More hurricanes

National Geographic has an article by Michael Greshko about this rotten hurricane season:

NASA-NOAA's Suomi NPP satellite captured this view of Hurricane Irma as a Category 4 hurricane approaching the Leeward Islands.

After Harvey, Irma, and Maria, we look at why this hurricane season has been so active.
Just as Hurricane Harvey wrapped up its devastation of Houston, Texas, Irma got into line behind it and quickly built into the strongest Atlantic hurricane in recorded history. Now, Maria leaves a broken Caribbean in its wake: Dominica's rooftops and rainforests have been ripped to shreds, and Puerto Rico may be without power for months as a result of the storm.
It’s hard to avoid comparisons to the last time two such powerful storms threatened US landfall in the catastrophic 2005 hurricane season, twelve years ago.
As in 2005, when Katrina and Rita devastated the Gulf Coast in rapid succession, the country is staring down the barrel of multiple hurricanes making landfall. In the face of multiple major storms, a reasonable person might wonder why this season seems worse for US cities, and why the last dozen years brought fewer large hurricanes to US shores.

If you have a question about this hurricane season compared with recent years, we’ve got you covered:

How active was 2017's hurricane season forecast to be?
Above average. The National Oceanic and Atmospheric Administration (NOAA), Colorado State University, and the Weather Channel all estimated that, this year, we’d likely see more hurricanes than usual spawning in the Atlantic Ocean. NOAA’s Climate Prediction Center forecasted that we’d see between fourteen and nineteen named storms and five to nine hurricanes this season.
In comparison, the average hurricane season from 1981 to 2010 featured twelve named storms and six hurricanes. 
Why is this season so active?In short: atmospheric conditions were hurricane-friendly, and surface sea temperatures were warmer than usual. The Climate Prediction Center says that multiple conditions, such as a strong west African monsoon, have aligned to make the Caribbean Sea and part of the tropical Atlantic, a storm-spawning area called the Main Development Region, particularly well-suited to hurricanes.
Kerry Emanuel, an atmospheric scientist at MIT who studies hurricanes, says that two factors stand out. For one, there’s currently little difference in wind speeds near the surface and those roughly ten miles up, which ensures that miles-tall hurricanes can form and remain stable. What’s more, the tropical Atlantic is exhibiting high “thermal potential”, meaning that water can rapidly evaporate into the atmosphere.
“Thermal potential is a thermodynamic speed limit on hurricanes,” Emanuel says. “The greater the speed limit, the more favorable conditions are for hurricanes to form, and the more powerful they can get.”
What’s more, El Niño is stuck in neutral this year, improving Atlantic hurricanes’ prospects. When this warming of the equatorial Pacific is active, there tends to be more wind shear and less thermal potential over the Atlantic, hurting hurricanes’ chances of survival.
IT FEELS LIKE IT HAS BEEN A LONG TIME SINCE WE’VE HAD A YEAR LIKE THIS. WAS THERE REALLY A “DROUGHT” OF HURRICANES FOR MORE THAN A DECADE?
The phrase “landfall drought” refers to the fact that, before Harvey, it had been nearly twelve years since a hurricane rated Category 3 or above made landfall in the US, going back to Hurricane Wilma in 2005. Depending on how you define a Category 3 hurricane, this stretch had been the longest since at least 1900.
WHAT CAUSED THE DROUGHT?
Largely, it’s an artifact of how we measure hurricanes. As Hart and colleagues demonstrated in a 2016 study, if you slightly tweak the definitions of hurricane categories, the “drought” mostly vanishes.
It’s worth noting that the category system, which bins hurricanes by their maximum wind speeds, is just one way to measure hurricane intensity. By loss of life or economic toll, there’s been less of a drought to speak of.
“Tell the folks who survived 2007’s Category 2 Hurricane Ike that that wasn’t a major hurricane; it destroyed a large part of the Texas coastline,” says Emanuel. “Tell folks that Sandy wasn’t a major event… and it wasn’t even a hurricane.” 

ALL THAT SAID, IS THIS SEASON UNUSUAL?
The longer it goes, the more severe it seems to get.
For starters, Irma made landfall in the Florida Keys as a Category 4 hurricane, the second Category 4 storm to make landfall on the continental US this year. Such a vicious one-two punch hasn't hit the US in over a century, though 1954 came close, says Florida State University meteorologist Robert Hart. That year, the Category 4 Hurricane Hazel devastated the Carolinas, and two Category 3 hurricanes just missed landfall. 

Hurricane damages giant radio telescope at Arecibo

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Puerto Rico's iconic Arecibo Observatory has sustained some significant damage from Hurricane Maria, officials reported today (Sept. 22).
The storm hit the island as a Category 4 hurricane Wednesday (Sept. 20) and left widespread destruction in its wake. Without power, phones or internet service, the Arecibo Observatory has been offline since the storm hit.

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The Arecibo Observatory houses the world's second-largest radio telescope. While the overall structure of the telescope is still standing, it sustained some pretty serious damage from Hurricane Maria, according to an update from the Universities Space Research Organization (USRA), which helps to operate the Arecibo Observatory.

One telescope operator at Arecibo managed to contact USRA officials Thursday (Sept. 21) via a short-wave radio transmission. National Geographic's Nadia Drake reports that Pennsylvania State University professor Jim Breakall spoke with the telescope operator, who was identified as Ángel Vazquez, and said staff members and their families were safe after sheltering at the facility. Vazquez also detailed some of the damage done to the iconic telescope.

The Arecibo Observatory sustained serious damage during Hurricane Maria. The pointy object protruding downward from the suspended platform is a 96-foot (29-meter) antenna that broke off during the hurricane, puncturing the telescope dish below.
The Arecibo Observatory sustained serious damage during Hurricane Maria. The pointy object protruding downward from the suspended platform is a 96-foot (29-meter) antenna that broke off during the hurricane, puncturing the telescope dish below.
Credit: Pedrik/Flickr, CC BY-SA 2.0
A 96-foot (29 meters) antenna that was suspended above the telescope's 1,000-foot (305 m) dish was lost as a result of the hurricane, "with falling debris puncturing the dish in several places," USRA officials said in the update. The antenna was affixed to a platform suspended about 500 feet (150 meters) above the dish, which is made out of a metal mesh. [The Arecibo Observatory: Puerto Rico's Giant Radio Telescope in Photos]

"Also, a separate 12-meter [40-foot] dish used as a phase reference for Very Long Baseline Interferometry (VLBI) was lost," the USRA update states. VLBI is a method astronomers use to make radio telescopes more powerful by creating a network of smaller telescopes that work together.

Officials have yet to conduct "a full assessment of the damage, repairs that are needed and when the observatory can resume observations," Nicholas White, senior vice president for science at the USRA, said in a statement.

Getting to the observatory will be a challenge, though, as roads are covered in debris and are inaccessible at this time, USRA officials said.

Before the hurricane, Arecibo officials tweeted that the observatory would reopen to staff today, though it seems unlikely that anyone who didn't take shelter at the observatory will be able to report for work.

The visitor center is officially closed through Wednesday (Sept. 27), Arecibo representatives said on Twitter Monday (Sept. 18) as the observatory was preparing for the second major hurricane to hit Puerto Rico this month. Hurricane Irma also hit the island on Sept. 6, though the Arecibo Observatory made it through that storm relatively unscathed.

Original article on Space.com.

Hurricanes
But focusing on only the continental US obscures the utter devastation that this hurricane season has brought to the Caribbean. Irma effectively wiped out civilization on the island of Barbuda, which had been continually inhabited for three hundred years. The American and British Virgin Islands have suffered horrific damage. Puerto Rico faced massive power outages in the wake of Irma—and Maria may deprive the island of power for months.
Now, in less than a day, Maria has intensified from Category 1 to Category 5, battering Dominica at full strength, with Puerto Rico still in its sights. According to meteorologist Eric Holthaus, no Category 5 hurricane has struck Dominica since at least 1851. Prior to Maria, no Category 4 storm had made landfall in Puerto Rico since 1932.
“There are no words left to describe this season,” Holthaus said on Twitter hours before Maria slammed into Dominica.
DOES A MORE ACTIVE SEASON MEAN THAT MORE HURRICANES WILL HIT LAND?
Not quite. Forecasters caution that within a single year, there’s no solid relationship between the number of storms in a hurricane season and the number of landfalls. That’s because local weather conditions govern hurricanes’ specific paths to landfall—and forecasters can estimate those only a few days in advance.
That said, Emanuel notes that on 100-year timescales, the number of hurricanes and number of landfalls correlate. However, he emphasizes the year-to-year role of chance.
“Andrew, which occurred in 1992, was at the time the most expensive hurricane ever to hit the U.S., [and] that occurred in one of the quietest years we’ve seen the Atlantic, as a whole,” he says.
SO DO SEASONAL HURRICANE FORECASTS REALLY MEAN ANYTHING?
Yes, but they’re more useful to forecasters than the public, says Emanuel. He laments that people sometimes base key decisions, such as whether to purchase insurance, on forecasts calling for “quiet” seasons—despite the fact that even seasons with few hurricanes can yield highly destructive storms, such as 1992’s Hurricane Andrew. “The seasonal forecast is so widely misinterpreted that it’s actually counterproductive,” he says.
"While the seasonal forecasts are useful across a broad range of demographics, one can’t simply and solely determine their own personal preparation based on those seasonal forecasts," adds Hart, who also points to Andrew's example.
Forecasts predicting landfall for a current hurricane, however, are another matter.
“People who are potentially in the path of a hurricane really need to pay attention and absolutely need to follow direction of emergency managers,” Emanuel says. “If you’re told to get out, get out—don’t mess around.”
HOW DOES CLIMATE CHANGE FIGURE INTO THE PICTURE?
It’s complicated, but there’s reason to think that a changing climate will have at least some impact on hurricane season activity. (Discover how climate change likely strengthened Hurricane Harvey.)
"When it comes to next year’s hurricane activity seasonal prediction, anthropogenic impacts are not a primary concern, as the change due to anthropogenic effects from this year to next year is obviously small," says Hart. "However, in the decades and century to come, it could easily become the primary concern for driving hurricane activity."
The sweeping U.S. Climate Science Special Report, prepared ahead of the U.S. government’s 2018 National Climate Assessment, says that detecting climate change’s fingerprints on hurricane behavior is challenging. Because hurricanes are rare events, there aren’t many data points for scientists to examine for a trend.
That said, in coming decades, predictions based on warming suggest that average-intensity tropical cyclones—Atlantic hurricanes included—will likely get more intense. Emanuel adds that there’s “pretty good consensus” that high-intensity (Category 3, 4, or 5) hurricanes will also become more common in coming decades.
It's unclear whether the total number of hurricanes will increase or decrease. More than 70 percent of tropical cyclones worldwide are Category 1 or 2 storms, and these weaker storms may or may not become less common in future decades.
Emanuel and the report both say that on average, individual hurricanes will drop more precipitation in the future, since warmer air can hold more water vapor. We are likely beginning to see this act out today: Every scientist contacted by National Geographic for a previous story agreed that Hurricane Harvey’s record-breaking rain was almost certainly shaped by rising temperatures from human activity.
Future storm surges may also worsen, says Emanuel, partly because the intense hurricanes that cause them will be more numerous, and partly because of sea level rise.

Rico says WHAT