Geoscience Reference
In-Depth Information
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The vorticity, or circulation, about a rotating circular
fluid disc is given by the product of the rotation on
its boundary (ωR) and the circumference (2πR)
where R = radius of the disc. The vorticity is then
2ωpR
2
, or 2ω per unit area.
type are appropriate for regions of limited latitudinal
extent as shown in Figure 10.2.
13 CLIMATE CHANGE
1
Statistics commonly reported for climatic data are:
the
arithmetic mean
,
7 PLANETARY-SCALE MOTIONS
IN THE ATMOSPHERE AND
OCEAN
x
i
¯
=
∑
---
n
1
The geostrophic wind concept is equally applicable
to contour charts. Heights on these charts are given
in geopotential metres (g.p.m.) or decametres (g.p.
dam). Geopotential and geometric heights are for
most purposes identical.
where
= sum of all values for i = l to n
x
i
= an individual value
n
= number of cases
∑
2
The World Meteorological Organization recom-
mends an arbitrary lower limit of 30 m s
-1
.
and the
standard deviation
,
s
(pronounced sigma).
3
Equatorial speed or rotation is 465 m s
-1
.
4
Note that, at the equator, an east/west wind of 5 m s
-1
represents an absolute motion of 460/470 m s
-1
towards the east.
∑
(
x
i
-
¯
)
2
σ
=
---------
n
which expresses the variability of observations.
For precipitation data, the
coefficient of variation
,
CV is often used:
9 MID-LATITUDE SYNOPTIC AND
MESOSCALE SYSTEMS
1
Resultant wind is the vector average of all wind
directions and speeds.
σ
CV
= ---
100 (%)
¯
2
This latter term is tending to be restricted to the
tropical (hurricane) variety.
For a
normal
(or Gaussian) bell-shaped symmet-
rical frequency distribution, the arithmetic mean is
the central value; 68.3 per cent of the distribution of
values are within ± 1
10 WEATHER AND CLIMATE IN MIDDLE
AND HIGH LATITUDES
σ
of the mean and 94.5 per cent
within ± 2
of the mean.
The frequency distribution of mean daily tem-
peratures is usually approximately normal. However,
the frequency distribution of annual (or monthly)
totals of rainfall over a period of years may be
'skewed' with some years (months) having very
large totals whereas most years (months) have low
amounts. For such distributions the median is a more
representative average statistic; the
median
is the
middle value of a set of data ranked according to
magnitude. Fifty per cent of the frequency distribu-
tion is above the median and 50 per cent below it.
The variability may be represented by the 25 and 75
per centile values in the ranked distribution.
σ
1
Standard indices of continentality developed by
Gorcynski (see p. 215), Conrad and others are based
on the annual range of temperature, scaled by
the sine of the latitude angle as a reciprocal in the
expression. This index is unsatisfactory for several
reasons. The small amplitude of annual temperature
range in humid tropical climates renders it unwork-
able for low latitudes. The latitude weighting is
intended to compensate for summer-winter differ-
ences in solar radiation and thus temperatures, which
were thought to increase uniformly with latitude.
For North America, the differences peak at about
55°N. It should be noted that indices of the Gorcynski
