DICTIONARY OF GEOPHYSICS, ASTROPHYSICS, and ASTRONOMY

Weber– Davis Model
physical instability properties of cloud and fog.
For example, there are abundant unfrozen water
droplets in cold clouds whose temperature is
lower than 0 ◦ C, so seeding glacigenic catalysts
can cause the unfrozen water droplets to freeze
to ice crystals and speed the forming processes
of waterdrops. The released condensational latent
heat in such processes can change the thermal
and dynamical structure of clouds, to enhance
precipitation, to reduce hail, to disperse
clouds, to clear fog, or to reduce wind speed
of hurricanes. Seeding appropriate sized salt
powder can enhance the forming process of rain
drops in warm clouds, and thus can enhance precipitation
or disperse cloud and fog. Currently,
the techniques of artificial cloud dispersal and
fog clearing are mature and used widely on airports,
whileotherweathermodificationmethods
often lead to unexpected results.
Weber–Davis Model The magnetohydrodynamic
theory of angular momentum transport a
steady, axially symmetric, magnetized, rotating
astrophysical wind.
The theory was originally developed as a
tool for analysis of the transfer of angular momentum
in the solar wind but has found much
broader application in theoretical investigations
of stellar winds and star formation. In the
most general form of the theory, the flow velocity
and magnetic field are separated into
poloidal and toroidal parts, B = Bp+Bt eφ and
V = Vp +Vt eφ, where eφ is the unit vector in
the azimuthal direction; these quantities are related
through conservation laws corresponding
to conservation of mass and magnetic flux,
Bp =λρVp
conservation of total (mechanical plus magnetic)
angular momentum
ϖ(Vt −λBt/4π)=J(λ) ,
and Faraday’s law for a frozen-in magnetic field
1
ϖ (Bt/λρ−Vt)=Q(λ) .
These invariants hold for each streamline, which
may be labeled byλ. ϖ is distance from the rotation
axis, ρ is the mass density, and J and Q
© 2001 by CRC Press LLC
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are functions ofλ to be determined from boundary
conditions.
If the flow describes expansion of a wind, the
velocity on each streamline passes through an
“Alfvén point”ϖA(λ) where the (poloidal) flow
and Alfvén speeds are equal, Vp = Bp/ √ 4πρ.
It can be shown that the constant of the second
conservation equation is J (λ) =ϖA 2 , where
is the rotation rate of the central object, and
that the angular momentum per unit mass of gas
approaches J (λ) infinitely far from the source
of the wind. Thus, the magnetic field enhances
angular momentum transport by providing an
effective lever arm of length ϖA.
weight The force of gravity; the force with
which an object is attracted to the center of a
planet;
W = mg ,
where g is the magnitude of the local acceleration
of gravity g=GM/r 2 , where M is the
mass, and r is the distance to the center of the
planet; G is Newton’s gravitational constant
g∼ 980 cm/sec 2 = 9.8 m/sec 2 .
near the surface of the Earth.
weir A structure placed on the bottom of a
channel, across which water flows, to control
depth or to facilitate measurement of flowrate. A
variety of designs are available, including broadcrested
and short-crested.
well-graded A term used to describe the distribution
of particle sizes in a soil sample. A
well-graded soil has a wide range of particle
sizes.
wentworth size classification A classification
of sediments based on size (diameter).
Ranges from boulders (>256 mm) down to colloids
(