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as wind waves or sea. (Sea is also occasionally used
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to describe the combined motion of all the factors
disturbing the surface.) These waves tend to be a
chaotic mix of heights, periods, and wavelengths.
Because the wind causes the height to increase faster
than the wavelength, they often have relatively
steep, pointed crests and rounded troughs. With a
windspeed of 12 knots, the waves begin to break at
their crests and create foam.
The height of waves depends on three factors: windspeed,
length of time the wind blows over the water,
and the distance over which the wind acts on the water.
As waves move away from the area where they were
generated (called a fetch), they begin to sort themselves
by height and period, becoming regular and
evenly spaced. These waves often continue for thousands
of miles from where they were generated. Swell
is the term describing waves that persist outside the
fetch or in the absence of the force that generated them.
Aswell may be large or small, and does not indicate the
direction of the wind. The wake of a boat or ship is also
a swell.
Unlike wind and current, waves are not deflected much
by the rotation of the Earth, but move in the direction
in which the generating wind blows. When this wind
ceases, water friction and spreading reduce the wave
height, but the reduction takes place so slowly that a
swell persists until the waves encounter an obstruction,
such as a shore. Swell systems from many different
directions, even from different parts of the world, may
cross each other and interact. Often two or more swell
systems are visible on the surface, with a sea wave system
developing due to the current wind.
In lakes and sheltered waters, it is often easy to tell
wind direction by simply looking at the water’s surface.
There is usually a strip of calm water along the
upwind shore of a lake. Waves are perpendicular to the
wind direction. Windspeeds above approximately eight
knots leave wind streaks on the water, which are parallel
to the wind.
Land masses sculpt and channel the air as it moves over
them, changing the wind direction and speed. Wind
direction may change dramatically from one part of a
lake or bay to another, and may even blow in opposite
directions within a surprisingly short distance. Always
pay attention to the various wind indicators in the area,
especially when setting up for takeoff or landing.
Terms Used
by U.S.
Weather Service
Calm
Light Air
Light
Breeze
Gentle
Breeze
Moderate
Breeze
Fresh
Breeze
Strong
Breeze
Moderate
Gale
Velocity
m.p.h.
Less than 1
1 - 3
4 - 7
8 - 12
13 - 18
19 - 24
25-31
32-38
Estimating Velocities
on Land
Smoke rises vertically.
Smoke drifts; wind
vanes unmoved.
Wind felt on face; leaves
rustle; ordinary
vane moves by wind.
Leaves and small twigs
in constant motion;
wind extends light flag.
Dust and loose paper
raised; small branches
are moved.
Small trees begin to
sway; crested wavelets
form in inland water.
Large branches in motion;
whistling heard in telegraph
wires; umbrellas used
with difficulty.
Whole trees in motion;
inconvenience felt in
walking against the wind.
Sea like a mirror.
Ripples with the appearance
of scales are formed but
without foam crests.
Small wavelets, still short
but more pronounced; crests
have a glassy appearance
and do not break.
Large wavelets; crests begin to
break. Foam of glassy appearance.
(Perhaps scattered whitecaps.)
Small waves, becoming longer;
fairly frequent whitecaps.
Moderate waves; taking a more
pronounced long form; many
whitecaps are formed.
(Chance of some spray.)
Large waves begin to form;
white foam crests are more
extensive everywhere.
(Probably some spray.)
Sea heaps up and white foam
from breaking waves begins
to be blown in streaks along
the direction of the wind.
Estimating Velocities
on Sea
Check your glassy water
technique before water flying
under these conditions.
Ideal water flying
characteristics in
protected water.
This is considered rough
water for seaplanes and
small amphibians,
especially in open water.
This type of water condition
is for emergency only in small
aircraft in inland waters and
　
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