Here's some info on the whole hurricane and tornado and cyclone subject. Enjoy =D
- Non-frontal synoptic scale low-pressure system over tropical or sub-tropical waters with organized convection (i.e. thunderstorm activity) and definite cyclonic surface wind circulation.
- Less than 17 m/s (34kmph) are tropical depressions
- Reached 34kmph are tropical storms and are assigned a name
- Over/reached (64kmph) are called ~:
• "hurricane" (the North Atlantic Ocean, the Northeast Pacific Ocean east of the dateline, or the South Pacific Ocean east of 160E)
• "typhoon" (the Northwest Pacific Ocean west of the dateline)
• "severe tropical cyclone" (the Southwest Pacific Ocean west of 160E or Southeast Indian Ocean east of 90E)
• "severe cyclonic storm" (the North Indian Ocean)
• "tropical cyclone" (the Southwest Indian Ocean)
- Storm system
- Driven by heat energy released as moist air drawn in over ocean waters rises and condenses
- A closed circulation
- Around a center of low pressure
Environmental factors / Conditions for the formation / tropical cyclogenesis of a Tropical Cyclone:
- Warm ocean waters (at least 26.5 C) to fuel the heat engine of the Tropical Cyclone
- Water depth must be sufficient (at least 50m)
- Atmosphere that cools fast as it increases in height such that is potentially unstable to moist convection.???
- Thunderstorm activity allows the heat stored in the ocean waters to be liberated for the tropical cyclonic development
- Relatively moist layers near the mid-troposphere (5km high)
(Dry mid-levels are mot conductive enough to allow the continuing development of widespread thunderstorm activity)
- A minimum distance from the Equator (at least 500km)
For tropical cyclogenesis to occur, there is a requirement for non-negligible amounts of the Coriolis force to provide for near gradient wind balance to occur. Without the Coriolis force, the low pressure of the disturbance cannot be maintained.
- A pre-existing near-surface disturbance with sufficient vorticity and convergence. Tropical cyclones cannot be generated spontaneously. To develop, they require a weakly organized system with sizable spin and low level inflow.
- Low values (less than about 20 kmph) of vertical wind shear between the surface and the upper troposphere. Vertical wind shear is the magnitude of wind change with height. Large values of vertical wind shear disrupt the developing tropical cyclone and can prevent genesis, or, if a tropical cyclone has already formed, large vertical shear can weaken or destroy the tropical cyclone by interfering with the organization of deep convection around the cyclone center.
Having these conditions met is necessary, but not sufficient as many disturbances that appear to have favorable conditions do not develop.
Recent work has identified that large thunderstorm systems (called mesoscale convective complexes (MCC) often produce an inertially stable, warm core vortex in the trailing altostratus decks of the MCC. These mesovortices have a horizontal scale of approximately 100 to 200 km [75 to 150 mi], are strongest in the mid-troposphere (5 km [3 mi]) and have no appreciable signature at the surface. Hypothesizes that genesis of the tropical cyclones occurs in two stages:
• stage 1 occurs when the MCC produces a mesoscale vortex.
• stage 2 occurs when a second blow up of convection at the mesoscale vortex initiates the intensification process of lowering central pressure and increasing swirling winds.
Why are tropical cyclones named?
Tropical cyclones are named to provide ease of communication between forecasters and the general public regarding forecasts, watches, and warnings. Since the storms can often last a week or longer and that more than one can be occurring in the same basin at the same time, names can reduce the confusion about what storm is being described.
The North Indian Ocean region tropical cyclones are not named.
The Southwest Indian Ocean tropical cyclones were first named during the 1960/1961 season.
The Australian and South Pacific region (east of 90E, south of the equator) started giving women's names to the storms in 1964 and both men's and women's names in 1974/1975.
Tropical cyclones ( facts )
During the twentieth century, it has become noticed that 800 to 1000 tropical cyclones have developed in the Australian region (105-165°E) with as few as one and as many as nineteen in a single season.
Tropical cyclones gain their name from their origin in the tropics and their cyclonic nature.
- high winds
- torrential rain
- Storm surges into coasts
- flash floods
- wind damage
Effects can be catastrophic
- carry away heat that builds up in the tropics
- have been known to relieve and end droughts in impacted areas
- are part of the larger atmospheric circulation (maintains equilibrium in the environment)
- Circulating storm systems
- Consist of multiple bands of intense showers and thunderstorms and high winds
- Develop over warm ocean waters
- Develop in the tropical regions that lie within about 25° latitude of the equator
- May begin as isolated thunderstorms
- In favorable conditions, they intensify to become Cyclones
- Fully developed, a tropical cyclone is a circular complex of thunderstorms
- About / Over 650km in diameter
- About / Over 12km high
- Winds near core can exceed 50 ps
- At centre of the storm is a region about 15-20km across called the eye
- In eye, winds are light and skies are often clear
- After forming and reaching peak of strength tropical cyclones may blow inland
- It could cause significant damage and loss of life
- Could force rapid rise in sea level that flood low lying areas
- Forecasting and Emergency planning has helped to lower death toll in recent years
- Several ocean areas adjacent to the equator possess all the necessary conditions for forming tropical
- Such places are: …South of the equator in the southern Indian Ocean and the south Pacific near Australia, where the peak cyclone months are January to March. Note that in each area the peak season is during late summer (in the Southern Hemisphere, summer runs from December to March). Tropical cyclones require warm surface waters at least 80°F (27°C). During the late summer months, sea surface temperatures reach their highest levels and provide tropical cyclones with the energy they need to develop into major storms.
- Cyclonic circulation is caused by two forces acting on the air: the pressure gradient and the Coriolis force
- Air rises at the center, creating a region of lower air (barometric) pressure.
- Air is fluid so it rushes in to fill the emptiness left by air that is rising off the surface
- Similar to effect given when you pull your plug out of the bathtub
- Water going down the drain is replaced by water rushing in from other parts of the tub
- This effect is called pressure gradient force because air moves from places of high pressure to lower pressure
- As the air moves toward low pressure in a straight line, the Coriolis force pushes the moving air to the left of the low pressure system
- The two forces are balanced and when the air circles around the low pressure zone it creates a stable cyclone rotating counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere.
- All large-scale air movements operate in this cyclonic circulation manner
- Examples are hurricanes, typhoons, extratropical cyclones, and large thunderstorms
- The larger the system, the more likely the rotation will be cyclonic due to the Coriolis Force prevailing
- The Coriolis force is a consequence of the rotation of Earth. Moving air masses, like any other physical body, tend to move in a straight line. However, we observe them moving over Earth's surface, which is rotating underneath the moving air. From our perspective, the air appears to be turning even though it is actually going in a straight line, and it is we who are moving.
- The rising air at the cyclone center causes clouds and precipitation to form
- The eye is where air is descending
- Descending air in associated with clear skies, so in the eye skies are clear and winds are nearly calm
- In the tropics tropical cyclones move at the speed of the prevailing winds, normally around 16 – 32kmph
- Several conditions are necessary to create a tropical cyclone. Warm sea surface temperatures, which reach a peak in late summer, are required to create and maintain the warm, humid air mass in which tropical cyclones grow. This provides energy for storm development through the heat stored in humid air, called latent heat. It takes energy to change water into vapor; that is why one must add heat to boil a kettle of water.
- The reverse is also true: when vapor condenses back to form liquid water, heat is released that may heat up the surrounding air. In a storm such as a hurricane, many hundreds of tons of humid air are forced to rise and cool, condensing out tons of water droplets and liberating a vast quantity of heat. This warms the surrounding air, causing it to expand and become even more buoyant, that is, more like a hot air balloon. More air begins rising, causing even more humid air to be drawn into the cyclone. This process feeds on itself until it forms a cyclonic storm of huge proportions. The more humid air available to a tropical cyclone, the greater its upward growth and the more intense it will become.
- For storm growth to begin, air needs to rise
- Because tropical air masses are uniformly warm and humid, the atmosphere over much of the tropics is stable; that is, it does not support rising air and the development of storms
- Thunderstorms occasionally develop but tend to be short-lived and small in scale, unlike the severe thunderstorms in the middle latitudes. During the late summer, this peaceful picture changes.
- Tropical disturbances begin to appear. These can take the form of a cluster of particularly strong thunderstorms or perhaps a storm system moving westward off of the African continent and out to sea. Tropical disturbances are regions of lower pressure at the surface. As we have seen, this can lead to air rushing into the low pressure zone and setting up a vortex, or rotating air column, with rising air at its core.
- Are a type of severe spinning (rotating) storm that occurs over the ocean near the tropics
- The word cyclone means ‘turning wind with one eye’
- It relates to the word Cyclops, a one-eyed creature in Greek Mythology
- They must have a wind speed greater than 119km/h
- Additional element needed: constant wind direction with height throughout the lower atmosphere
- This allows the growing vortex to stretch upward throughout the atmosphere without being sheared apart
- Not every tropical disturbance results in a full tropical cyclone, even with these elements present
- Once the tropical disturbance begins to intensify, it begins a chain reaction.
- It draws in humid air and air begins to rise and it eventually begins to condense into water droplets
- This releases latent heat which warms to air making it less dense and more buoyant
- The air begins to rise faster
- The pressure drops and humid air moves towards to storm
- It begins cyclonic rotation and surface winds begin to increase
- The tropical disturbance forms a circular ring of low air pressure and becomes known as a tropical depression
- More heat energy is liberated and updrafts increase inside the vortex
- Internal barometric pressure continues to drop and incoming winds increase
- When wind speeds increase beyond (60 kph), the depression is upgraded to a tropical storm
- If the winds reach (120 kph), the tropical storm is officially classified as a hurricane (or typhoon, cyclone, etc., depending on location)
- The chain reaction driving this storm growth is efficient. About 50–70% of tropical storms intensify to hurricanes.