Research » Infrastructure and Equipment » Rudolf Geiger climate site » Visibility and Present Weather Sensor

Visibility and Present Weather Sensor

Figure 1

Operating Principles

At our Rudolf-Geiger site, we operate a Biral HSS VPF-730 device (see Figure 1). It measures the extinction in forward scattering, and from this, visibility and present weather can be derived. Its measuring range is 10m - 75000m. A Visibility and Present Weather Sensor is a meteorological instrument to measure visibility and to determine the present weather (i.e. precipitation situation).

The sensor measures the amount of light scattered by small particulates suspended in the air (i.e. fog droplets, haze particles and smoke aerosols) or larger particles (i.e. rain drops, snow flakes, ice grains, drizzle and mist droplets) which pass through the sample volume between the instrument 'heads'. Afterwards, the sensor automatically calculates the atmosphere extinction coefficient, from which the Meteorological Optical Range (MOR) (visibility) is derived. The present weather data differentiates over 20 types of present weather (i.e. precipitation types and intensities) and includes both precipitation type and rate with drop size distributions and precipitation accumulation.

The measurements are unaffected by other light sources, as the HSS VPF-730 uses an infra-red light source.

Measurement Results

The first plot indicates the water precipitation amount in mm/min by precipitation type (different colours for each precipitation type (RA=rain, DZ=drizzle, SN=snow, GR=hail) and different symbols for light (-), moderate ( ), or heavy (+)). It is empty in Figure 3 as the measured 24 hours were a time of no rainfall. In Figure 4, moderate to heavy snowfall from about 3 to 6 UTC and three short rain periods after 6, after 9 and before 12 UTC are seen.Figures 2 & 3 show typical 24-hour time series of the visibility and present weather sensor.

In the last plot, the black line indicates the visibility on a logarithmic scale (shown on the left hand side of the plot), and the blue line (if present, i.e. in case of precipitation) shows the number of precipitation particles per minute on an equidistant scale (shown on the right hand side of the plot). On the visibility axes, a horizontal line at 1000m helps identify the presence or absence of fog (commonly defined as conditions with a visibility of less than 1000m).

References

HSS Visibility Sensor Brochure (© Biral 2010)

www.biral.com/meteorological-sensors/visibility-and-present-weather/hss-vpf-730-visibility-and-present-weather-sensor (access: 2015-02-05)

weather.cod.edu/notes/metar.html (access: 2015-02-23)

Figure 2
Figure 3