Press release archive: 2006

Helsinki Testbed's fifth campaign studies convection and storms

2.8.2006 12:00

The Finnish Meteorological Institute (FMI), Vaisala and partners are launching the fifth phase of the Helsinki Testbed project, where a dense weather observation network is utilized for various measurement campaigns. The new campaign looks at convective phenomena, such as storms and lightning. The campaigns produce information for research on small scale weather phenomena.

In Finland, the most dangerous weather phenomena in August are storms that include heavy rain, wind and lightning. Many remember the World Championships in Athletics in Helsinki a year ago, when heavy convective rains caused the event to be interrupted at times.

Convection causes storms
Convection is caused by temperature and density differences in matter, which produces an upward movement of air. The upward motion in turn creates clouds and rain. During the summer season this mechanism is strong as the ground warms substantially due to heat radiating from the sun. As a result, heated air bubbles ascend, being lighter than the surrounding air. In summer, the temperature differences between the ground and upper air layers are intensified. Convection balances these differences. As the air bubbles ascend, they cool down, the climb slows down and the motion weakens until it stops altogether. Rains produced by convection can be heavy, and at times include lightning or hail.

Practical consequences
Convection is what enables e.g. glider flying. The upward movement is substantially greater in convective clouds than in large layered clouds.

All around the world, lightning detection is important for the protection of life and property. It enables preventative measures and helps in limiting damages. Lightning observations are important for short-term weather forecasts and warnings. Users and applications include e.g. power plants, power transmission, explosives handling, insurance companies, aviation and ground operations, flammable materials handling, boating, and other outdoor activities.

Occurrence of convective weather phenomena
In Finland, the thunderstorm season typically lasts from May until September. The occurrence of storms usually declines after the first weeks of August. Approximately 16 thunderous days are observed in southern Finland annually (average for the whole country is 12).

The annual total of cloud to ground lightning in all of Finland is approximately 150 000, which corresponds to 0,4 lightning per square kilometer. Severe thunder is often connected to large and durable mesoscale thunder cloud systems, particularly in mid and late summer.

The most thunderous air masses usually come from the south or southeast. To observe the origin, movements and dying out of convective rain, go to the Helsinki Testbed project website at and view the radar images provided.

Further information:

Finnish Meteorological Institute
Senior Analyst Jani Poutiainen
tel +358 9 1929 4140

Vaisala Oyj
Director of Business Development
Jussi Mykkänen
tel +358 9 8949 2207