"Phenomena” are defined as the collection of simultaneous events that together characterize the weather phenomena. Simultaneous means that the events occurred within the time and space definitions of the phenomena and as a result are part of the same weather phenomena.”
Notes to definition:
• Phenomena (event)-duration
• Each phenomenon has a unique duration as determined by the meteorological
processes that generate it. Severe thunderstorm events typically have duration
of less than 30 minutes.
• Phenomena (event)-scale
• Each phenomenon has a unique spatial scale as determined by the meteorological
processes that generate it. Severe thunderstorm events typically have a
spatial diameter of less than 30 kilometres.
• Note that phenomena (event)-scale is related to Phenomena (event)-duration
through the typical speed of the phenomena. Scale should be approximately
the duration times the speed of the phenomena. Using these concepts, the
speed for severe thunderstorm events should be 60 kilometres per hour,
which is quite reasonable based on observations.
• The time of the phenomena (events) is dependent upon whether it is comprised
of occurrence or accumulation events. The meteorology of the events and
the requirements of the clients are required to determine the best method
of
assigning times to phenomena (events).