Some of this year’s storms themselves were unusual: Hurricane Harvey moved slowly across the ocean as a Category 4 storm, collecting enormous amounts of moisture, before making landfall in Texas on 26 August. It then stayed at almost the same place for a long period, collecting water vapour all the while. That is what shattered the rainfall records: more than 1000 litres of rain per square metre fell across the eastern coast of Texas (with more than 1300 litres in some locations), a phenomenon which, statistically, occurs roughly once in a thousand years. The result was severe flooding.
Irma has been one of the strongest hurricanes since record-taking began, and it spent the longest time with wind speeds of the highest category (5; >250 km/h), before it was downgraded one level on 8 September.
The most costly cyclone season so far was 2005, with overall losses of approximately US$170bn. The value of losses caused by this year’s storms cannot yet be estimated, but will certainly rank among the highest in recent years.