Geoscience Reference
In-Depth Information
Table 2.2 Some useful units
SI (mks)
cgs
Length
meter
centimeter
m
cm
Mass
kilogram
gram
kg
g
Time
second
second
s
s
Force
newton
dyne
N
=
kgms
−
2
dyn
=
gcms
−
2
Pressure
pascal
microbar
Pa
=
Nm
−
2
μ
bar
=
dyn cm
−
2
Energy
joule
erg
J
=
N m
erg
=
dyn cm
Power
watt
W
=
Js
−
1
erg s
−
1
Table 2.3 Conversion factors
=
10
2
=
=
10
3
μ
=
10
3
dyn cm
−
2
Pressure
millibar
1 mb
Pa
1hPa
bar
millimeter mercury
1 mm Hg
=
1
.
333 224 hPa
atmosphere
1 atm
=
1
.
013 25 10
5
Pa
Energy
calorie (IT)
1 cal
=
4
.
1868 J
=
4
.
1868 10
7
erg
(Energy/area)
(langley)
(1 ly
=
1 cal cm
−
2
)
The precipitable water is the total mass of water vapor in a vertical atmospheric column;
if it is assumed that the pressure is negligible at the top of the atmosphere, it can be written
as
p
o
W
p
=
qdp
/
g
(2.11)
0
where
p
o
is the surface pressure. Recall that the basic dimensions of these variables are
[
q
]
=
[M
w
/
M
a
], [
p
]
=
[M
a
L
−
1
T
−
2
], and [
g
]
=
[LT
−
2
], in which it is convenient to dis-
tinguish between the mass of air M
a
and the mass of water substance M
w
. Therefore
the basic dimensions of precipitable water are [
W
p
]
=
[M
w
L
−
2
], i.e. water mass per
unit area. In SI units this can be expressed in kg m
−
2
, which is roughly equivalent
with mm of vertical liquid water column, because the density of liquid water is around
1000 kg m
−
3
.
For convenient reference, some common units and conversion factors are listed in
Tables 2.2 and 2.3.
