Hydro
meteorological Disasters: Cyclones and Tsunami: Structure and nature of
cyclones and tsunamis, characteristics, factors, hazard potential; Frost, heat
and cold waves: cause, intensity and extent of frost, heat and cold waves and
its impact on agricultural crops.
Cyclone
Structure
There are a number of structural characteristics
common to all cyclones. A cyclone is a low-pressure area. A cyclone's
center (often known in a mature tropical cyclone as the eye), is the area
of lowest atmospheric pressure in the region. Near the center,
the pressure gradient force(from the pressure in the center of the cyclone
compared to the pressure outside the cyclone) and the force from
the Coriolis effect must be in an approximate balance, or the cyclone
would collapse on itself as a result of the difference in pressure.
Because of the Coriolis effect, the wind flow
around a large cyclone is counterclockwise in the Northern Hemisphere and
clockwise in the Southern Hemisphere. In the Northern Hemisphere, the
fastest winds relative to the surface of the Earth therefore occur on the
eastern side of a northward-moving cyclone and on the northern side of a
westward-moving one; the opposite occurs in the Southern Hemisphere. In
contrast to low pressure systems, the wind flow around high pressure systems
are clockwise (anticyclonic) in the northern hemisphere, and counterclockwise
in the southern hemisphere
Cyclogenesis is the
development or strengthening of cyclonic circulation in the
atmosphere. Cyclogenesis is an umbrella term for several different
processes that all result in the development of some sort of cyclone. It can
occur at various scales, from the microscale to the synoptic scale.
Extratropical cyclones begin as waves
along weather fronts before occluding later in their life cycle as
cold-core systems. However, some intense extratropical cyclones can become
warm-core systems when a warm seclusion occurs.
Tropical cyclones form as a result of significant
convective activity, and are warm core. Mesocyclones form as warm core
cyclones over land, and can lead to tornado formation. Waterspouts can
also form from mesocyclones, but more often develop from environments of high
instability and low vertical wind shear. Cyclolysis is the opposite
of cyclogenesis, and is the high-pressure system equivalent, which deals with
the formation of high-pressure areas—Anticyclogenesis.
A surface low can form in a variety of ways.
Topography can create a surface low. Mesoscale convective systems can
spawn surface lows that are initially warm core. The disturbance can grow
into a wave-like formation along the front and the low is positioned at
the crest. Around the low, the flow becomes cyclonic. This rotational flow
moves polar air towards the equator on the west side of the low, while warm air
move towards the pole on the east side. A cold front appears on the west side,
while a warm front forms on the east side. Usually, the cold front moves at a
quicker pace than the warm front and "catches up" with it due to the
slow erosion of higher density air mass out ahead of the cyclone. In addition,
the higher density air mass sweeping in behind the cyclone strengthens the
higher pressure, denser cold air mass. The cold front over takes the warm
front, and reduces the length of the warm front. At this point
an occluded front forms where the warm air mass is pushed upwards
into a trough of warm air aloft, which is also known as a trowal.
Tropical cyclogenesis is the development and
strengthening of a tropical cyclone. The mechanisms by which tropical
cyclogenesis occurs are distinctly different from those that produce
mid-latitude cyclones. Tropical cyclogenesis, the development of
a warm-core cyclone, begins with significant convection in
a favorable atmospheric environment. There are six main requirements for
tropical cyclogenesis:
1. sufficiently warm sea surface
temperatures,
2. atmospheric instability,
3. high humidity in the
lower to middle levels of the troposphere
4. enough Coriolis
force to develop a low-pressure center
5. a preexisting low-level focus or
disturbance
6. low vertical wind shear.
An average of 86 tropical cyclones of tropical
storm intensity form annually worldwide, with 47 reaching hurricane/typhoon
strength, and 20 becoming intense tropical cyclones (at least Category 3
intensity on the Saffir–Simpson Hurricane Scale).
Synoptic scale
The following types of cyclones
are identifiable in synoptic charts.
Surface-based types
There
are three main types of surface-based cyclones: Extratropical
cyclones, Subtropical cyclones and Tropical cyclones
Extratropical cyclone
An extratropical
cyclone is a synoptic scale of low-pressure weather
system that does not have tropical characteristics, as it is
connected with fronts and horizontal gradients (rather than
vertical) in temperature and dew point otherwise known as
"baroclinic zones"
"Extratropical" is
applied to cyclones outside the tropics, in the middle latitudes. These systems
may also be described as "mid-latitude cyclones" due to their area of
formation, or "post-tropical cyclones" when a tropical cyclone has
moved (extratropical transition) beyond the tropics. They are often
described as "depressions" or "lows" by weather forecasters
and the general public. These are the everyday phenomena that, along with anti-cyclones,
drive weather over much of the Earth.
Although
extratropical cyclones are almost always classified
as baroclinic since they form along zones of temperature and dewpoint
gradient within the westerlies, they can sometimes
become barotropic late in their life cycle when the temperature
distribution around the cyclone becomes fairly uniform with radius. An
extratropical cyclone can transform into a subtropical storm, and from there
into a tropical cyclone, if it dwells over warm waters sufficient to warm its
core, and as a result develops central convection. A particularly intense
type of extratropical cyclone that strikes during winter is known colloquially
as a nor'easter.
Polar low
A polar
low is a small-scale, short-lived atmospheric low-pressure
system (depression) that is found over the ocean areas poleward of the
main polar front in both the Northern and Southern Hemispheres. Polar
lows were first identified on the meteorological satellite imagery that became
available in the 1960s, which revealed many small-scale cloud vortices at high
latitudes. The most active polar lows are found over certain ice-free maritime
areas in or near the Arctic during the winter, such as the Norwegian Sea,
Barents Sea, Labrador Sea and Gulf of Alaska. Polar lows dissipate rapidly when
they make landfall. Antarctic systems tend to be weaker than their northern
counterparts since the air-sea temperature differences around the continent are
generally smaller. However, vigorous polar lows can be found over the Southern
Ocean. During winter, when cold-core lows with temperatures in the mid-levels
of the troposphere reach −45 °C (−49 °F) move over open waters, deep
convection forms, which allows polar low development to become
possible. 4] The systems usually have a horizontal length scale
of less than 1,000 kilometres (620 mi) and exist for no more than a couple
of days. They are part of the larger class of mesoscale weather
systems. Polar lows can be difficult to detect using conventional weather
reports and are a hazard to high-latitude operations, such as shipping and gas
and oil platforms. Polar lows have been referred to by many other terms, such
as polar mesoscale vortex, Arctic hurricane, Arctic low, and cold air
depression. Today the term is usually reserved for the more vigorous systems that
have near-surface winds of at least 17 m/s.
Subtropical
A subtropical
cyclone is a weather system that has some characteristics of
a tropical cyclone and some characteristics of an extratropical
cyclone. They can form between the equator and the 50th parallel. As early
as the 1950s, meteorologists were unclear whether they should be characterized
as tropical cyclones or extratropical cyclones, and used terms such as
quasi-tropical and semi-tropical to describe the cyclone hybrids. By 1972,
the National Hurricane Center officially recognized this cyclone
category. Subtropical cyclones began to receive names off
the official tropical cyclone list in the Atlantic Basin in
2002. They have broad wind patterns with maximum sustained winds located
farther from the center than typical tropical cyclones, and exist in areas of
weak to moderate temperature gradient.
Since they form from extratropical cyclones, which
have colder temperatures aloft than normally found in the tropics, the sea
surface temperatures required is around 23 degrees Celsius (73 °F)
for their formation, which is three degrees Celsius (5 °F) lower than for
tropical cyclones. This means that subtropical cyclones are more likely to
form outside the traditional bounds of the hurricane season. Although
subtropical storms rarely have hurricane-force winds, they may become tropical
in nature as their cores warm
Tropical
A tropical
cyclone is a storm system characterized by a low-pressure center
and numerous thunder storms that produce strong winds and flooding rain.
A tropical cyclone feeds on heat released when moist air rises,
resulting in condensation of water vapour contained in the moist
air. They are fueled by a different heat mechanism than other cyclonic
windstorms such as nor'easters, European windstorms, and polar
lows, leading to their classification as "warm core" storm systems.
The term "tropical" refers to both the
geographic origin of these systems, which form almost exclusively
in tropicalregions of the globe, and their dependence on Maritime
Tropical air masses for their formation. The term "cyclone"
refers to the storms' cyclonic nature, with counterclockwise rotation
in the Northern Hemisphere and clockwise rotation in
the Southern Hemisphere. Depending on their location and strength, tropical
cyclones are referred to by other names, such as hurricane, typhoon,
tropical storm, cyclonic storm, tropical depression, or simply as a cyclone.
While tropical cyclones can produce extremely
powerful winds and torrential rain, they are also able to produce high
waves and a damaging storm surge. Their winds increase the wave size,
and in so doing they draw more heat and moisture into their system, thereby
increasing their strength. They develop over large bodies of warm water, and
hence lose their strength if they move over land. This is the reason
coastal regions can receive significant damage from a tropical cyclone, while
inland regions are relatively safe from strong winds. Heavy rains, however, can
produce significant flooding inland. Storm surges are rises in sea level caused
by the reduced pressure of the core that in effect "sucks" the water
upward and from winds that in effect "pile" the water up. Storm
surges can produce extensive coastal flooding up to 40 kilometres
(25 mi) from the coastline. Although their effects on human populations
can be devastating, tropical cyclones can also
relieve drought conditions. They also carry heat and energy away
from the tropics and transport it toward temperate latitudes, which makes
them an important part of the global atmospheric circulation mechanism. As
a result, tropical cyclones help to maintain equilibrium in the
Earth's troposphere.
Many tropical cyclones develop when the
atmospheric conditions around a weak disturbance in the atmosphere are favorable.
Others form when other types of cyclones acquire tropical
characteristics. Tropical systems are then moved by steering winds in
the troposphere; if the conditions remain favorable, the tropical
disturbance intensifies, and can even develop an eye. On the other end of
the spectrum, if the conditions around the system deteriorate or the tropical
cyclone makes landfall, the system weakens and eventually dissipates. A
tropical cyclone can become extratropical as it moves toward higher latitudes
if its energy source changes from heat released by condensation to differences
in temperature between air masses. A tropical cyclone is usually not considered
to become subtropical during its extratropical transition
Upper level types
Polar cyclone
A polar, sub-polar,
or Arctic cyclone (also known as a polar vortex) is
a vast area of low pressure that strengthens in the winter and weakens in the
summer. A polar cyclone is a low-pressure weather system, usually
spanning 1,000 kilometres (620 mi) to 2,000 kilometres (1,200 mi), in
which the air circulates in a counterclockwise direction in the northern
hemisphere, and a clockwise direction in the southern hemisphere. The Coriolis
acceleration acting on the air masses moving poleward at high altitude, causes
a counterclockwise circulation at high altitude. The poleward movement of air
originates from the air circulation of the Polar cell. The polar low is
not driven by convection as are tropical cyclones, nor the cold and warm air
mass interactions as are extratropical cyclones, but is an artifact of the
global air movement of the Polar cell. The base of the polar low is in the mid
to upper troposphere. In the Northern Hemisphere, the polar cyclone has two
centers on average. One center lies near Baffin Island and the other over
northeast Siberia. In the southern hemisphere, it tends to be located near
the edge of the Ross ice shelf near 160 west longitude. When the
polar vortex is strong, its effect can be felt at the surface as a westerly
wind (toward the east). When the polar cyclone is weak, significant cold
outbreaks occur.
TUTT cell
Under specific circumstances, upper level cold lows
can break off from the base of the Tropical Upper Tropospheric Trough (TUTT),
which is located mid-ocean in the Northern Hemisphere during the summer months.
These upper tropospheric cyclonic vortices, also known as TUTT cells or TUTT
lows, usually move slowly from east-northeast to west-southwest, and their
bases generally do not extend below 20,000 feet (6,100 m) in altitude. A
weak inverted surface trough within the trade wind is generally found
underneath them, and they may also be associated with broad areas of high-level
clouds. Downward development results in an increase of cumulus
clouds and the appearance of a surface vortex.
In rare cases, they become warm-core tropical
cyclones. Upper cyclones and the upper troughs that trail tropical cyclones can
cause additional outflow channels and aid in their intensification. Developing
tropical disturbances can help create or deepen upper troughs or upper lows in
their wake due to the outflow jet emanating from the developing tropical
disturbance/cyclone.
Mesoscale
The
following types of cyclones are not identifiable in synoptic charts.
Mesocyclone
A mesocyclone is
a vortex of air, 2.0 kilometres (1.2 mi) to 10 kilometres
(6.2 mi) in diameter (the mesoscale of meteorology), within
a convectivestorm. Air rises and rotates around a vertical axis,
usually in the same direction as low-pressure systems in both northern and
southern hemisphere. They are most often cyclonic, that is, associated with a
localized low-pressure region within a supercell. Such storms can
feature strong surface winds and severe hail. Mesocyclones often occur
together with updrafts in supercells, where tornadoes may form.
About 1,700 mesocyclones form annually across the United States, but only
half produce tornadoes.
Tornado
A tornado is a violently rotating column of air
that is in contact with both the surface of the earth and a cumulonimbus cloud
or, in rare cases, the base of a cumulus cloud. Also referred to as twisters, a
colloquial term in America, or cyclones, although the word cyclone is used in
meteorology, in a wider sense, to name any closed low-pressure circulation.
Dust devil
A dust devil is a strong, well-formed, and
relatively long-lived whirlwind, ranging from small (half a metre wide and a
few metres tall) to large (more than 10 metres wide and more than 1000 metres
tall). The primary vertical motion is upward. Dust devils are usually harmless,
but can on rare occasions grow large enough to pose a threat to both people and
property.
Waterspout
A waterspout is a columnar vortex forming over
water that is, in its most common form, a non-supercell tornado over
water that is connected to a cumuliform cloud. While it is often weaker
than most of its land counterparts, stronger versions spawned
by mesocyclones do occur.
Steam devil
A gentle vortex over calm water or wet land made
visible by rising water vapour.
Fire whirl
A fire whirl – also colloquially known as a fire
devil, fire tornado, firenado, or fire twister – is a whirlwind induced by a
fire and often made up of flame or ash.
Climate
change
Scientists
warn that climate change could increase the intensity of typhoons as
climate change projections show that the difference in temperature between the
ocean – the heat source for cyclones – and the storm tops – the cold parts of
cyclones – are likely to increase. Climate change is predicted to increase
the frequency of high-intensity storms in selected ocean basins. While the
effect changing climate is having on tropical storms remains largely unresolved
scientists and president of Vanuatu Baldwin Lonsdale say the
devastation caused by Pam, was aggravated by climate change
Other
planets
Cyclones are not unique to Earth. Cyclonic storms
are common on Jovian planets, such as the Small Dark
Spot on Neptune. It is about one third the diameter of the Great
Dark Spot and received the nickname "Wizard's Eye" because it
looks like an eye. This appearance is caused by a white cloud in the middle of
the Wizard's Eye. Mars has also exhibited cyclonic
storms. Jovian storms like the Great Red Spot are usually
mistakenly named as giant hurricanes or cyclonic storms. However, this is
inaccurate, as the Great Red Spot is, in fact, the inverse phenomenon,
an anticyclone.
Tsunami
Tsunami/tidal
wave/seismic sea wave, is a series of waves in a water body caused by
the displacement of a large volume of water, generally in an ocean or
a large lake. Earthquakes, volcanic eruptions and other
underwater explosions (including detonations, landslides, glacier
calvings, meteorite impacts and other disturbances above or below
water all have the potential to generate a tsunami. Unlike
normal ocean waves, which are generated by wind, or tides, which are
generated by the gravitational pull of the Moon and the Sun, a tsunami is
generated by the displacement of water.
Tsunami waves do not resemble normal undersea
currents or sea waves because their wavelength is far
longer. Rather than appearing as a breaking wave, a tsunami may instead
initially resemble a rapidly rising tide. For this reason, it is
often referred to as a "tidal wave", although this usage is not
favoured by the scientific community because it might give the false impression
of a causal relationship between tides and tsunamis.
Tsunamis generally consist of a series of waves,
with periods ranging from minutes to hours, arriving in a so-called
"internal wave train". Wave heights of tens of metres can be
generated by large events. Although the impact of tsunamis is limited to
coastal areas, their destructive power can be enormous, and they can affect
entire ocean basins. The 2004 Indian Ocean tsunami was among the
deadliest natural disasters in human history, with at least 230,000 people
killed or missing in 14 countries bordering the Indian Ocean.
The Ancient
Greek historian Thucydides suggested in his 5th century BC History
of the Peloponnesian War that tsunamis were related to submarine
earthquakes, but the understanding of tsunamis remained slim until the
20th century and much remains unknown. Major areas of current research include
determining why some large earthquakes do not generate tsunamis while other
smaller ones do; accurately forecasting the passage of tsunamis across the
oceans; and forecasting how tsunami waves interact with shorelines.
While
Japan may have the longest recorded history of tsunamis, the sheer destruction
caused by the 2004 Indian Ocean earthquake and tsunami event mark it
as the most devastating of its kind in modern times, killing around 2,30,000
people. The Sumatran region is also accustomed to tsunamis, with
earthquakes of varying magnitudes regularly occurring off the coast of the
island.
The principal generation mechanism (or cause) of a
tsunami is the displacement of a substantial volume of water or perturbation of
the sea. This displacement of water is usually attributed to either
earthquakes, landslides, volcanic eruptions, glacier calvings or more rarely by
meteorites and nuclear tests. The waves formed in this way are then
sustained by gravity.
Landslides
In general, landslides generate displacements
mainly in the shallower parts of the coastline, and there is conjecture about
the nature of large landslides that enter the water. This has been shown to
subsequently affect water in enclosed bays and lakes, but a landslide large
enough to cause a transoceanic tsunami has not occurred within recorded
history. Susceptible locations are believed to be the Big
Island of Hawaii, Fogo in the Cape Verde Islands, La
Reunion in the Indian Ocean, and Cumbre Vieja on the island
of La Palma in the Canary Islands; along with other volcanic
ocean islands. This is because large masses of relatively unconsolidated
volcanic material occurs on the flanks and in some cases detachment planes are
believed to be developing. However, there is growing controversy about how
dangerous these slopes actually are.
Characteristics
Tsunamis cause damage by two mechanisms: the
smashing force of a wall of water travelling at high speed, and the destructive
power of a large volume of water draining off the land and carrying a large
amount of debris with it, even with waves that do not appear to be large.
While everyday wind waves have
a wavelength(from crest to crest) of about 100 metres (330 ft) and a
height of roughly 2 metres (6.6 ft), a tsunami in the deep ocean has a
much larger wavelength of up to 200 kilometres (120 mi). Such a wave
travels at well over 800 kilometres per hour (500 mph), but owing to the
enormous wavelength the wave oscillation at any given point takes 20 or 30
minutes to complete a cycle and has an amplitude of only about 1 metre
(3.3 ft). This makes tsunamis difficult to detect over deep water,
where ships are unable to feel their passage.
The velocity of a tsunami can be calculated by
obtaining the square root of the depth of the water in metres multiplied by the
acceleration due to gravity (approximated to 10 m/s2). For
example, if the Pacific Ocean is considered to have a depth of 5000 metres, the
velocity of a tsunami would be the square root of √(5000 × 10) = √50000 = ~224
metres per second (735 feet per second), which equates to a speed of ~806
kilometres per hour or about 500 miles per hour. This is the formula used for
calculating the velocity of shallow-water waves. Even the deep ocean
is shallow in this sense because a tsunami wave is so long (horizontally from
crest to crest) by comparison.
As the tsunami approaches the coast and the waters
become shallow, wave shoaling compresses the wave and its speed decreases
below 80 kilometres per hour. Its wavelength diminishes to less than 20
kilometres and its amplitude grows enormously – in accord with Green's
law. Since the wave still has the same very long period, the tsunami may
take minutes to reach full height. Except for the very largest tsunamis, the
approaching wave does not break, but rather appears like a
fast-moving tidal bore. Open bays and coastlines adjacent to very
deep water may shape the tsunami further into a step-like wave with a
steep-breaking front.
When the tsunami's wave peak reaches the shore, the
resulting temporary rise in sea level is termed run up. Run up is
measured in metres above a reference sea level. A large tsunami may
feature multiple waves arriving over a period of hours, with significant time
between the wave crests. The first wave to reach the shore may not have the
highest run-up.
About 80% of tsunamis occur in the Pacific Ocean,
but they are possible wherever there are large bodies of water, including
lakes. They are caused by earthquakes, landslides, volcanic explosions, glacier
calvings, and bolides.
A typical wave period for a damaging tsunami is
about twelve minutes. Thus, the sea recedes in the drawback phase, with areas
well below sea level exposed after three minutes. For the next six minutes, the
wave trough builds into a ridge which may flood the coast, and destruction
ensues. During the next six minutes, the wave changes from a ridge to a trough,
and the flood waters recede in a second drawback. Victims and debris may be
swept into the ocean. The process repeats with succeeding waves.
Scales
of intensity and magnitude
As
with earthquakes, several attempts have been made to set up scales of tsunami
intensity or magnitude to allow comparison between different events.
Intensity scales
The
first scales used routinely to measure the intensity of tsunami were the Sieberg-Ambraseysscale,
used in the Mediterranean Sea and the Imamura-Iida intensity
scale, used in the Pacific Ocean. The latter scale was modified by
Soloviev, who calculated the Tsunami intensity I known
as the Soloviev-Imamura tsunami intensity scale, is used in the
global tsunami catalogues compiled by the NGDC/NOAA and the Novosibirsk
Tsunami Laboratory as the main parameter for the size of the tsunami.
In 2013, following the intensively studied tsunamis
in 2004 and 2011, a new 12 point scale was proposed, the Integrated Tsunami
Intensity Scale (ITIS-2012), intended to match as closely as possible to the
modified ESI2007 and EMS earthquake intensity scales.
Magnitude scales
The first scale that genuinely calculated a
magnitude for a tsunami, rather than an intensity at a particular location was
the ML scale proposed by Murty & Loomis based on the potential energy. Difficulties
in calculating the potential energy of the tsunami mean that this scale is
rarely used. Abe introduced the tsunami magnitude scale Mt,
calculated from,
Mt = a log h + b log = D
where h is the maximum tsunami-wave amplitude (in m)
measured by a tide gauge at a distance Rfrom the epicentre, a, b and D are
constants used to make the Mt scale match as closely as
possible with the moment magnitude scale.
Tsunami
heights
Several
terms are used to describe the different characteristics of tsunami in terms of
their height:
·
Amplitude, Wave Height, or Tsunami Height:
Amplitude of Tsunami refers to its height relative to the normal sea level. It
is usually measured at sea level, and it is different from the crest-to-trough
height which is commonly used to measure other type of wave height.
·
Run-up Height, or Inundation Height: The height
reached by a tsunami on the ground above sea level, Maximum run-up height
refers to the maximum height reached by water above sea level, which is
sometimes reported as the maximum height reached by a tsunami.
·
Flow Depth: Refers to the height of tsunami above
ground, regardless of the height of the location or sea level.
·
(Maximum) Water Level: Maximum height above sea
level as seen from trace or water mark. Different from maximum run-up height in
the sense that they are not necessarily water marks at inundation line/limit.
Warnings
and predictions
Tsunami
warning sign
A tsunami cannot be precisely predicted, even if
the magnitude and location of an earthquake is known. Geologists, oceanographers,
and seismologists analyse each earthquake and based on many factors may or may
not issue a tsunami warning. However, there are some warning signs of an
impending tsunami, and automated systems can provide warnings immediately after
an earthquake in time to save lives. One of the most successful systems uses
bottom pressure sensors, attached to buoys, which constantly monitor the
pressure of the overlying water column.
Regions with a high tsunami risk typically
use tsunami warning systems to warn the population before the wave
reaches land. On the west coast of the United States, which is prone to Pacific
Ocean tsunami, warning signs indicate evacuation routes. In Japan, the
community is well-educated about earthquakes and tsunamis, and along the
Japanese shorelines the tsunami warning signs are reminders of the natural
hazards together with a network of warning sirens, typically at the top of the
cliff of surroundings hills.
The Pacific Tsunami Warning System is
based in Honolulu, Hawaiʻi.
It monitors Pacific Ocean seismic activity. A sufficiently large earthquake
magnitude and other information triggers a tsunami warning. While the
subduction zones around the Pacific are seismically active, not all earthquakes
generate a tsunami. Computers assist in analysing the tsunami risk of every
earthquake that occurs in the Pacific Ocean and the adjoining land masses.
Computer models can predict tsunami arrival,
usually within minutes of the arrival time. Bottom pressure sensors can relay
information in real time. Based on these pressure readings and other
seismic information and the seafloor's shape (bathymetry) and
coastal topography, the models estimate the amplitude and surge height of
the approaching tsunami. All Pacific Rim countries collaborate in the
Tsunami Warning System and most regularly practise evacuation and other
procedures. In Japan, such preparation is mandatory for government, local
authorities, emergency services and the population.
Some zoologists hypothesise that some animal
species have an ability to sense subsonic Rayleigh waves from an
earthquake or a tsunami. If correct, monitoring their behaviour could provide
advance warning of earthquakes, tsunami etc. However, the evidence is
controversial and is not widely accepted. There are unsubstantiated claims about
the Lisbon quake that some animals escaped to higher ground, while many other
animals in the same areas drowned. The phenomenon was also noted by media
sources in Sri Lanka in the 2004 Indian Ocean
earthquake. It is possible that certain animals (e.g., elephants) may
have heard the sounds of the tsunami as it approached the coast. The elephants'
reaction was to move away from the approaching noise. By contrast, some humans
went to the shore to investigate and many drowned as a result.
Along the United States west coast, in addition to
sirens, warnings are sent on television and radio via the National Weather
Service, using the Emergency Alert System.
In some tsunami-prone countries, earthquake
engineering measures have been taken to reduce the damage caused onshore.
Japan, where tsunami science and response measures
first began following a disaster in 1896, has produced ever-more elaborate
countermeasures and response plans. The country has built many tsunami
walls of up to 12 metres (39 ft) high to protect populated coastal areas.
Other localities have built floodgates of up to 15.5 metres
(51 ft) high and channels to redirect the water from an incoming tsunami.
However, their effectiveness has been questioned, as tsunami often overtop the
barriers.
The Fukushima Daiichi nuclear
disaster was directly triggered by the 2011 Tōhoku earthquake and
tsunami, when waves exceeded the height of the plant's sea wall. Iwate
Prefecture, which is an area at high risk from tsunami, had tsunami barriers walls
(Taro sea wall) totalling 25 kilometres
long at coastal towns. The 2011 tsunami toppled more than 50% of the
walls and caused catastrophic damage.
The Okushiri, Hokkaidō tsunami which
struck Okushiri Island of Hokkaidō within two to five
minutes of the earthquake on July 12, 1993, created waves as much as 30 metres
(100 ft) tall—as high as a 10-storey building. The port town of Aonae was
completely surrounded by a tsunami wall, but the waves washed right over the
wall and destroyed all the wood-framed structures in the area. The wall may
have succeeded in slowing down and moderating the height of the tsunami, but it
did not prevent major destruction and loss of life.
Frost
Frost is
a thin layer of ice on a solid surface, which forms
from water vapor in an above freezing atmosphere coming in
contact with a solid surface whose temperature is below freezing, and
resulting in a phase change from water vapor (a gas)
to ice (a solid) as the water vapor reaches the freezing point.
In temperate climates, it most commonly appears on surfaces near the
ground as fragile white crystals; in cold climates, it occurs in a greater
variety of forms. The propagation of crystal formation occurs by the
process of nucleation.
The ice crystals of frost form as the
result of fractalprocess development. The depth of frost crystals varies
depending on the amount of time they have been accumulating, and the
concentration of the water vapor (humidity). Frost crystals may be invisible
(black), clear (translucent), or white; if a mass of frost crystals scatters
light in all directions, the coating of frost appears white.
Types of frost include crystalline frost (hoar
frost, hoarfrost, radiation frost) from deposition of water
vapor from air of low humidity, white frost in humid
conditions, window frost on glass surfaces, advection
frost from cold wind over cold surfaces, black frost without
visible ice at low temperatures and very low humidity,
and rime under supercooledwet conditions.
Plants
that have evolved in warmer climates suffer damage when the temperature falls
low enough to freeze the water in the cells that make up the
plant tissue. The tissue damage resulting from this process is known as
"frost damage". Farmers in those regions where frost damage
is known to affect their crops often invest in substantial means to protect
their crops from such damage.
Formation
If a solid surface is chilled below the dew
point of the surrounding humid air and the surface itself is colder than
freezing, ice will form on it. If the water deposits as a liquid that then
freezes, it forms a coating that may look glassy, opaque, or crystalline,
depending on its type. Depending on context, that process also may be
called atmospheric icing. The ice it produces differs in some ways from
crystalline frost, which consists of spicules of ice that typically
project from the solid surface on which they grow.
The main difference between the ice coatings and
frost spicules arises from the fact that the crystalline spicules grow directly
from desublimation of water vapour from air, and desublimation is not a factor
in icing of freezing surfaces. For desublimation to proceed the surface must be
below the frost point of the air, meaning that it is sufficiently
cold for ice to form without passing through the liquid phase. The air
must be humid, but not sufficiently humid to permit the condensation of liquid
water, or icing will result instead of desublimation. The size of
the crystals depends largely on the temperature, the amount of water
vapor available, and how long they have been growing undisturbed.
As a rule, except in conditions where supercooled
droplets are present in the air, frost will form only if the deposition surface
is colder than the surrounding air. For instance frost may be observed around
cracks in cold wooden sidewalks when humid air escapes from the warmer ground
beneath. Other objects on which frost commonly forms are those with low specific
heat or high thermal emissivity, such as blackened metals; hence the
accumulation of frost on the heads of rusty nails.
The apparently erratic occurrence of frost in
adjacent localities is due partly to differences of elevation, the lower areas
becoming colder on calm nights. Where static air settles above an area of
ground in the absence of wind, the absorptivityand specific heat of the
ground strongly influence the temperature that the trapped air attains.
Types
of frost
Hoar frost
Hoar frost,
also hoarfrost, radiation frost, or pruina, refers to white ice
crystals deposited on the ground or loosely attached to exposed objects, such
as wires or leaves. They form on cold, clear nights when conditions are such
that heat radiates out to the open air faster than it can be replaced
from nearby sources, such as wind or warm objects. Under suitable
circumstances, objects cool to below the frost point of the surrounding
air, well below the freezing point of water. Such freezing may be promoted by
effects such as flood frost or frost pocket. These occur when
ground-level radiation losses cool air until it flows downhill and accumulates
in pockets of very cold air in valleys and hollows. Hoar frost may freeze in
such low-lying cold air even when the air temperature a few feet above ground
is well above freezing.
Hoar
frost may have different names depending on where it forms:
·
Air hoar is a deposit of hoar frost on objects above the surface, such as
tree branches, plant stems, and wires.
·
Surface hoar refers to fern-like ice crystals directly deposited on snow, ice
or already frozen surfaces.
·
Crevasse hoar consists of crystals that form in glacial crevasses where water
vapour can accumulate under calm weather conditions.
·
Depth hoar refers to faceted crystals that have slowly grown large within
cavities beneath the surface of banks of dry snow. Depth hoar crystals grow
continuously at the expense of neighbouring smaller crystals, so typically are
visibly stepped and have faceted hollows.
When surface hoar covers sloping snowbanks, the
layer of frost crystals may create an avalancherisk; when heavy layers of
new snow cover the frosty surface, furry crystals standing out from the old
snow hold off the falling flakes, forming a layer of voids that prevent the new
snow layers from bonding strongly to the old snow beneath. Ideal conditions for
hoarfrost to form on snow are cold clear nights, with very light, cold air
currents conveying humidity at the right rate for growth of frost crystals.
Wind that is too strong or warm destroys the furry crystals, and thereby may
permit a stronger bond between the old and new snow layers. However, if the
winds are strong enough and cold enough to lay the crystals flat and dry,
carpeting the snow with cold, loose crystals without removing or destroying
them or letting them warm up and become sticky, then the frost interface
between the snow layers may still present an avalanche danger, because the
texture of the frost crystals differs from the snow texture and the dry
crystals will not stick to fresh snow. Such conditions still prevent a strong
bond between the snow layers.
In very low temperatures where fluffy surface hoar
crystals form without subsequently being covered with snow, strong winds may
break them off, forming a dust of ice particles and blowing them over the
surface. The ice dust then may form yukimarimo, as has been observed in
parts of Antarctica, in a process similar to the formation of dust
bunnies and similar structures.
Hoar frost and white frost also occurs in
man-made environments such as in freezers or industrial cold
storage facilities. If such cold spaces or the pipes serving them are not
well insulated and are exposed to ambient humidity, the moisture will
freeze instantly depending on the freezer temperature. The frost may
coat pipes thickly, partly insulating them, but such inefficient insulation
still is a source of heat loss.
Advection frost
Advection frost (also
called wind frost) refers
to tiny ice spikes that form when very cold wind is blowing over tree
branches, poles, and other surfaces. It looks like rimming on the edges of
flowers and leaves and usually forms against the direction of the wind. It
can occur at any hour, day or night.
Window frost
Window frost (also
called fern frost or ice flowers) forms when a glass pane
is exposed to very cold air on the outside and warmer, moderately moist air on
the inside. If the pane is not a good insulator (for example, if it
is a single pane window), water vapour condenses on the glass forming frost
patterns. With very low temperatures outside, frost can appear on the bottom of
the window even with double pane energy efficient windows because the air
convection between two panes of glass ensures that the bottom part of the
glazing unit is colder than the top part.
On unheated motor vehicles the frost will usually
form on the outside surface of the glass first. The glass surface influences
the shape of crystals, so imperfections, scratches, or dust can modify the way
ice nucleates. The patterns in window frost form a fractal with
a fractal dimensiongreater than one but less than two. This is a
consequence of the nucleation process being constrained to unfold in two
dimensions, unlike a snowflake which is shaped by a similar process but forms
in three dimensions and has a fractal dimension greater than two.
If the indoor air is very humid, rather than
moderately so, water will first condense in small droplets and then
freeze into clear ice. Similar patterns of freezing may occur on other
smooth vertical surfaces, but they seldom are as obvious or spectacular as on
clear glass.
White frost
White frost is
a solid deposition of ice that forms directly from water
vapour contained in air. White frost forms when there is
a relative humidityabove 90% and a temperature below −8 °C
(18 °F) and it grows against the wind direction, since air
arriving from windward has a higher humidity than leeward air, but
the wind must not be strong or it damages the delicate icy structures as they
begin to form. White frost resembles a heavy coating of hoar frost with big,
interlocking crystals, usually needle-shaped.
Rime
Rime is
a type of ice deposition that occurs quickly, often under heavily
humid and windy conditions. Technically speaking, it is not a type of frost,
since usually supercooled water drops are involved, in contrast to
the formation of hoar frost, in which water vapour desublimates slowly and
directly. Ships travelling through Arctic seas may accumulate large quantities
of rime on the rigging. Unlike hoar frost, which has a feathery appearance,
rime generally has an icy, solid appearance.
Black frost
Black frost (or
"killing frost") is not strictly speaking frost at all, because it is
the condition seen in crops when the humidity is too low for frost to form, but
the temperature falls so low that plant tissues freeze and die, becoming
blackened, hence the term "black frost". Black frost often is called
"killing frost" because white frost tends to be less cold, partly
because the latent heat of freezing of the water reduces the
temperature drop.
Effect
on plants
Damage
Many plants can be damaged or killed by freezing
temperatures or frost. This varies with the type of plant, the tissue exposed,
and how low temperatures get: a "light frost" of −2 to 0 °C (28
to 32 °F) will damage fewer types of plants than a "hard frost"
below −2 °C (28 °F).
Plants likely to be damaged even by a light frost
include vines—such as beans, grapes, squashes, melons—along
with nightshades such as tomatoes, eggplants and peppers. Plants that
may tolerate (or even benefit) from frosts include:
·
root vegetables (e.g. beets, carrots, parsnips,
onions)
·
leafy greens (e.g. lettuces, spinach, chard,
cucumber)
·
cruciferous vegetables (e.g. cabbages,
cauliflower, bok choy, broccoli, Brussels sprouts, radishes, kale, collard,
mustard, turnips, rutabagas)
Even those plants that tolerate frost may be
damaged once temperatures drop even lower (below −4 °C or
25 °F). Hardy perennials, such as Hosta, become dormant
after the first frosts and regrow when spring arrives. The entire visible plant
may turn completely brown until the spring warmth, or may drop all of its
leaves and flowers, leaving the stem and stalk only. Evergreen plants, such as
pine trees, withstand frost although all or most growth stops. Frost
crack is a bark defect caused by a combination of low temperatures and
heat from the winter sun.
Vegetation is not necessarily damaged when leaf
temperatures drop below the freezing point of their cell contents. In the absence
of a site nucleatingthe formation of ice crystals, the leaves remain in
a supercooled liquid state, safely reaching temperatures of −4 to
−12 °C (25 to 10 °F). However, once frost forms, the
leaf cells may be damaged by sharp ice crystals. Hardening is
the process by which a plant becomes tolerant to low temperatures. See
also Cryobiology.
Certain bacteria, notably Pseudomonas
syringae, are particularly effective at triggering frost formation, raising
the nucleation temperature to about −2 °C (28 °F). Bacteria
lacking ice nucleation-active proteins (ice-minus bacteria) result in
greatly reduced frost damage.
Protection methods
To prevent frost or reduce its severity powerful
blowers may be deployed to simulate wind, thereby preventing the formation of
accumulations of cold air. Trees may be wrapped
and grow cover crops. Spraying crops
with a layer of water that releases latent heat, preventing harmful freezing of
the tissues of the plants that it coats.
Heat wave
A heat wave is a period of excessively hot weather,
which may be accompanied by high humidity,
especially in oceanic climate countries. While definitions
vary, a heat wave is usually measured relative to the usual weather in the
area and relative to normal temperatures for the season. Temperatures that
people from a hotter climate consider normal can be termed a heat wave in a
cooler area if they are outside the normal climate pattern for that
area.
The term is applied both to hot weather variations
and to extraordinary spells of hot which may occur only once a century. Severe
heat waves have caused catastrophic crop failures, thousands of deaths
from hyperthermia, and widespread power outages due to increased use of
air conditioning. A heat wave is considered extreme weather, and a danger
because heat and sunlight may overheat the human body. Heat waves can usually
be detected using forecasting instruments so that a warning
call can be issued.
Heat Wave Duration Index is that a heat wave occurs
when the daily maximum temperature of more than five consecutive days exceeds
the average maximum temperature by 5 °C
To be a heat wave such a period
should last at least one day, but conventionally it lasts from several days to
several weeks. In 1900, A. T. Burrows more rigidly defined a “hot wave” as a
spell of three or more days on each of which the maximum shade temperature
reaches or exceeds 90 °F (32.2 °C). More realistically, the comfort
criteria for any one region are dependent upon the normal conditions of that
area.
In
the Netherlands, a heat wave is defined as a period of at least 5
consecutive days in which the maximum temperature in De Bilt exceeds
25 °C (77 °F), provided that on at least 3 days in this period the
maximum temperature in De Bilt exceeds 30 °C (86 °F). This definition
of a heat wave is also used in Belgium and Luxembourg.
In Denmark, a national heat wave is defined as
a period of at least 3 consecutive days of which period the average maximum
temperature across more than fifty percent of the country exceeds 28 °C
(82.4 °F) – the Danish Meteorological Institute further defines
a "warmth wave" when the same criteria are met for a 25 °C
temperature, while in Sweden, a heat wave is defined as at least 5
days in a row with a daily high exceeding 25 °C.
In the United Kingdom, the Met Office operates
a Heat Health Watch system which places each Local Authority region into one of
four levels. Heatwave conditions are defined by the maximum daytime temperature
and minimum nighttime temperature rising above the threshold for a particular
region. The length of time spent above that threshold determines the particular
level. Level 1 is normal summer conditions. Level 2 is reached when there is a
60% or higher risk that the temperature will be above the threshold levels for
two days and the intervening night. Level 3 is triggered when the temperature
has been above the threshold for the preceding day and night, and there is a
90% or higher chance that it will stay above the threshold in the following
day. Level 4 is triggered if conditions are more severe than those of the
preceding three levels. Each of the first three levels is associated with a
particular state of readiness and response by the social and health services,
and Level 4 is associated with more widespread response.
A more general indicator that allows comparing heat
waves in different regions of the World, characterized by different climates,
has been recently developed. This was used to estimate heat waves
occurrence at the global scale from 1901 to 2010, finding a substantial and
sharp increase in the amount of affected areas in the last two decades.
Formation
of heat wave
High pressure in the upper
atmosphere traps heat near the ground, forming a heat wave
Heat waves form when high pressure aloft (from
10,000–25,000 feet (3,000–7,600 metres)) strengthens and remains over a region
for several days up to several weeks. This is common in summer (in both
Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the
equator side of the jet stream, in the upper layers of the atmosphere, is the
high pressure area.
Summertime weather patterns are generally slower to
change than in winter. As a result, this upper level high pressure also moves
slowly. Under high pressure, the air subsides (sinks) toward the surface,
warming and drying adiabatically. This warmer sinking air creates a high level
inversion that acts as a dome capping the atmosphere, inhibiting convection,
thereby trapping high humidity warm air below it. Typically, convection is
present along the periphery of the cap where the pressure becomes less. This
peripheral convection, however, can add to the high pressure dome by
ventilating the upper level outflow of the thunderstorms into it. The end
result is a continual build-up of heat at the surface that people experience as
a heat wave.
In the Eastern United States a heat wave can occur
when a high pressure system originating in the Gulf of Mexico becomes
stationary just off the Atlantic Seaboard (typically known as a Bermuda
High). Hot humid air masses form over the Gulf of Mexico and the Caribbean Sea
while hot dry air masses form over the desert Southwest and northern Mexico.
The SW winds on the back side of the High continue to pump hot, humid Gulf air
northeastward resulting in a spell of hot and humid weather for much of the
Eastern States.
In the Western Cape Province of South
Africa, a heat wave can occur when a low pressure offshore and high pressure
inland combine to form a Bergwind. The air warms as it descends from the
Karoo interior, and the temperature will rise about 10 °C from the
interior to the coast. Humidities are usually very low, and the temperatures
can be over 40 °C in summer. The highest official temperatures recorded in
South Africa (51.5 °C) was recorded one summer during a bergwind occurring
along the Eastern Cape coastline.
Global warming boosts the probability of extreme
weather events, like heat waves, far more than it boosts more moderate
events
Cold
wave
A cold wave (known
in some regions as a cold snap or cold spell) is a weather phenomenon
that is distinguished by a cooling of the air. Specifically, as used by the
U.S. National Weather Service, a cold wave is a rapid fall in temperature
within a 24-hour period requiring substantially increased protection to
agriculture, industry, commerce, and social activities. The precise criterion
for a cold wave is determined by the rate at which the temperature falls, and
the minimum to which it falls. This minimum temperature is dependent on the
geographical region and time of year.
In the United States, a cold spell is
defined as the national average high temperature dropping below 20 °F
(−7 °C). A cold wave of sufficient magnitude and duration may be
classified as a cold air outbreak (CAO).
Effects
A cold wave can cause death and injury to livestock
and wildlife. Exposure to cold mandates greater caloric intake for
all animals, including humans, and if a cold wave is accompanied by heavy and
persistent snow, grazing animals may be unable to reach needed food and die
of hypothermia or starvation. They often necessitate the
purchase of foodstuffs to feed livestock at considerable cost to farmers.
Cold spells are associated with increased mortality
rates in populations around the world. Both cold waves and heat waves
cause deaths, though different groups of people may be susceptible to different
weather events. In developed countries, more deaths occur
during a heat wave than in a cold snap, though the mortality rate is higher in
undeveloped regions of the world. Globally, more people die during cold weather
than hot weather, due to the rise in diseases like cold, flu, and pneumonia.
Extreme winter cold often causes poorly insulated water pipelines and
mains to freeze. Even some poorly protected indoor plumbing ruptures
as water expands within them, causing much damage to property and costly
insurance claims. Demand for electrical power and fuels rises
dramatically during such times, even though the generation of electrical power
may fail due to the freezing of water necessary for the generation of hydroelectricity.
Some metals may become brittle at low temperatures. Motor vehicles may fail
when antifreeze fails or motor oil gels, producing a failure
of the transportation system. To be sure, such is more likely in places
like Siberia and much of Canada that customarily get very
cold weather.
Fires become even more of a hazard during extreme
cold. Water mains may break and water supplies may become unreliable, making firefighting more
difficult. The air during a cold wave is typically denser and thus contains
more oxygen, so when air that a fire draws in becomes unusually cold it is
likely to cause a more intense fire.
Winter cold waves that aren't considered cold in
some areas, but cause temperatures significantly below average for an area, are
also destructive. Areas with subtropical climates may recognize unusual cold,
perhaps barely freezing, temperatures, as a cold wave. In such places, plant
and animal life is less tolerant of such cold as may appear rarely. The same
winter temperatures that one associates with the norm for Colorado, Ohio,
or Bavaria are catastrophic to winter crops in places
like Florida, California, or parts of South America that
grow fruit and vegetables in winter.
Cold waves that bring
unexpected freezes and frosts during the growing season in mid-latitude zones
can kill plants during the early and most vulnerable stages of growth,
resulting in crop failure as plants are killed before they can be harvested economically.
Such cold waves have caused famines. At times as deadly to plants as drought,
cold waves can leave a land in danger of later brush and forest fires that
consume dead biomass. One extreme was the so-called Year Without a
Summer of 1816, one of several years during the 1810s in which numerous
crops failed during freakish summer cold snaps after volcanic
eruptions that reduced incoming sunlight.
Countermeasures
In some places, such
as Siberia, extreme cold requires that fuel-powered machinery
to be used even part-time must be run continuously. Internal plumbing can be
wrapped, and persons can often run water continuously through pipes. Energy
conservation, difficult as it is in a cold wave, may require such measures as
collecting people (especially the poor and elderly) in communal shelters. Even
the homeless may be arrested and
taken to shelters, only to be released when the hazard abates. Hospitals
can prepare for the admission of victims of frostbite and hypothermia; schools
and other public buildings can be converted into shelters.
People can stock up on food, water, and other
necessities before a cold wave. Some may even choose to migrate to places of
milder climates, at least during the winter. Suitable stocks of forage can be
secured before cold waves for livestock, and livestock in vulnerable areas
might be shipped from affected areas or even slaughtered. Smudge pots can
bring smoke that prevents hard freezes on a farm or grove. Vulnerable crops may
be sprayed with water that will paradoxically protect the plants by freezing
and absorbing the cold from surrounding air.
Most people can dress appropriately and can even
layer their clothing should they need to go outside or should their heating
fail. They can also stock candles, matches, flashlights, and portable fuel for
cooking and wood for fireplaces or wood stoves, as necessary. However caution
should be taken as the use of charcoal fires for cooking or heating within an
enclosed dwelling is extremely dangerous due to carbon
monoxide poisoning. Adults must remain aware of the exposure that
children and the elderly have to cold.
No comments:
Post a Comment