy rainfall. Therefore, areas affected by a storm not only experience strong winds but also dense clouds and significant rainfall.
A strong typhoon is making landfall. (Illustrative image).
Differences Between Tropical Depression and Tropical Storm
When a tropical depression gains enough strength and continues to develop sufficient energy to attract strong winds and moisture, a tropical storm is formed. The term tropical storm is translated from the English term “tropical storm.” The distinction between a tropical depression and a tropical storm is primarily based on wind speed.
According to the wind classification system developed by Irish naval officer Francis Beaufort, wind is categorized into 13 levels from 0 to 12. When wind speeds reach levels 6-7, it is referred to as a tropical depression.
Today, storm classifications are sometimes described using Beaufort scale levels from 12 to 16, closely corresponding to the wind speed categories of the Saffir-Simpson hurricane scale, where actual storms are measured. Level 1 on the Saffir-Simpson scale corresponds to level 12 on the Beaufort scale. However, levels beyond 13 on the Beaufort scale do not align with the Saffir-Simpson hurricane scale.
- Internationally, a tropical storm must have wind speeds exceeding 64 km/h (or 35 knots), which is above level 8. If the winds are weaker than 63 km/h, it is classified as a tropical depression.
- If wind speeds exceed 118 km/h (level 12, 64 knots), the storm is classified as a major typhoon.
- Additionally, there are very large storms or super typhoons (super typhoon, intense major hurricane, super cyclonic storm, very intense tropical cyclone) with wind speeds exceeding 240 km/h (or over 130 knots), corresponding to level 4 on the Saffir-Simpson scale or level 15 (Beaufort scale) and above.
The term “typhoon” is used in the East China Sea and the Northwest Pacific; “hurricane” refers to storms in the Atlantic; and “tropical cyclone” is the term used in the Indian Ocean.
