Geoscience Reference
In-Depth Information
6
Measuring Snow with Weather Radar
Elena Saltikoff
Finnish Meteorological Institute
Finland
1. Introduction
People of warm climate tend to think of snow as something rather rare and exotic. However,
most weather radars operating at mid-latitudes measure snow every day, at some altitude.
Even at relatively low elevation angles the edges of a PPI image are often measured above
the freezing level. Fig. 1 shows radar measurements from a night when surface minimum
temperature was +13 ºC - still, the majority of radar measurement volume was filled with
snow.
In many meteorological classifications hydrometeors are divided in two or three classes
(rain, wet snow and dry snow, see Fig. 1.). On the other hand, we have been told that there
are not two identical snowflakes. Between these extremes are the snowflake type
classifications such as those by Ukichiro Nakaya (Nakaya, 1954). From his work we can
learn how, based on temperature and humidity, snow crystals can take the shape of needles,
columns, plates, stars, rosettes and dendrites, only to name a few. They can also join each
other in a process called aggregation, and they can be covered in icing in a process called
riming.
Fig. 1. Hydrometeor classification 30 August 2009 00:45 UTC in Vantaa, Finland. RHI to 150
km in range, 12 km in height (left). PPI to 160 km in range, 0.5 degrees in elevation (middle)
and 1.5 degrees in elevation (right). Cyan for dry snow, dark blue for melting snow, light
blue for rain.
