Chapter 3________________________________________________________ Functional Description
VAISALA ________________________________________________________________________ 19
CHAPTER 3
FUNCTIONAL DESCRIPTION
This chapter describes the functionality of the product.
Vaisala Present Weather Detector PWD22 is an optical sensor that
measures visibility (meteorological optical range, MOR), precipitation
intensity, and precipitation type. The detector measures visibility
using the principle of forward scatter measurement. Light scatters
from particles whose diameter is in the order of magnitude of the
wavelength of the light. The amount of scatter is proportional to the
attenuation of the light beam.
Larger particles behave as reflectors and refractors and their effect on
the MOR must be handled separately. Usually these particles are
precipitation droplets. The optical arrangement of PWD22 is such that
individual droplets can be detected from rapid signal changes. The
PWD22 software calculates the precipitation intensity by analyzing
the amplitudes of these changes. This intensity estimate is
proportional to the volume of the precipitation droplets.
The optical signal also contains some information about the
precipitation type but not sufficiently for reliable identification.
Additional information is needed, especially in conditions such as
very light precipitation or windy weather. As the extra parameter,
PWD22 measures an estimate of the water content of precipitation
with Vaisala RAINCAP
®
Rain Sensor. In rain, the water equivalent
and the volume are equal whereas in snow the optical volume estimate
is about ten times larger. This difference of approximately one decade
is used to distinguish between rain and snow.
