BEAUFORT WIND SPEED
-----------------
John Pazmino
NYSkies Astronomy Inc
www.myskies.org
nyskies@nyskies.org
2020 August 4
Introduction
-----------
Two tropical storms, so far in 2020, crossed the NYSkies region.
storm Fay passed by in July; Isaias, August.
Readers asked if there is a way to estimate the wind speed without
regular weather instruments. Readers want to do their own measurements
to follow the weather broadcasts or simply to amuse themselfs while
outdoors.
These is such a means, invented over two hundred years ago in
England. And it's still in use today by home astronomers and others
weather-widely people. It's the Beaufort eind speed scale, adapted
here from the wikipedia article aout it.
Beaufort scale
--------------
In 1805 Baeufort, officer in the British navy, devised a method of
estimating wind speed from the state of the sea. It was modified and
standardized over the centuries and is used worldwide to capture
approximate wind speed data in absence of proper instruments.
In 1923 the nautical scale was supplemented by a scale based on
conditions on land. Adjustments were made from time to time since
then.
The estimated wind speed is stated in several equivalent ways:
knots (~1.8 kilometer/hour), meter/second, kilometer/hour. There is an
other Beaufort scale that lists only the speeds for the several
classes of levels of wind, but with no criteria to assess the speed.
This scale is no good for observing wind speed.
Wind pressure
-----------
When wind hits a surface, face-on to it, it presses on the surface
with a force/area, in newton/meter2. The pressures in the table below
are the surcharge, above normal atmospheric pressure. If the back face
of the surface has still sir, like inside a building, the wind load is
an unbalanced pressure.
The nominal sea level atmospheric pressure is about 100
kilonewton/meter2. common structures like wood-frame, sheet metal,
plywood, brick & mortar can not withstand a surcharge by wind load of
even one0.01 atmosphere, some 1,000 newton/meter2. Ordinary variations
of air pressure, not caused by wind, are far less than this. This is
why so much structural damage occurs during strong winds.
pressure, force per area. is id the same as the kinetic energy
per volume. The nominal air density at sea level and 20C temperature
is 1.226kg/m3. This varies slightly with temperature, 'warm air rises,
cold air sinks'.
pressure = (air density) * (wind speed)^2 / 2
= (1.226 kg/m3) * (wind speed)^2 / 2
= (0.613 kg/m3) * (wind speed)^2
The units work out as:
N/m2 = kg/m3 * (m/s)^2
= kg.m2/m3.s2
= (kg.m/n3) * (m/s2)
= (kg.m/n3) * (N/kg ) =
= kg.m.N/kg.m3
= N/m2
I didn't fine a Beaufort table with this pressure. I worked it our
and added it to the table below.
The explicit statement of wind load makes it easier to picture the
effect of wind on people and structures. By dividing the newton/meter2
by 10N/kg, a rounded value for the Earth's gravity field strength, we
get a 'kilogram-force', what is red off of a weighing scale.
For a wind load of 40N/m2 we have a kilogram-force of 40kg/m2. An
adult human has a frontal area of about 3/4 square meter. Lie on your
back, belly up. Then this wind load on you is equivalent to a 30kg
weight (3/4 of 40kg), a large sack of sand, resting head to toe on
you. It may be hard to get up, specially if the sack is strapped to
you so it does not pull or slide off.
Beaurot wind speed table
----------------------
The rows for each level of wind speed are:
* Beaufort number and wind name
* Wind speed in knots, km/h, m/s (1 knot = ~1.8 km/h)
* Wave height in open sea and wind load or pressure
* Sea conditions for the wind level
* Land conditions for the wind level
------------------------------------
0 - Calm
< 1 knot, < 2 km/h, < 0.5 m/s
0 m, 0 N/m2
Sea like a mirror
Smoke rises vertically
- - - - -
1- Light air
1-3 knots, 2-5 km/h, 0.5-1.5 m/s
0-0.3 m, 0.1-1.4 N/m2
Ripples with appearance of fish scales are formed, without foam
crests.
Direction of wind shown by smoke drift but not by wind vanes.
- - - - -
2 - Light breeze
4-6 knots, 6-11 km/h, 1.6-3.3 m/s
0.3-0.6 m, 1.5-6.7 N/m2
Small wavelets still short but more pronounced; crests have a
glassy appearance but do not break.
Wind felt on face; leaves rustle; wind vane moved by wind.
- - - - -
3 - Gentle breeze
7-10 knots 12-19 km/h, 3.4-5.5 m/s
0.6-1.2 m, 7.1-18.5 N/m2
Large wavelets; crests begin to break; foam of glassy appearance;
perhaps scattered white horses.
Leaves and small twigs in constant motion; light flags extended.
- - - - -
4 - Moderate breeze
11-16 knots, 20-28 km/h, 5.6-7.9 m/s
1-2 m, 19.2-38.2 N/m2
Small waves becoming longer; fairly frequent white horses
Raises dust and loose paper; small branches moved.
- - - - -
5 - Fresh breeze
17-21 knots, 29-38 km/h, 8.0-10.7 m/s
2-3 m, 39.1-70.0 N/m2
Moderate waves taking a more pronounced long form; many white
horses are formed; chance of some spray.
Small trees in leaf begin to sway; crested wavelets form on inland
waters.
- - - - -
6 - Strong breeze
22-27 knots, 39-49 km/h, 10.8-13.8 m/s
3-4 m, 75.3-116.4 N/m2
Large waves begin to form; the white foam crests are more
extensive everywhere; probably some spray.
Large branches in motion; whistling heard in telegraph wires;
umbrellas used with difficulty.
- - - - -
7 - High wind, moderate gale, near gale
28-33 knots, 50-61 km/h, 13.9-17.1 m/s
4-5.5 m, 118.5-179.2 N/m2
Sea heaps up and white foam from breaking waves begins to be blown
in streaks along the direction of the wind; spindrift begins to be
seen.
Whole trees in motion; inconvenience felt when walking against the
wind.
- - - - -
8 - Gale, fresh gale
34-40 knots, 62-74 km/h, 17.2-20.7 m/s
5.5-7.5 m, 181.3-261.9 N/m2
Moderately high waves of greater length; edges of crests break
into spindrift; foam is blown in well-marked streaks along the
direction of the wind.
Twigs break off trees; generally impedes progress.
- - - - -
9 - Strong/severe gale
41-47 knots, 75-88 km/h, 20.8-24.4 m/s
7-10 m, 265.2-365.0 N/m2
High waves; dense streaks of foam along the direction of the wind;
sea begins to roll; spray affects visibility.
Slight structural damage (chimney pots and slates removed).
- - - -
10 - Storm, whole gale
48-55 knots, 89-102 km/h, 24.5-28.4 m/s
9-12.5 m, 368.0-494.4 N/m2
Very high waves with long overhanging crests; resulting foam in
great patches is blown in dense white streaks along the direction of
the wind; on the whole the surface of the sea takes on a white
appearance; rolling of the sea becomes heavy; visibility affected.
Seldom experienced inland; trees uprooted; considerable structural
damage.
- - - - -
11 - Violent storm
56-63 knots, 103- 117 km/h, 28.5-32.6 m/s
11.5-16 m, 497.9-651.5 N/m2
Exceptionally high waves; small- and medium-sized ships might be
for a long time lost to view behind the waves; sea is covered with
long white patches of foam; everywhere the edges of the wave crests
are blown into foam; visibility affected.
Very rarely experienced; accompanied by widespread damage.
- - - - -
12 - Hurricane force
ò 64 knots, ò 118 km/h, ò 32.7 m/s
ò 14 m, ò 655.5 N/m2
The air is filled with foam and spray; sea is completely white
with driving spray; visibility very seriously affected.
Devastation.
----------------
Note well that for sea conditions the criteria to assess wind
speed are the conditions of the actual sea, not those experienced on
the shop.. The observer's ship is has its own 'ground' speed that
combines with the wind speed. Measuring wind speed by an instrument on
the ship would give an erroneous reading.
The criteria for land observation assume the observer can stand
.still on the ground to make an assessment.
The scale is prolonged to levels 13-15 for interiors of hurricanes
and for tornados. These levels are likely not survived by observers to
ell about their experience.
Composite speeds
--------------
The typical application of the Beaufort scale in NYSkies is
hurricanes and tropical storms. In NYSkies the hurricane season is
June-November each year with usually one instance per year.
Occasionally we get no storms or we get several.
These storms rotate counterclockwise as they move along their
paths. The speed determined by from the Beaufort scale is the vector
addition of the ground speed of the storm and the circulating speed of
the air within the storm.
The sketch here shows the large range of potential wind speed over
an observer. North is up; east, right.
A
<-
/ \
/ \
/ \
D | /|\ ^
V | | B
\ | /
\ /
\ /
->
C
Hurricanes over NYSkies travel generally south to north, trending
along the east coast of the US. This ground path is severely modified
for each storm by geophysical forces we do not well understand, whence
the erratic predictions for the arrival and duration of these storms.
Staying with the northward movement, indicated by the large arrow
in the middle of the sketch, we have four cardinal cases of observed
wind speed.
A is the north, front, side of the storm. The circulating wind is
at right angles to the storm speed. The vector addition of the two
makes a moderate observed wind speed.
B is the east side, where the storm speed is parallel to the
circular speed. The vector addition has the maximum value. For an
example storm speed of 50 KPH and circular speed of 80 KPH, the
observer records a ground wind speed of 50+80 = 130 K{H. On the whole
when a storm center passes west of the observer he gets the strongest
wind and likely the worst damage from the storm.
C is the south, rear,side of the storm. The two speeds are at
right angles where their vector addition makes a moderate observed
ground wind speed.
D is the west side, where the two speeds are antiparallel. The
vector addition is a minimum value. For the two speeds used for case
A, the observer records a ground wind speed of 50-80 = -30 KPH, The
wind blows southward. This seems, for this instance, to be
inconsistent with the storm's northward motion. In general when a
storm passes to the east of an observer he gets the weakest wind,
sometimes well below hazardous values.
The observed wind speed is a composite of the forward speed of the
storm and the circular speed of the air within it. Weather forecasts
generally do not clearly state what their 'wind speed' is. Some may
give the circular speed from aerial or satellite monitoring of the
storm. . The stated speed may be the vector addition for a given
location, like that of the weather broadcast.
For noncircular storms, the nor'easters of NYSkies winter, there
is no definite decomposition of wind speed. The observed round speed
is the full statement of the wind speed.
Conclusion
--------
Many home astronomers heard of the Beaufort scale, perhaps from
camping or boating. Yet it is annoying to hunt up the table when high
winds are on the way. Some versions are weak, with sparse details or
obsolete with oldstyle measures. And so far i didn't find any with the
wind load.
The table above works for sea and land, has speed in three modes,
and the wind pressure. Keep it handy when ever you are in or have a
prospect on the outdoors