KNOTS, PLUS OTHER WIND INFORMATION
 
METEOROLOGIST JEFF HABY
It seems strange that a knot is so close to miles per hour. There are 1.15 miles per hour in 1.00 knots. Why is
this unit needed when we already have miles per hour? Who invented knots? For what purpose were knots invented?
Knots are the unit of wind measurement given on weather charts. Why then are they expressed
as miles per hour to the general
public? Why not just keep them as knots? Is it because the average person knows what a mile per hour is but has less
of an understanding of knots? I'm contemplating the reaction a broadcast meteorologist would get if they suddenly
began expressing wind speed in knots. Why are wind speeds over water expressed in knots while they are expressed
in miles per hour over land? This is probably due to land vehicles giving speed in miles per hour and water going
vehicles giving speeds in knots. Also, the units of knots were invented over a water surface to help with navigation.
Being able to convert miles per hour to knots and vice versa is important.
The surface wind speed in knots is multiplied by 1.15 to get the speed in miles per hour. A 100 knot wind is equal
to 115 miles per hour while a 10 knot wind is equal to 11.5 miles per hour. Therefore, the stronger the wind the
greater the difference between the numerical value of miles per hour and knots. Here are some of the answers to
the questions posed above. A knot is exactly equal to 1/60th of a degree of latitude. A 1/60th of a degree of
latitude is known as a minute of latitude. Therefore a knot is equal to one minute of distance. There are 90 degrees
from the equator to the pole. Therefore a knot is 1/5,400th the distance from the equator to the pole. This number
is found by multiplying 90 (degrees from equator to pole) * 60 (number of minutes in a degree of latitude). The nautical
mile is based on how our system of ocean navigation evolved, using a circle of 360 degrees, with each degree divided
into 60 minutes and 60 seconds. At sea, one's position is always based on degrees, minutes and seconds, so therefore
how fast you traverse the ocean is based on "minutes" traveled per unit time (hour); hence, the nautical mile per
hour or knot. In centuries past, mariners determined the speed of their ships using a knotted "log line." The buoyant
line was let out freely as the ship sailed along, and then the number of knots let out during a given time gave
the shipmaster a measure of his vessel's speed.
VECTORS: Many graphical model outputs denote the wind speed at all
levels in the troposphere by the use of
wind vectors. The longer the winds
vector, the stronger the wind. The
disadvantage is that exact wind speeds are not given. It can however, give you a relative sense to which locations
are experiencing strong or weak wind. Wind vectors can be used to analyze
vorticity advection,
divergence, convergence, confluence, and diffluence. These all have important meteorological implications.


