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22 How do I convert from mph to knots (to m/s) and from inches of mercury to mb (to hPa)?




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This article is from the Storms FAQ, by Chris Landsea landsea@aoml.noaa.gov with numerous contributions by others.

22 How do I convert from mph to knots (to m/s) and from inches of mercury to mb (to hPa)?

For winds:  1 mile per hour (mph) = 0.864 knots (kt)
            1 mph = 1.609 kilometers per hour (kph)
            1 mph = 0.4470 meters per second (m/s)
            1 kt = 1.853 kph
            1 kt = 0.5148 m/s
            1 m/s = 3.600 kph

For pressures: 1 inch of mercury = 33.86 mb = 33.86 hPa

For distances: 1 ft = 0.3048 m

Subject: 41) How does the damage that hurricanes cause increase as a function of wind speed?

Or to rephrase the question: Would a minimal 74 mph hurricane cause one
half of the damage that a major hurricane with 148 mph winds? No, the
amount of damage (at least experienced along the U.S. mainland) does not
increase linearly with the wind speed. Instead, the damage produced
increases exponentially with the winds. The 148 mph hurricane (a category
4 on the Saffir-Simpson Scale) may produce - on average - up to 100
times the damage of a minimal category 1 hurricane!

Landsea (1993) analyzed the damage caused by various categories of
tropical storms and hurricanes after normalizing by both the inflation
rate and population changes. Tropical cyclones from 1944 through 1990
were tabulated in terms of 1990 U.S. dollars. The following table
summarizes the findings:

Intensity (cases)                   Median Damage       "Potential Damage"
Tropical/Subtropical Storm (75)       <$1,000,000               0
Hurricane Cat. 1 (34)                 $24,000,000               1
Hurricane Cat. 2 (14)                $218,000,000              10
Hurricane Cat. 3 (24)              $1,108,000,000              50
Hurricane Cat. 4 (6)               $2,274,000,000             100
Hurricane Cat. 5 (1)               $5,933,000,000             250

The "Potential Damage" values just provide a reference value if one assigns
the median damage caused by a category 1 hurricane to be "1". The rapid
increase in damage as the categories go up is apparent.

Note that this study was done in mid-1992 (i.e. before Andrew) and thus
the median and potential damage values for the category 4 and 5
hurricanes may be on the conservative side.

Other interesting findings:

* Mean annual damage in mainland US is $1,857,000,000. (Again, this value
is pre-Andrew.)

* The damage is nearly evenly divided between that caused on the US Gulf
Coast (Florida panhandle to Texas) and the US East Coast (Florida
peninsula to Maine).

* Even though the intense hurricanes (the category 3, 4 and 5 storms)
comprise only 20% of all US landfalling tropical cyclones, they account
for 71% of all of the damage. (Again, the figure is pre-Andrew. With
Andrew included, the damage percentage is likely 75 to 80%.)

 

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