1.3 Hurricanes and storms
Of quite different origin are those disasters associated with the weather:
They too are interrelated, a hurricane being a vast area of low atmospheric pressure, which effectively grows into a giant vortex sometimes hundreds of miles wide (Figure 1). A storm is a smaller version of the same phenomenon, still associated with low pressure – a depression.
Both can be created when hot tropical seas give birth to a column of warm air saturated with water vapour. When such a column cools, the heat released by condensation of the water vapour further warms the air and provides a positive feedback loop for yet further growth of the storm.
Tornadoes are a more local phenomenon, a vortex of air spinning down from thunderclouds and, as yet, of inexplicable origin. Condensation of water vapour gives rise to growth of water droplets and hence rain, which in the tropics can be intense and long lasting from hurricanes or typhoons, so giving rise to floods of greater or lesser severity.
Although such phenomena are little understood, there are ways, at least, of classifying their severity, such as the Beaufort scale for wind speed (see Input 2, linked below).
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The hurricane that struck Galveston, Texas on 8 September 1900 is the worst natural disaster ever in the United States for the loss of life and destruction of property. It hit land during the morning and grew in intensity as the afternoon came, with winds of 84 miles per hour measured on anemometers at 5 pm. Beach homes had at this stage been washed away by giant combers from the sea. With nightfall, wind speed had increased to more than 120 mph, with a maximum of up to 150 mph. The winds only died down at about 10 pm, by which time most of the structural damage had occurred.
The US Weather Bureau and local meteorologists believed they could predict the track of the hurricane. They said the storm would veer away from the city, but they were wrong.
Half of the buildings in the city were totally destroyed (Figures 2 and 3) and between 6000 and 10 000 people lost their lives. The clapboard houses of the city were torn away by the winds during the event. The intense low pressure at its centre created a surge of the sea that inundated the stricken city, so compounding the disaster.
Using the pressure data provided in Table C1 from Input 2, estimate the maximum wind pressure likely to be produced at the height of the Galveston hurricane. Either draw a graph from the data provided, or identify the relation between the two variables concerned by calculation.
The relationship between wind speed s measured in mph at 33 feet and pressure p measured in lbf ft−2 can be estimated from Table C1 simply by drawing the curve of the one variable against the other, as shown in Figure 4 below.
It is clearly not a linear relation, so try the following equation.
Let p = k sn, a simple power law relation, a reasonable relation because pressure is zero when the wind speed is zero, and pressure grows faster than speed. k is a constant, hence
The value of n can be estimated by plotting log p against log s as shown in Figure 5.
The graph is a straight line, with a gradient of 14 / 7.5 or about 2 and the equation is thus p ≈ ks2.
Using the relation developed in the previous question, estimate the pressure exerted by the maximum wind experienced during the hurricane at Galveston. Estimate the force exerted on a one-storey, low-cambered-roof, clapboard house with a side 50 feet long and 10 feet high.
Assume the wind acts at right angles to the windward side of the house. The side of the house is supported by 4-inch by 4-inch vertical posts spaced evenly at 2.5-foot intervals and inserted into the foundations. A post will snap off when subjected to a horizontal force of 1000 lbf ft−2 delivered 5 feet above foundation level. Predict the failure mode of the house.
Using Figure 5 developed in the previous question, then for a maximum wind speed of about 150 mph
so extrapolating onto the straight line on the figure gives
The force acting on a flat area a is given by
If there are vertical supports every 2.5 feet, there will be 21 supports along a 50 foot stretch of wall, so if the force is exerted uniformly, then the bending force per support will be about
As this exceeds the strength in bending of the support, the wall will be blown over, probably by breakage at the foundations. If the wall is made from overlapping slats, they may be stripped first if the connections between them are weak. The wall therefore falls in either case, the roof and other walls failing progressively.
As a direct result of the power of the winds, more than 2600 houses were totally destroyed (Figure 6), with an estimated 10 000 people made homeless. One reason for the high level of destruction was the low-lying position of the port, with most of the city built on an island off the coast.
Yet the port was reconstructed quickly from the remains of the old, although many protective measures were then incorporated into structures. Many buildings were raised seven feet to give greater height above the sea level, and a sea wall built to limit the effects of any future storms.
Prediction of storms and hurricanes is fraught with difficulty, but once a hurricane is formed, it can be tracked closely and warnings issued to communities likely to be in its path (Figure 7 shows the path of the Galveston hurricane of 1902). The prediction of an exact path is still a problem. Large areas are now cleared ahead of a detected hurricane in the predicted path, although a hurricane often behaves unpredictably.
The historical frequency of these weather phenomena varies in a way that is presently not well understood, although some climatologists suggest that storm frequency may rise, apparently due to global warming. The control of storms is correspondingly impossible, because of the sheer size of even small storms.
By contrast, disasters associated with manufactured structures or machines is potentially soluble, if for no other reason than that human beings can make rational choices about their design, maintenance and use.