Tropical cyclones

How a tropical storm forms

Tropical cyclones require warm ocean water with surface temperatures of at least 26°C to form and strengthen. When warm and moist air rises above the ocean, it is called convection. In layers of air up to 5 km high, it cools again and condenses, releasing heat energy that drives the storm. The released heat energy remains in the troposphere and the air pressure there increases. The higher air pressure spreads out and creates a suction effect that pulls in more moist air from below. The Earth's rotational motion causes the storm to spin and develop into a tropical cyclone.

Classification

Tropical cyclones are classified into different categories from 1 to 5 using the Saffir-Simpson scale. The decisive criterion here is the wind speed. However, since weather phenomena are observed before they develop into full-blown storms, there are additional subdivisions:

Tropical depression	< 61 km/h	< 38 mph
Tropical storm	62 - 118 km/h	39 - 73 mph
Category 1	119 - 153 km/h	74 - 95 mph
Category 2	154 - 177 km/h	96 - 110 mph
Category 3	178 - 208km/h	111 - 129 mph
Category 4	209 - 251 km/h	130 - 156 mph
Category 5	> 251 km/h	> 156 mph

In addition to sustained wind speed, other factors such as storm surges, precipitation and damage potential are also taken into account. Even a Category 1 storm can cause significant damage if the storm system carries large masses of water that are discharged over land as heavy rainfall.

Typhoons in the Pacific Ocean that reach a speed of 150 km/h (92 mph) are officially designated as "severe typhoons" by the Hong Kong Observatory. Above 190 km/h (= 118 mph), one speaks of a super typhoon. The Hawaiian Joint Typhoon Warning Center, on the other hand, has a different definition: Here, the minimum speed for a super typhoon is 240 km/h (150 mph). In practice, however, these different limits often do not play a role, since the Hong Kong Observatory measures the speed in a 10-minute average, while in Hawaii a 1-minute average is used.

The most frequently affected countries

From the past 36 months, we have identified the most frequently affected countries. The table below shows the number of tropical storms per category that made direct landfall. The category indicated corresponds to the Saffir-Simpson scale at the time of impact.

Hurricane, cyclone or typhoon?

Which of the words is used depends on where a storm has formed. The word "cyclone" is often confusing because it is also used as a generic term for all rotating storms that originate on the ocean. At the same time, it is equally the term for these storms in the Indian Ocean and on both sides of Australia. Basically, this classification applies:

• Hurricanes: Atlantic and Northeast Pacific
• Typhoons: Northwest Pacific
• Cyclones: Indian Ocean and South Pacific Ocean

With this classification it is now clear why in North America one speaks almost only of hurricanes, which are called just that on both sides of the continent. In the Asian region they are cyclones or typhoons. However, the weather phenomenon itself is always the same. The question of whether a typhoon or hurricane is stronger also cannot be answered in a generalized way. On average, typhoons are certainly somewhat larger and faster, because the water in the Pacific is often warmer. However, this classification is purely geographical.

When is hurricane season?

The peak season for tropical storms depends on the region and the ocean currents with which warm and cold water masses move in the oceans: