Cumulus clouds are thick with vertical during the day when the surface is heated
development. Their upper surfaces are dome significantly and then dissipate at night when
shaped and exhibit protuberances, while the the land cools.
base is nearly horizontal.
Temperature and Humidity
Cumulonimbus clouds are composed of
heavy masses with great vertical develop-
Although not particularly useful alone,
ment. The summits rise in the form of moun-
when used in conjunction with other weath-
tains or towers, and the upper parts have a
er indicators, variations in temperature and
fibrous texture and often spread out in the
humidity may provide you with some clues
shape of an anvil.
to forthcoming changes in weather. A heavy
The extent of cloud cover is sometimes
dew or frost indcateds that the sky was clear
expressed on a scale of zero to eight. These
during the night, but this alone does not
numbers represent eighths of sky covered and
necessarily mean that good weather will
are called oktas. Zero represents a sky that
continue throughout the day. Haze prob-
is free of clouds; eight represents an entirely
ably indicates the presence of a temperature
overcast sky with no patches of visible blue.
inversion (the air is colder at the surface than
Interpreting Clouds: Of all cloud types,
aloft) and is generally caused by sinking air
pay particular attention to cirrus and cir-
around the perimeter of a high-pressure
rostratus. These clouds can be your “early
area. This often occurs during dry summer
warning system,” the first precursors of an
weather and indicates fair weather attending
advancing warm front associated with a
the high pressure.
low-pressure system that may still be hun-
Sea Surface Temperature: Because the sea
dreds of miles away.
surface is in direct contact with the overlying
Extended and well-defined cirrus contrails
air mass, sea temperature is particularly im-
indicate very cold upper-level air. High cirrus
portant in meteorology. Water temperatures
clouds whose movement across the sky can
change more slowly than air, so water tem-
be detected without reference to a station-
perature can be used as a short-term “con-
ary object indicate 100-knot (or greater) jet
stant.” The difference between sea and air
r
stream winds. Winds of this speed can bring
temperatures can be a useful guide in identi-
mixed air and rapid changes within the at-
fying air masses. Polar air is colder than the
mosphere. If fine cirrus clouds increase and
sea; tropical air is warmer.
e
athe
thicken into a sheet and form a halo around
When the air temperature is rising and the
W
the sun or moon, a high probability exists
sea is calm, the surface water temperature
that inclement weather will follow within 12
may be several degrees warmer than water
to 24 hours. This prognosis is more reliable if
a few feet deep. When the air temperature is
falling and/or there are waves or tidal cur-
r
the barometer is falling steadily and the wind
changes direction and freshens.
rents, the water temperature will be uniform
Small- or medium-sized cumulus clouds
from the surface downward. Waves and cur-
e
athe
form when air is lifted in response to heating
rents mix the water, and as the surface water
from the surface land, or water, and indicate
cools, it becomes denser and sinks, which
fair weather. Towering cumulus clouds are
assists in the mixing.
formed in response to pronounced surface
If you don’t have a through-hull tempera-
heating and an abundance of rising hot air.
ture sensor, you must draw a water sample
They can produce showers and squalls. In
to determine the surface water temperature.
summer, ragged and vertically developed
Draw water in a nonconductive canvas
r
standing W
cumulonimbus clouds with very high tops
or rubber bucket from over the side (not
through your ship’s plumbing). Read the
nde
often indicate thunderstorms.
U
There is not much change in sea-surface
temperature as soon as you’ve drawn the
temperature between day and night; there-
water and be sure your thermometer is fully
fore, large, active cumulus clouds can form
immersed. Most thermometers will give ac-
over the ocean whenever the sea surface is
curate readings within 30 seconds of draw-
warm enough to lift the air above it. In con-
ing the water.
trast, cumulus clouds often form over land
Identifying Currents with a Thermom-
eter: You can use sea temperature as a useful
guide to identifying warm and cold currents.
795