New York and its history of storms
New York City is no stranger to the effects of tropical storms and hurricanes. In fact, being located on something of a meteorological crossroads, lying in the zone where cold, Canadian Arctic air masses meet the warm Gulf Stream current, the Big Apple is in the firing line for both extreme winter storms and tropical cyclones.
One particularly notable storm that hit New York is the blizzard of 11 March 1888, which is considered one the USA's worst ever blizzards. As for hurricanes and tropical cyclones, a number of tropical cyclones have clipped New York as they worked their way northwards, three making a direct hit (or landfall) over New York City: the 1821 Norfolk and Long Island Hurricane, the 1893 New York Hurricane and Tropical Storm Irene in 2011. The 1938 New England Hurricane came very close, making landfall on nearby Long Island.
Meanwhile, several hurricanes and tropical storms have just clipped New York City, including Hurricane Agnes, which passed just west of New York in June 1972 and killed 24. Hurricane Hazel brought record-breaking gusts of 113 mph to Battery Park, Manhattan Island, in October 1954. More recently Tropical Storm Floyd brought 60 mph winds and flash flooding to New York City in September 1999, whilst Hurricane Irene made landfall on Coney Island in August 2011, sparking the first-ever mandatory evacuation of coastal residents as a precaution.
2012's Hurricane Sandy broke no wind or rainfall records in the Big Apple, but this massive hurricane proved one of the costliest ever to affect the USA. It brought winds up to 100 mph and widespread flooding from the associated storm surge. The surge flooded large parts of lower Manhattan, including subways and tunnels, caused mass power outages and destroyed thousands of homes and businesses, not just in New York but also in neighbouring New Jersey.
Before we look at how Sandy developed into one of New York City's most notorious visitors it's worth taking a closer look at some general aspects of tropical cyclones and hurricanes.
A tropical cyclone is the generic name given to a weather system over tropical or sub-tropical waters containing an organised area of thunderstorms, with cyclonic winds (anticlockwise in the northern hemisphere) around a low pressure centre. The tropical cyclone spectrum ranges from relatively small, weak storms called tropical depressions, with surface wind speeds less than 38 mph, to powerful hurricanes with surface wind speeds in excess of 160 mph. They are among the most dangerous natural hazards on earth and every year they cause considerable loss of life and damage to property.
Tropical cyclones typically start life over tropical oceans, forming when tropical thunderstorms are able to cluster and merge together in areas where the sea surface temperature is 27 ºC or more, where wind speed does not vary greatly with height and where winds near the ocean surface blow from different directions.
The sea provides a constant source of heat and moisture to 'fuel' the tropical cyclone. Winds near the ocean surface blowing from different directions help the warm, moist air rise and form cloud, and as wind speeds do not vary greatly with height, the cloud is able to grow into the giant thunderstorms.
When reaching land (known as 'making landfall'), tropical cyclones will quickly tend to weaken because their 'fuel source' has been cut off. They will also weaken if they move over areas of cooler sea. Or they can weaken if wind speeds near the upper parts of the tropical cyclone cloud increase - 'blowing' the tops of the cloud downstream, destroying some of the cyclone's organised structure and weakening it. Sometimes, however, a tropical cyclone will move away from the tropics and sub-tropics into the mid-latitudes and merge with existing mid-latitude weather systems. When this happens large and very powerful storms can form from the merger of the two systems.
There are various categories of tropical cyclone based on their wind speed. Weak tropical cyclones are called tropical depressions. When winds reach 39 mph they become known as tropical storms and they are then also given a name, which helps weather forecasters talk about them. Tropical cyclones can last more than a week and there can be more than one over any ocean at once, so giving them different names helps prevent confusion in weather forecasts. When winds reach 74 mph tropical storms over the Atlantic and north-east Pacific become known as hurricanes, and it is usually not until a storm becomes a hurricane that an 'eye' (an area of calm in the centre of a storm) becomes visible. In North America the Saffir-Simpson scale is used to categorise hurricane intensity - there are five categories and a hurricane is known as a 'major hurricane' if it reaches category 3 or higher.
Categories of tropical cyclone
Figure One (below) shows where Atlantic hurricanes tend to occur. They usually take place between early June and late November, though a few have been known in both May and December. The peak in the Atlantic hurricane season is mid-August to around mid-October. Climatologically a powerful hurricane tracking close the USA's eastern seaboard becomes more likely later in the summer and during the autumn; later in the year such storms will tend to be steered away north-eastwards into the Atlantic Ocean.
Typically tropical cyclones move forward at speeds of around 10 to 15 mph, though they can move both more slowly or much quicker, perhaps as fast as 40 mph under some circumstances. Movement can also be erratic, making forecasting their track even more challenging. A typical hurricane is around 300 to 400 miles in diameter, though as we shall see later they can be much bigger. The highest wind speeds will be wrapped around the core of the hurricane, extending out 25 to 50 miles from the core in smaller hurricanes, and 150 to 200 miles in larger ones.
The size of tropical cyclones is such that they will tend to steer around larger scale weather systems. In the case of Hurricane Sandy we shall see that this played an important role in determining her track.
Sandy started life as a cluster of thunderstorms which left western Africa on 11 October 2012 and moved westward to reach the Caribbean Sea on 18 October. This cluster of thunderstorms then gradually intensified to become a tropical storm on the 22nd. It moved towards Jamaica and on 24 October officially became a hurricane, called Sandy, just south of Jamaica. Sandy then moved across Jamaica, bringing with it winds up to 85 mph, before crossing eastern Cuba on the 25th. Sandy was at its most intense as it crossed eastern Cuba and moved towards the Bahamas, sustaining winds of around 115 mph. Sandy hit the Bahamas on the 26th and then weakened a little, briefly dropping back to a tropical storm before re-intensifying to a hurricane on the 27th. During the 26th and 27th Sandy was also able to grow much bigger in size whilst tracking almost parallel to the east coast of the USA.
Evolution of Hurricane Sandy
An area of high pressure developing over Ontario on the 28th spread eastwards on the 29th and 30th, and acted as a block to Sandy's path. Instead of curving north-eastwards into the Atlantic Ocean as many hurricanes do, Sandy was instead forced to turn north-westwards towards north-eastern USA. At the same time it interacted with a mid-latitude weather system which helped it to re-intensify and become much larger.
Sandy made landfall near Atlantic City, New Jersey, during the early evening of 29 October as one the most intense and damaging storms ever to affect the east coast of the USA. Sustained surface winds at landfall were close to 80 mph with gusts between 85 and 95 mph. After making landfall Sandy moved north-westwards, bringing heavy snow and blizzards to parts of the central Appalachian Mountains, and by the morning of 31 October no discernible storm centre could be found as the remnants of Sandy pressed on towards the Great Lakes and eastern Canada.
Impacts of Hurricane Sandy
As Sandy was so big, wind damage covered a much larger area than would usually be expected from a hurricane. A larger area of strong winds led to a larger than usual storm surge. Sandy's arrival into the US coast on the 29th also coincided with both high tide and spring tide, meaning that the tide would be at around its highest level. In New York City this added an extra 20 to 50 cm to the high water mark.
The extensive damage Sandy caused was the result of a number of unfortunate coincidences. It was able to grow particularly big, it was steered by the weather pattern developing over Canada, its landfall coincided with one of the highest tides of the month, worsening the impact of the storm surge, and it was pushed into the New York area rather than the less densely populated area further north.
Last updated: 23 May 2014