Mehr hierzu und wie ein Beobachter einen Tropischen Sturm erlebt finden Sie in diesem Sonderartikel über Hurrikane.
Cyclone, that sounds kind of brutal, don’t you think? A single-eyed
giant, trampling behind a fleeing group of dwards with the ground
Well, that’s not too far from reality in fact, although that you
seem to mix up cyclone with cyclops. The word cyclone is derived from
cyclonal, meaning counterclockwise rotation. So now with my first
posting it’s for me to explain:
What is a cyclone?
Basically, a cyclone is a tropical storm with a certain wind speed. The
strength of a cyclone is defined by the speed, the wind is blowing your
roof away. Does this wind blow with more than 73 mph, and is the system
born off Australia or the Indian Ocean, then this is what you call a
cyclone. Has it formed over other oceans, it would be called a typhoon
or hurricane. The difference between all these powerful candidates is
solely their place of origin.
Born From Waves
How exactly a cyclone forms is not yet known by every detail. The first
jerking though, that the ball gets rolling are probably irregularities
in a wind band in the upper atmosphere known as the “jet stream”. These
irregularities creates waves, just like when you tip your toe into a
These (air-)waves propagate northeastwards within the cyclone season
from April on. Sometimes, these disturbances lead to thunderstorms,
that are transported by trade winds.
Trade Winds? What Was That Again…
Your first association with “Trades” is your personal stock portfolio?
Or is there something flickering from your geography lessons? Very
briefly, trade winds arise because the air in the tropics rises up and
drops down again in the subtropics.
Cyclone: Born From Depression
Maybe this headline sounds a bit melodramatic, but if you take
the very meaning of the word, this is really what you can call it.
Because this is the first of several stages of development of a cyclone.
- Tropical Depression: Thunderstorms are organised in clusters and rotate around a vertical axis
- Tropical Storm :
pimp my Tropical Depression. Winds are increasing and reaching means
between 39 to 73 mph. This is the stage where a storm is being named,
just like “Nargis”.
- Cyclone: with
speeds more than 73 mph a Tropical Storm is becoming a cyclone.
Cyclones, just like any other strong tropical storms, are characterized
by the Saffir-Simpson scale, defining the categories 1 to 5. In the
following animation you get a general impression of the destructive
power of a cyclone:
Where Is This Whole Power Coming From?
Have you ever taken a shower? Hopefully. After taking the shower, you
will clearly have noticed that you freeze. The air is deprived of
energy necessary for evaporation, and this is what you recognize you
with your shivering.
So there is energy in the water, in hot
water even more. When the air is moving over large contiguous areas of
water, then it is gathering vast quantities of energy. The surface sea
temperature (SST) needed to form a cyclone has been defined earlier
with almost 80°F.
Today oe knows that it is not the SST itself that counts, but the
decrease of temperature with height. This is what makes an air parcel
rise to reach the level of condensation. So it is also possible, that a
cyclone is forming over SSTs with 68°F as long as the upper air is cold
The Eye Of A Cyclone
If you have seen the one or other Hollywood movie dealing with storms,
you will have heard of the eye of the hurricane. After the storm is at
its top, suddenly a quiet and blue sky appears. After a while, the
storm will continue just as extreme as he left off.
The whole process how eye and eyewall (the wall of clouds just
around the eye with the highest wind speeds) are formed are still not
known in detail. Most likely, there is air missing in the center
because of the centrifugal force. So new air is flowing in from above.
But there is one thing with air masses sinking: the clouds dissolve
because of the increase of temperature.
Eye and eyewall in a cyclone (Credit: NASA)
So now you have a general idea about the process how a cyclone is forming. You will need:
- high sea surface temperatures over a vast area
- “disturbance” in the air flow patterns
- no big differences of wind speed or direction with height
With this it is also clear why the Tropical Cyclones always weaken over
land: You need constant replenishment in form of warm water, otherwise
you are simply lacking fuel. Mostly though the storme surge causes
greater damage than the wind itself, because the water from the sea
being pushed by the wind on land is destroying everything, just like
you saw in Burma.1 / 2