The southeast of the United States, Florida in particular, is awaiting the passage of Hurricane Milton with great concern, just ten days after the significant damage caused by Hurricane Helene. In France, it’s the Kirk storm which is making headlines this Wednesday by copiously watering half of the country. In recent days, typhoons Yagi and Bebinca have shaken Southeast Asia, particularly China and the Philippines. But what is the difference between these weather phenomena?
From depression to cyclone
It all starts with an area of very low pressure, above a very warm ocean to maximize water evaporation. According to Météo-France, the transfer of humidity from the ocean to the atmosphere “is at its maximum at the end of summer when surface waters reach 28 to 29°C”. As warm air is drawn in by the low pressure at the center of the phenomenon, the winds begin to swirl.
If this wind is measured at 63 km/h or less, it is called a tropical depression. Between 64 and 118 km/h, the phenomenon becomes a tropical storm, and is given a name by the meteorology center of the area following it. From 119 km/h, we speak of a cyclone.
An eye, a zone devoid of clouds and wind, approximately 30 to 60 km wide, forms in the center, surrounded by the “wall” of the cyclone, made up of cumulonimbus clouds rising up to 15 km in altitude. It is within the wall that the cyclone is most devastating: “the winds blow there up to 300 km/h and the rains are torrential”, describes Météo-France.
Thunder in the tropics
This functioning and this hierarchy are the same wherever the cyclone is raging. But depending on where it occurs, the meteorological phenomenon is called differently: cyclone in the Indian Ocean and the South Pacific, hurricane in the North Atlantic and the Northeast Pacific (on both sides of the United States), and typhoon in the northwest Pacific. But wherever they are located, the power of these phenomena should be reinforced by global warming, according to IPCC simulations.
The associated phenomena, such as the swell and the strong tide which precedes the arrival of the cyclone, or the heavy rains, are also the same. Furthermore, all these phenomena, massive and going upstream, draw their power from the evaporation of ocean water, and quickly lose power when they touch land. Nothing to do with a tornado, a very localized phenomenon which can form above the ground from storm cells, in the event of violent winds and a temperature which drops rapidly with altitude.