This book and accompanying CD ROM offer a descriptive survey of atmospheric sciences. While the approach is not physical or mathematical, some knowledge of the 'physical world' is necessary. This prerequisite knowledge of physics (and problem solving) is summarized below.
The world around us can be completely described by means of 7 basic units. These units are: length, mass, time, temperature, electrical current, light intensity and amount. Each of these units has its own unique dimension. For our purpose, the atmosphere can be fully represented by the first 4 of these units:
basic unit name dimension
length meter m
mass kilogram kg
time second s
temperature Kelvin K
These units conform to the Systeme International d' Unites (or the SI system), also referred to as the mks system, for the first letter of the dimensions of the first three basic units. Other systems, such as the cgs (cm - g - s) system, or any non metric system (for instance feet - pounds - o Fahrenheit) are not accepted in this class.
All other units within the SI system are derived units, because they can be derived from the basic units. For instance, velocity is a derived unit, because it is a displacement (m) per unit time (s), so its dimensions are m/s (written alternatively as m s-1). Similarly, acceleration is a derived unit, because an acceleration implies a change in velocity (m s-1) per unit time (s), so its dimensions are m/s/s (written less ambiguously as m s-2).
Here is a list of derived units (and their dimensions) used in the book:
derived unit name symbol dimensions
force Newton N kg.m/s2
pressure Pascal Pa N/m2 = kg/(m.s/2)
energy Joule J N.m = kg.m2/s2
power Watt W J/s = kg.m2/s3
radiance W/m2 = kg/s3
heat flux W/m2 = kg/s3
frequency hertz Hz 1/s
temperature degree °C K - 273.15
Celcius
Notice that K and °C are different by a constant amount. This difference is the absolute minimum temperature, i.e. - 273.15 °C is the lowest temperature any material can obtain in any circumstance (therefore K is referred to as absolute temperature) (Note 1.K). Since K and °C differ by a constant amount, a temperature difference (DT) can be expressed identically in K or °C units, e.g. the temperature difference between the boiling and the freezing of water under normal atmospheric pressure is 100K or 100°C.
A full list of derived units within the SI system as well as other systems is given in Note 1.J. The use of units of other systems is discouraged. For instance, we suggest to use Joules in lieu of calories, and Watt instead of horsepower. One exception is the millibar (mb), which is still widely used in atmospheric sciences as the unit of pressure:
1 mb = 100 Pa = 1 hPa (hectopascal)
Another exception is the evaporation (or rainfall) rate, which is often expressed in millimeters per day (or mm/d), and can be converted to SI as follows:
1 mm/d = [10-3 m/mm] x [1/86400 d/s] x [1 mm/d] = 1.16 x 10-8 m/s
A wide range of sizes occurs in nature; this is especially true in atmospheric sciences, which studies phenomena as small as a an ice crystal or a wind gust, and as large as the El Nino oscillation. Physical principles which determine small (often microscale) scale features (e.g. the scattering of sunlight in a cloud) also ultimately control the global climate.
As a first consequence, the wide range of sizes demands the use of multiples of basic or derived units. The following prefixes are used to construct decimal multiples of units:
multiple prefix symbol
109 giga G
106 mega M
103 kilo k
102 hecto h
101 deca da
10-1 deci d
10-2 centi c
10-3 milli m
10-6 micro m
10-9 nano n
Note: you may notice our use of a single slash in defining units. It seems clearer to write
Joule/(m2.s),
than the more common
Joule.m-2.s-1.