2008 REED’s East Coast Edition - Weather
Weather Instruments
tate at a rate that corresponds to wind speed.
Your observation of clouds and sea state,
The RPMs are translated electronically into
obviously, is visual. The basic instruments
wind velocity that can be read in a vessel’s
for observing the other indicators are a
cockpit or navigation station. Some an-
barometer, a windvane/anemometer, a ther-
emometer and windvanes on moving vessels
mometer, and a hygrometer. You should
measure relative or apparent, not true, wind
always obtain information from several of
speed and direction. When using their read-
these instruments and you should never
ings in weather analysis, convert the readings
depend on single observations or read-
to true wind speed and direction.
ings. Multiple readings over time show the
Thermometer: The liquid in common
amount and rate of change, which are more
liquid-in-glass thermometers (either mer-
accurate indicators of weather trends than a
cury or alcohol) expands when heated and
single reading. Changes in barometric read-
contracts when cooled to a greater extent
ings are especially important. A prudent nav-
than the glass that contains it. You sight the
igator will enter barometric readings, wind
expansion and contraction on the Fahrenheit
direction and strength, cloud characteristics,
or Celsius scale (see the table on page 346
temperature, sea state, and possibly humidity
for Fahrenheit and Celsius conversions).
into the log at regular intervals.
Bimetal thermometers indicate the differing
thermal expansion rates of two thin strips of
r
Barometer: A barometer measures at-
mospheric pressure. A mercury barometer
dissimilar metals welded together. The feel
consists of a tube containing mercury in a
of hot or cold air on your skin is not a reli-
able measure of temperature or its change
e
athe
vacuum. The level of the mercury rises and
falls as the atmospheric pressure increases
because humidity and wind speed affect that
and decreases. The height of this column of
feel and distort your senses.
mercury is measured in inches or millibars
Hygrometer: A hygrometer measures the
(see the barometric conversion table on
water content of air using two thermom-
page 342).
eters—one containing a dry bulb and the
other a wet bulb. As water evaporates from
r
standing W
Most modern barometers are “aneroid”
models, which means there is no fluid in-
the surface of the wet bulb, heat is extracted
and that thermometer reads less than the
nde
volved. An aneroid barometer thus contains
dry-bulb thermometer. The difference in
U
no mercury; its indicator is moved directly
by atmospheric pressure. The works of this
readings indicates the humidity, or damp-
type of barometer consist of a bellows made
ness, of the air. When the air contains a rela-
of thin corrugated metal in a “box,” from
tively high amount of water, the temperature
which almost all the air has been removed.
difference between the wet and dry bulb
The pressure exerted by the atmosphere on
thermometers is slight because little or no
the outside of the box is magnified by a series
evaporation is occurring at the wet bulb.
of levers that activate a pointer on a dial. The
When an air mass is saturated (100 percent
readout is expressed in inches or millibars,
humidity), a drop in temperature will cause
just as in the case of a mercury barometer.
the water vapor in the atmosphere to con-
You don’t need to correct an aneroid ba-
dense, forming rain, snow, or fog. The point
rometer for temperature or gravity, but you
at which the temperature is cool enough so
should correct it for height above sea level
that condensation begins is called the dew
when you’re using it on a body off fresh wa-
point. Using the dew-point table, you can
ter that’s not at sea level. Otherwise, to deter-
determine what decrease in temperature for
mine if your barometer’s reading is correct,
a given humidity level will cause rain, snow
compare it to known sea-level pressure (local
or fog.
weather forecasts provide sea-level baromet- Forecasting Rules of Thumb
ric pressure). If your barometer’s reading
Fair Weather: When the barometer is
does not match, adjust the indicator by turn-
steady or rising at a steady rate, and very high
ing a screw on the back of the instrument
cirrostratus clouds (mares’ tails) or cumu-
case using a thin-ended screwdriver.
lus clouds are present, you can expect fair
Windvane/anemometer: The typical ane-
weather to continue for a while (see the color
mometer consists of an impeller formed by a
plates on clouds beginning on page 301).
set of “cups” that scoop up the wind and ro-
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