CONDITONS OF DEVELOPMENT
The storms are generated by horizontal
thermic contrasts in the air and may grow stronger on earth
unlike cyclones. Storms occur especially during the fall
and winter months. But they can in spring and summer even
if it is rare. When the oceans are still so warm and the
polar air already cold, the gradient
of temperature between the two air masses involved is important.
The higher the gradient is the greater the power of the
storm will be.
Some storms may be the remains of an Atlantic
tropical cyclone in later life that has retained
sufficient energy and taken by western currents which bring
them to Europe. On their way, the weather conditions they
will encounter can sometimes provide them sufficient energy
to be qualified of storms.
initially stationary front is affected by a ripple.
Hot air rises pushed by cold air_;
By Coriolis effect a vortex is
formed around a depression
between a warm front at ahead
and a cold front behind ;
The warm moist air cools and condenses resulting
in formation of clouds of rain, the pressure drops
more and more ;
front compresses the hot air which increases
the rains and winds and makes pressure fallen. The
warm air mass that rises has no more contact with
the ground cut off its source by the cold front (formation
of an occluded front) ;
Birth and evolution of a storm
Then, when the
warm front disappears precipitations drain the last
drops, the storm subsided and the stationary front is reformed,
waiting for the next cycle.
Some storms absorb their energy in the
reservoir of thermal potential energy of the
jet stream as it was the case for the storms of 1999,
2010... in France.
As this diagram shows us a vortex
exists at the surface another in altitude
slightly shifted, this thermal energy associated to the
powerful jet stream
winds at 10 km altitude promotes wind power in the storm
to the surface
Storms occur mostly when the N.A.O.
index (North Atlantic Oscillation) is positive as the pressures
of the Azores are high and low in Iceland what affects the
speed of the wind, sea temperature, heat flux, height of
waves, storm track, and profiles of evaporation and precipitation.
They mainly depend on the position of the jet stream hardly
here to enlarge map of the trajectory of the
major storms passed over Western Europe since 1950