Lecture 3

Weather forecasting

Units and Variables

It is important to express observational data

and derived meteorological variables in a

standard measurement – unit system, so that

they can be understood by every one.

The MKS System

the system of units endorsed by the American

Meteorological society is called the SI ( Le

Système International ) or International

System of Units. It is an MKS system.

The basic units are as follows:

Variable Unit Abbreviation

for unit

Length meter m

Mass Kilogram kg

Time second s

The derived unit of Pressure in this system

take the form

Force / area = mass x acceleration / area =

= kg m-1 s-2 = Pa

where Pa is the Pascal, and

1000 Pa = 1 kPa = 10 hPa

Where kPa is the kilopascal and hPa is the

hectropascal.

The MTS System

is a system of units hardly used at all, even

though it is easier to use than the MKS

system. The basic units are as follows:

variable Unit Abbreviation For unit

Length meter m

Mass ton t

Time second s

Other Useful conversion factors

The unit of Velocity is usually given in Knots

( nautical miles per hour) or meter per second.

1 meter = 3.2808 feet ( ft )

1 m s-1 = 1.94 knots ( kts )

1o latitude = 111.137 km

TemperatureoC = 5/9 * ( oF – 32 )

K = oC + 273.15

( C degree centigrade, F degree Fahrenheit , and K Kelvin )