DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

western boundary currents Ocean currents
flowing on the western boundary at speeds much
higher than those in the rest of an ocean basin.
This western intensification of ocean currents is
a combined effect of rotation and surface curvature
of the Earth, or the so-called beta-effect.
The Gulf Stream and Kuroshio Current are the
surface western boundary currents of the North
Atlantic and North Pacific, respectively. In the
Atlantic, deep western boundary currents are
also observed, transporting deep water formed
in the Nordic and Labrador Seas.
western boundary intensification The intensification
of current toward the western
boundary of the ocean due to the variation of the
Coriolis parameter with latitude (the β-effect).
west Greenland current A branch of the east
Greenland current that branches northwestward
along the southwest coast of Greenland.
westward drift The movement of features
of the geomagnetic field to the west. This was
first noticed by Halley in the 17th century, who
hypothesized that the Earth’s magnetic field emanates
from magnetized layers in the Earth’s interior
that are in rotation with respect to the surface.
It is now understood that the magnetic
field is generated in a molten iron outer core,
and that changes in the magnetic field observed
at the Earth’s surface are related either to flows
at the top of the core, or diffusion of the magnetic
field. If the former is responsible for the
westward drift, then it would be related to a general
westward flow of the core’s surface. The
magnitude of the drift is around 0.2 to 0.4 ◦ per
year, although it appears to vary with time and
latitude. It is better determined in some places
than others: for example, the field in the Pacific
region is relatively plain, so it is difficult to tell
whether or not it is drifting, and some models of
core flow show flows to the east there. Part of
the time variation of the westward drift may be
related to torsional oscillations. See core flow.
wet-bulb temperature The temperature to
which a parcel of air is cooled by evaporating
water into it gradually, adiabatically, and at constant
pressure until it is saturated. It is measured
© 2001 by CRC Press LLC
Weyl tensor
directly by a thermometer whose bulb is covered
by a moist cloth over which air is drawn.
wetness (or degree of saturation) (S) The
proportion of pore space that contains water:
S =
Vw
= θ
φ
Va + Vw
where Vw is the volume of water in the sample,
and Va is the volume of air in the sample. When
a soil or rock sample is saturated with water the
volume of air goes to zero, the volumetric water
content equals porosity, and wetness equals one.
wetted perimeter A linear measure of the
length of a river or canal cross-section that is
wetted by flowing fluid.
Weyl space-times (1917) The static and axially
symmetric vacuum metrics of the form
ds 2 = −e 2U dt 2 + e −2U
e 2γ (dρ 2 + dz 2 ) + ρ 2 dφ 2
where ρ and z are generalized cylindrical coordinates.
The function U = U(ρ,z)satisfies the
Laplace equation U = 0 as follows from the
vacuum Einstein equations. The linearity of the
Laplace equation allows a superposition of solutions.
The function γ = γ(ρ,z)is determined
by the line integral in the (ρ, z) plane
∂U 2
2
∂U
γ = ρ
− dρ
∂ρ ∂z
+2 ∂U ∂U
∂z ∂ρ dz
.
The solution U = m/(ρ 2 + z 2 ) 1/2 yields the
axisymmetric Curzon metric (rather than the
spherical Schwarzschild solution).
Weyl tensor A tensor, components of which
are the following linear combination of the components
of the curvature tensor:
Cαβρσ =Rαβρσ − 2
n − 2
Rα[ρ gσ ]β − Rβ[ρ gσ ]α
2 R
+
(n − 1)(n − 2) gα[ρ gσ ]β .
513