Chapter 3 ______________________________________________________ Functional Description
VAISALA _______________________________________________________________________ 19
CHAPTER 3
FUNCTIONAL DESCRIPTION
Vaisala Present Weather Detector PWD22/52 optical sensor measures
visibility (Meteorological Optical Range, (MOR)), precipitation intensity,
and precipitation type.
PWD22/52 measures visibility using 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 PWD22/52 allows
individual droplets to be detected from rapid signal changes. The
PWD22/52 software calculates the precipitation intensity by analyzing
the amplitudes of these changes. The intensity estimate is proportional to
the volume of the precipitation droplets.
The optical signal also contains precipitation type information. However,
additional information is required to reliably identify precipitation types,
especially in conditions such as very light precipitation or windy weather.
PWD22/52 provides an extra parameter by estimating the precipitation’s
water content with Vaisala RAINCAP
®
Rain Sensor. In rain, the water
equivalent and the optical volume are equal, while the optical volume in
snow is about ten times larger than its water equivalent. PWD22/52 uses
this difference to distinguish between rain and snow.
Identifying freezing conditions uses indirect methods that are affected by
shortcomings in temperature measurement. Reporting temperatures and
freezing conditions should be complemented with an expert familiar with
local conditions.
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