12
Spectrometers imaging through thermal windows in the infrared (IR) band,
through which radiation escapes absorption by greenhouse gases, provide a skin
temperature representing the uppermost molecular boundary water layer with
depths measured in micrometers. Vicarious calibration to in situ temperature measurements taken by thermistor-equipped fixed and drifting data buoys (see below) is
employed to derive equivalence to bulk temperature, the temperature of the topmost
0.5–2.0 m from which approximate depth buoy measurements are performed. Skin
to bulk temperature differentials average 0.3 °K but range between −1.4 and + 1.2
and depend on solar irradiance (time of day), wind conditions, and cloud cover
(Emery et al. 1995).
2.2.2 Seawater Pressure
Pressure measurements are performed to estimate depth and to record waves and
tides. Initially, commercial electronic pressure sensors employed strain-gauge
transducers incorporating a bonded metallic wire grid whose resistance varies with
applied stress. Instrumental resolution improved markedly upon the introduction in
1972 of the quartz crystal resonator, a sensor whose frequency of oscillation varies
with strain applied to the crystal. Modern CTDs incorporate either type of sensor
but the crystal resonator offers higher precision and decreased hysteresis.
Pressure sensors are also used for wave and tide measurements but in neither
case are they the primary choice for routine operational observations. Rather they
are favored for research and survey in specialized applications such as surf zone
deployments.
2.2.3 Ocean Currents
Coastal ocean currents normally span speeds to about 5 knots (2.57 m.s
−1
). However,
nearshore currents can achieve significantly higher speeds in narrow inlets and
along prominent capes, especially under the influence of large amplitude tides.
Tidal bores have been reported at speeds of up to 15 knots. The Bay of Fundy in
eastern Canada is an outstanding example. Oceanic mass transport is measured in
Sverdrup units (Sv) representing millions of cubic meters per second. The Florida
current for example averages 30 Sv, transporting most of the waters exiting the
Caribbean Sea and Gulf of Mexico through the relatively narrow Florida Straits
between Florida and the Bahama Islands. Surface velocities in certain locations of
the Straits can exceed 5 knots.
Historically, two methods have been used for direct current measurement both
named after prominent eighteenth-century mathematicians: the Eulerian system
relying on fixed-point measurements of current strength and direction and the
2 Electronic Sensors and Instruments for Coastal Ocean Observing
Spectrometers imaging through thermal windows in the infrared (IR) band,
through which radiation escapes absorption by greenhouse gases, provide a skin
temperature representing the uppermost molecular boundary water layer with
depths measured in micrometers. Vicarious calibration to in situ temperature measurements taken by thermistor-equipped fixed and drifting data buoys (see below) is
employed to derive equivalence to bulk temperature, the temperature of the topmost
0.5–2.0 m from which approximate depth buoy measurements are performed. Skin
to bulk temperature differentials average 0.3 °K but range between −1.4 and + 1.2
and depend on solar irradiance (time of day), wind conditions, and cloud cover
(Emery et al. 1995).
2.2.2 Seawater Pressure
Pressure measurements are performed to estimate depth and to record waves and
tides. Initially, commercial electronic pressure sensors employed strain-gauge
transducers incorporating a bonded metallic wire grid whose resistance varies with
applied stress. Instrumental resolution improved markedly upon the introduction in
1972 of the quartz crystal resonator, a sensor whose frequency of oscillation varies
with strain applied to the crystal. Modern CTDs incorporate either type of sensor
but the crystal resonator offers higher precision and decreased hysteresis.
Pressure sensors are also used for wave and tide measurements but in neither
case are they the primary choice for routine operational observations. Rather they
are favored for research and survey in specialized applications such as surf zone
deployments.
2.2.3 Ocean Currents
Coastal ocean currents normally span speeds to about 5 knots (2.57 m.s
−1
). However,
nearshore currents can achieve significantly higher speeds in narrow inlets and
along prominent capes, especially under the influence of large amplitude tides.
Tidal bores have been reported at speeds of up to 15 knots. The Bay of Fundy in
eastern Canada is an outstanding example. Oceanic mass transport is measured in
Sverdrup units (Sv) representing millions of cubic meters per second. The Florida
current for example averages 30 Sv, transporting most of the waters exiting the
Caribbean Sea and Gulf of Mexico through the relatively narrow Florida Straits
between Florida and the Bahama Islands. Surface velocities in certain locations of
the Straits can exceed 5 knots.
Historically, two methods have been used for direct current measurement both
named after prominent eighteenth-century mathematicians: the Eulerian system
relying on fixed-point measurements of current strength and direction and the
2 Electronic Sensors and Instruments for Coastal Ocean Observing