Hurricane Barry is hitting Louisiana as the first hurricane of 2019. Here's why storms are getting stronger, slower, and wetter.

FILE - This file satellite image provided by NASA and taken by U.S. astronaut Christina Koch on Thursday, July 11, 2019 at the International Space Station, shows Tropical Storm Barry as it bears down on Texas, Louisiana, Alabama and the panhandle of Florida as it makes its way through the Gulf of Mexico. Barry could harm the Gulf Coast environment in a number of ways. But scientists say it’s hard to predict how severe the damage will be. (Christina Koch/NASA via AP, File)NASA via Associated PressA satellite image taken by US astronaut Christina Koch on July 11, 2019 at the International Space Station, shows Tropical Storm Barry over the Gulf of Mexico.

Hurricane Barry is moving over the coast of Louisiana – the first hurricane of 2019 and only the third time in the last 168 years that such a storm has hit the Gulf region in July.

Barry threatens to dump up to 20 inches of rain along coastline, with 25 inches expected in some places.

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A tropical storm in the Gulf of Mexico is approaching Louisiana. It will put New Orleans’ river levees to the test.

Scientists can’t definitely say whether Barry was directly caused by climate change, but they agree that warming overall makes storms and hurricanes more devastating than they would otherwise be.

That’s because higher water temperatures lead to sea-level rise, which increases the risk of flooding during high tides and in the event of storms surges. Warmer air also holds more atmospheric water vapour, which enables tropical storms to strengthen and unleash more precipitation.

Here’s what to know about why storms are getting so much stronger, wetter, and slower.

How a hurricane forms

Hurricanes are vast, low-pressure tropical cyclones with wind speeds over 74 mph.

In the Atlantic Ocean, the hurricane season generally runs from June through November, with storm activity peaking around September 10. The storms form over warm ocean water near the equator, when sea surface temperature is at least 80 degrees, according to tthe National Hurricane Center.

As warm moisture rises, it releases energy, forming thunderstorms. As more thunderstorms are created, the winds spiral upward and outward, creating a vortex. Clouds then form in the upper atmosphere as the warm air condenses.

How hurricane forms infographicShayanne Gal/Business Insider

As the winds churn, an area of low pressure forms over the the ocean’s surface. At this point, hurricanes need low wind shear – or a lack of prevailing wind – to form the cyclonic shape associated with a hurricane.

Once the wind speed hits 74 mph, the storm is considered a Category 1 hurricane on the Saffir-Simpson scale. Hurricane Barry is now a Category 1 storm.

Saffir simpson hurricane scaleSamantha Lee/Business Insider

Hurricanes are moving more slowly and dropping more rain

Hurricanes use warm water as fuel, so once a hurricane moves over colder water or dry land, it usually weakens and dissipates.

However, because climate change is causing ocean and air temperatures to climb – last year was the hottest on record for the planet’s oceans – hurricanes are getting wetter and more sluggish. Over the past 70 years or so, the speed of hurricanes and tropical storms has slowed about 10% on average, according to a 2018 study.

But over land in the North Atlantic and Western North Pacific specifically, storms are moving 20% to 30% more slowly, the study showed.

Hurricane harveyDavid J. Phillip/APHouston residents navigate streets in boats due to flooding during Hurricane Harvey.

A slower pace of movement gives a storm more time to lash an area with powerful winds and dump rain, which can exacerbate flood problems. So its effects can wind up feeling more intense.

Hurricane Harvey is a prime example of this: After it made landfall, Harvey weakened to a tropical storm, then stalled for days. That allowed the storm to dump unprecedented amounts of rain on the Houston area – scientist Tom Di Liberto described it as the “storm that refused to leave.”

Climate scientist Michael Mann previously wrote on Facebook that Hurricane Harvey – which flooded Houston, killed more than 100 people, and caused $US125 billion in damages – “was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage, and a larger storm surge.”

Hurricane Barry, similarly, could dump up to 25 inches of rain onto some parts of Lousiana this weekend.

St. Bernard Parish Sheriff's Office inmate workers move free sandbags for residents in Chalmette, La., Thursday, July 11, 2019 ahead of ahead of Tropical Storm Barry from the Gulf of Mexico. (AP Photo/Matthew Hinton)Associated PressSt. Bernard Parish Sheriff’s Office inmate workers move free sandbags for residents in Chalmette, Louisiana, July 11, 2019 ahead of Hurricane Barry.

To make matters worse, a warmer atmosphere can hold more moisture, so a 10% slowdown in a storm’s pace could double the amount of rainfall and flooding that an area experiences. The peak rain rates of storms have increased by 30% over the past 60 years – that means up to 4 inches of water can fall in an hour.

“Precipitation responds to global warming by increasing,” Angeline Pendergrass, a project scientist at University Corporation for Atmospheric Research, said last year.

Storms are also getting stronger

A hurricaneFlickr/NASA’s Marshall Space Flight CenterA hurricane as seen from space.

As ocean temperatures continue to increase, we’re also likely to see more severe hurricanes because a storm’s wind speed is influenced by the temperature of the water below. A 1-degree Fahrenheit rise in ocean temperature can increase a storm’s wind speed by 15 to 20 miles per hour, according to Yale Climate Connections.

“With warmer oceans caused by global warming, we can expect the strongest storms to get stronger,” James Elzner, an atmospheric scientist at Florida State University, told Yale.

That can also mean that storms are able to intensify and develop into powerful hurricanes in a shorter time span.

Generally, a strong storm brings a storm surge: an abnormal rise of water above the predicted tide level. This wall of water can flood coastal communities – if a storm’s winds are blowing directly toward the shore and the tide is high, storm surges can force water levels to rise as rapidly as a few feet per minute.

So higher sea levels, of course, mean more destructive storm surges during a hurricane. Even if we were to cut emissions dramatically starting today, some sea-level rise is inevitable, since the planet’s oceans absorb 93% of the extra heat that greenhouse gases trap in the atmosphere. Water (like most things) expands when it’s heated, so even small changes in temperature results in a larger water volume.

Kevin Loria contributed to an earlier version of this story.

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