Sunday 27 August 2017

Hurricane Harvey causes at least three deaths in Texas.

Two people have now been confirmed dead after Hurricane Harvey made landfall near the city of Corpus Christi in Texas on Friday 25 August 2017. One of these deaths occurred in a house fire in the seaside town of Rockport in Aransas County, which has been particularly badly hit by the storm, and the others occurred in Houston, where a woman and child apparently drowned after leaving her car when it became trapped in floodwaters. 

High winds in Corpus Christi, Texas, as Hurricane Harvey made landfall on Friday 25 August 2017. Courtney Sacco/Caller-Times/USA Today.

Hurricane Harvey was a Category 4 Hurricane when it made landfall, making it the biggest storm to make landfall in the US since Hurricane Charley hit Florida in 2004, and the most powerful Hurricane to hit Texas since Hurricane Carla hit the coast near Galveston in 1961. A Category 4 Hurricane is defined as a storm with sustained winds in excess of 181 kilometres per hour, with speeds as high as 210 being recorded near Corpus Christi on Friday. Hurricanes lose energy and eventually dissipate over land, and since making landfall Harvey has dropped to a Category 2 Tropical storm (storm with sustained winds in excess of 154 kilometres per hour), still enough to cause severe problems.

The path and strength of Hurricane Harvey. Thick line indicates the past path of the storm (till 9.00 am GMT on Sunday 27 August 2017), while the thin line indicates the predicted future path of the storm, and the dotted circles the margin of error at six and twelve hours ahead. Colour indicated the severity of the storm. Tropical Storm Risk.

Despite the obvious danger of winds of this speed, which can physically blow people, and other large objects, away as well as damaging buildings and uprooting trees, the real danger from these storms comes from the flooding they bring. Each drop millibar drop in air-pressure leads to an approximate 1 cm rise in sea level, with big tropical storms capable of causing a storm surge of several meters. This is always accompanied by heavy rainfall, since warm air over the ocean leads to evaporation of sea water, which is then carried with the storm. These combined often lead to catastrophic flooding in areas hit by tropical storms.

 Damage to mobile homes on the Coastal Bend region of Texas on Saturday 26 August 2017. Gabe Hernandez/AP.

Hurricane Harvey has caused extensive direct damage in the Coastal Bend region around Corpus Christi, particularly in Rockport where many homes have been destroyed, but has caused flooding-related problems over a much wider area, with rainfall in excess of 75 cm falling within 24 hours in parts of Texas, and flooding occurring as far away as Houston, over 300 km from the area where the storm made landfall. Flooding has become a particular problem in rapidly expanding areas like Houston (now the USA's fourth largest city), which is built on an area of historic coastal wetlands. Samuel Brody of the Center for Texas Beaches and Shores at Texas A&M University at Galveston has calculated that the area of soil covered by paving in Houston has expanded by 25% in the last 15 years, preventing rainwater from draining into the soil and raising the dangers of flooding in the city.
Damaged buildings and flooding in Rockport, Texas in the aftermath of Hurricane Harvey. Joe Raedle/Getty Images.

Tropical storms are caused by the warming effect of the Sun over tropical seas. As the air warms it expands, causing a drop in air pressure, and rises, causing air from outside the area to rush in to replace it. If this happens over a sufficiently wide area then the inrushing winds will be affected by centrifugal forces caused by the Earth's rotation (the Coriolis effect). This means that winds will be deflected clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere, eventually creating a large, rotating Tropical Storm. They have different names in different parts of the world, with those in the northwest Atlantic being referred to as hurricanes.

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