The November 2005 measurement campaign of the Helsinki Testbed, located in the Greater Helsinki area, focuses on different precipitation types
Rain, snow and their mixture will be measured in 350 measurement points with different methods and equipment during 4 weeks.
The type of precipitation is often self-evident: in freezing temperatures it is snow, during the summer it is water.
However, when the temperature is close to 0C, precipitation can take different forms even within short distances: for instance, it can be snowing in one part of Helsinki while it is raining in another.
Precipitation type depends on the temperature in the upper atmosphere, as well as on the temperature and humidity close to the ground.
This is why it can snow in dry weather even in +4C.
Supercooled liquid precipitation can occur in freezing temperatures if the upper atmospheric layers produce such conditions.
In November, the temperature in the Greater Helsinki area is often close to 0C, and precipitation falls as rain or snow, or mixture of both.
The relatively great differences in temperature at sea and in inland areas cause significant differences in the type of precipitation that occurs in different areas within Greater Helsinki.
Rain in freezing temperatures is particularly troublesome for road traffic and aviation, as the water quickly freezes as it hits a cold surface, such as a highway.
The icing of aircraft wings alters the plane's flight capacity, so it is important that deicing is carried out before takeoff.
In addition to aviation authorities, precipitation information is important to authorities in charge of road and property maintenance, as they make decisions on snow plowing and road salting.
Dense observation network The November campaign of the Helsinki Testbed uses different methods for measuring precipitation types.
Temperature and humidity are measured with a dense observation network.
In locations where there is no measurement equipment, the temperature is estimated based on the environment and the measurements taken at stations nearby.
An acoustic sounding system located at Malmi airport, coupled with more-frequent-than-average soundings produce information on temperatures in the upper layers of the atmosphere.
The properties of snow flakes and raindrops are measured by a weather radar particularly developed for this purpose.
Helsinki Testbed, launched in early 2005, is a joint effort between the Finnish Meteorological Institute and Vaisala.
It consists of a particularly dense and versatile weather observation network, located in the Greater Helsinki area.
Helsinki Testbed produces data for research on small scale weather phenomena, and contributes to the development of new mesoscale weather services, providing information on local weather events.
The first measurement campaign was carried out in conjunction with the World Championships in Athletics, held in Helsinki in August.
The focus then was on gathering data for short-term weather forecasts.
The next campaign kicks off in January 2006 and focuses on the stable boundary layer.
