training course on weather radar systems - RTC, Regional Training ...

MODULE A- INTRODUCTION TO RADAR
n=c/u
where c is the speed of light in a vacuum, u is the speed of the light in the medium and n is the
refractive index.
The refractive index of the atmosphere depends upon atmospheric pressure, temperature and
vapour pressure. Although the number of free electrons present also affects the refractivity of the
atmosphere, this can be ignored in troposphere due to insufficient free electrons. At microwave
frequencies, the index of refraction n for air which contains water vapour is
77.6 p 3.73e10
(n-1)10 6 =N= +
2
T T
5
where p = barometric pressure in hPa, e = partial pressure of water vapour in hPa, and T = absolute
temperature in K. The parameter N is called refractivity. N is defined as follows:
N=(n-1) 10 6
If n=1.0003 then N will be 300.
These values may be gathered by radiosondes. The index of refraction normally decreases with
increasing altitude and is typically 1.000313 near the surface of the earth, i.e. N = 313. This means
that electromagnetic radiation travels approximately 0.0313 % slower there than in a vacuum.
3.3.2. Curvature
Curvature is defined as “the rate of change in the deviation of a given arc from any tangent to it.”
Stated another way, it is the angular rate of change necessary to follow a curved path. Another
definition of curvature is the reciprocal of the radius and expressed as follows:
C= δθ/ δS
Where δθ is the change in angle experienced over a distance δS. When we think about a circle
with a radius of R, expression becomes;
TURKEY RADAR TRAINING 1.0 / ALANYA 2005 33