Hurricane Harvey is set to be one of the most devastating natural disasters in American history. At the time of writing, it has fortunately claimed few lives, but its storm surge and its sustained, unprecedentedly concentrated rainfall have both triggered a powerful flood the likes of which the Lone Star State has never seen before.
The problem isn’t that the hurricane – now a tropical storm – made landfall as a Category 4 (out of 5) monster. It’s that, ever since, it’s stalled over Houston, and rainfall that should have been distributed across several states has actually been dumped pretty much over one city. It’s a disaster: the Federal Emergency Management Agency (FEMA) said that they will be there for “years”, and the economic and infrastructural damage is likely to be post-Katrina levels.
It’s difficult to visualize just how much water is being deposited on Houston though. Every day, the estimates keep increasing for this “one-in-1,000 year” event. The latest is that about 56.8 trillion liters of water (15 trillion gallons) have fallen on Houston, but it’s predicted that this will total 75.7-94.6 trillion liters (20-25 trillion gallons) of water will have descended by the time Harvey dissipates.
There are some excellent visualizations by Vox of what it might look like as a single drop. As they explain, one particularly terrifying comparison can be made with Hurricane Katrina: this record-breaking beast that ruined New Orleans in 2005 released 3.8 times less water than Hurricane Harvey will by the time it terminates.
Here’s another scary comparison: Harvey is dumping about 65 percent of the volume of the Dead Sea on Houston. It is enough to destroy a city, and make no mistake, that’s exactly what it’s doing. Even the emergency dams designed to handle such flooding are themselves overflowing, and the authorities are running out of options.
So why is it dumping so much water at once? Where’s it all coming from? Well, as we’ve explained here, hurricanes invest about 400 times more energy in precipitation than they do in their wind. As the hurricane moves over land and loses its primary heat source – the warm oceanic water – the water vapor it contains radiates out plenty of heat, which causes it all to condense and fall.
As mentioned, the hurricane has also stalled. Most tend to keep moving over land, but Harvey appears to be in the middle of several air currents; a calm zone in the middle of a chaotic system. That means that it’s barely moving, and all that water vapor is condensing out en masse.
To add insult to injury, Harvey is also potentially spinning up brief and powerful tornadoes – and will continue to do so until as late as Thursday morning.