Weather
The weather is the set of all extant phenomena in a given atmosphere at a
given time. The term usually refers to the activity of these phenomena over
short periods (hours or days), as opposed to the term climate, which refers
to the average atmospheric conditions over longer periods of time. When used
without qualification, "weather" is understood to be the weather of Earth.
Basic mechanism
Weather most often results from temperature differences from one place to
another. On large scales, temperature differences occur because areas closer
to the equator receive more energy per unit area from the Sun than do
regions closer to the poles. On local scales, temperature differences can
occur because different surfaces (such as oceans, forests, ice sheets, or
man-made objects) have differing physical characteristics such as
reflectivity, roughness, or moisture content.
Surface temperature differences in turn cause pressure differences. A hot
surface heats the air above it and the air expands, lowering the air
pressure. The resulting horizontal pressure gradient accelerates the air
from high to low pressure, creating wind, and Earth's rotation then causes
curvature of the flow via the Coriolis effect. The simple systems thus
formed can then display emergent behaviour to produce more complex systems
and thus other weather phenomena. Large scale examples include the Hadley
cell while a smaller scale example would be coastal breezes.
The strong temperature contrast between polar and tropical air gives rise to
the jet stream. Most weather systems in the mid-latitudes are caused by
instabilities of the jet stream flow (see baroclinic instability). Weather
systems in the tropics are caused by different processes, such as monsoons
or organized thunderstorm systems.
Because the Earth's axis is tilted relative to its orbital plane, sunlight
is incident at different angles at different times of the year. In June the
Northern Hemisphere is tilted towards the sun, so at any given Northern
Hemisphere latitude sunlight falls more directly on that spot than in
December (see Effect of sun angle on climate). This effect causes seasons.
Over thousands to hundreds of thousands of years, changes in Earth's orbital
parameters affect the amount and distribution of solar energy received by
the Earth and influence long-term climate (see Milankovitch cycles).
Terrestrial weather
On Earth, common weather phenomena include such things as wind, cloud, rain,
snow, fog and dust storms. Less common events include natural disasters such
as tornadoes, hurricanes and ice storms. Almost all familiar weather
phenomena occur in the troposphere (the lower part of the atmosphere).
Weather does occur in the stratosphere and can affect weather lower down in
the troposphere, but the exact mechanisms are poorly understood.[1]
The atmosphere is a chaotic system, so small changes to one part of the
system can grow to have large effects on the system as a whole. This makes
it difficult to accurately predict weather more than a few days in advance,
though weather forecasters are continually working to extend this limit
through the scientific study of weather, Meteorology. It is theoretically
impossible to make useful day-to-day predictions more than about two weeks
ahead, imposing an upper limit to potential for improved prediction
skill.[1] Chaos theory says that the slightest variation in the motion of
the air will grow with time. This idea is sometimes called the butterfly
effect, from the idea that the motions caused by the flapping wings of a
butterfly eventually could produce marked changes in the state of the
atmosphere. Because of this sensitivity to small changes it will never be
possible to make perfect forecasts, although there still is much potential
for improvement.
Shaping the planet
Weather is one of the fundamental processes that shape the Earth. The
process of weathering breaks down rocks and soils into smaller fragments and
then into their constituent substances. Almost all weather is the cause of
heat exchange. These are then free to take part in chemical
reactions that can affect the surface further (e.g., acid rain) or are
reformed into other rocks and soils. Weather also plays a major role in
erosion of the surface.
Human history
Weather has played a large and sometimes direct part in human history. Aside
from climatic changes that have caused the gradual drift of populations (for
example the desertification of the Middle East, and the formation of land
bridges during glacial periods), extreme weather events have caused smaller
scale population movements and intruded directly in historical events. One
such event is the saving of Japan from invasion by the Mongol fleet of
Kublai Khan by the Kamikaze winds in 1281. A series of great storms
throughout the 13th century caused the powerful English Cinque Ports to be
silted up and hence lose their influence. More recently, Hurricane Katrina
forced the temporary abandonment of the entire city of New Orleans in 2005.
Though weather affects people in drastic ways, it can also affect the human
race in simpler ways. It has been noted that the human immunity system is
affected in extreme heat or cold. Mood (psychology) can also be affected by
weather, hence the common scene of heavy downpour in Soap operas when a
person cries. Weather, in its power, however, cannot affect a person's
performance, at work, school or play. It is the person's own mindset that
leads to poor performance during times of bad weather, heat, cold or rain.
Forecasting
Weather forecasting is the application of science and technology to predict
the state of the atmosphere at a future time. Prior to the advent of
scientific methods of weather forecasting, a large body of weather folklore
developed to explain the weather. An example is the Groundhog Day
celebration near the end of winter in parts of the United States and Canada,
which forecasts whether spring is near or far depending on if the groundhog
sees his shadow or not. Today, weather forecasts are made by collecting data
that describe the current state of the atmosphere (particularly the
temperature, humidity and wind) and using physically-based mathematical
models to determine how the atmosphere is expected to change in the future.
The chaotic nature of the atmosphere means that perfect forecasts are
impossible, and that forecasts become less accurate as the range of the
forecast increases.
Weather modification and human impact
The wish to control the weather is evident throughout human history: from
ancient rituals intended to bring rain for crops to the U.S. Military
Operation Popeye, an attempt to disrupt supply lines by lengthening the
North Vietnamese monsoon. The most successful attempts at influencing
weather involve cloud seeding; they include the fog- and low stratus
dispersion techniques employed by major airports, techniques used to
increase winter precipitation over mountains, and techniques to suppress
hail.[2]
Whereas there is inconclusive evidence for these techniques' efficacy, there
is extensive evidence that human activity such as agriculture and industry
results in inadvertent weather modification:[3]
* Acid rain, caused by industrial emission of sulfur dioxide and nitrogen
oxides into the atmosphere, adversely effects freshwater lakes, vegetation,
and structures.
* Anthropogenic pollutants reduce air quality and visibility.
* Climate change caused by human activities that emit greenhouse gases into
the air is expected to affect the frequency of extreme weather events such
as drought, extreme temperatures, flooding, high winds, and severe
storms.[4]
The effects of inadvertent weather modification may pose serious threats to
many aspects of civilization, including ecosystems, natural resources, food
and fiber production, economic development, and human health.[5].
Extremes
On earth, temperatures usually range between ±40°C. However, the wide range
of climates and latitudes offer extremes of temperature well outside this
range. The coldest air temperature ever recorded on Earth is -89.2°C
(-127.8°F), at Vostok, Antarctica on 21 July 1983. The hottest air
temperature ever recorded was 57.7°C (135.9°F), at Al 'Aziziyah, Libya, on
13 September 1922. The highest recorded average annual temperature was
34.4°C (94°F) at Dallol, Ethiopia. The coldest recorded average annual
temperature is -50.6°C (-59°F) at Vostok, Antarctica. The coldest average
annual temperature in a permanently inhabited location is at Resolute,
Nunavut, in Canada.
Extra-terrestrial weather
Studying how the weather works on other planets has been seen as helpful in
understanding how it works on Earth.[6] Weather on other planets follows
many of the same physical principles as weather on Earth, but occurs on
different scales and in atmospheres having different chemical composition.
The Cassini-Huygens mission to Titan discovered clouds formed from methane
or ethane which deposit rain composed of liquid methane and other organic
compounds. Earth's atmosphere includes about six latitudinal circulation
zones, three in each hemisphere (see Hadley cell). In contrast Jupiter's
banded appearance shows over a dozen such zones, Titan has a single cell
covering its entire surface, and Venus appears to have no zones at all.
One of the most famous landmarks in the solar system, Jupiter's Great Red
Spot, is an anticyclonic storm known to have existed for at least 300 years.
On other gas giants the lack of a surface allows the wind to reach enormous
speeds: gusts of up to 400 metres per second (about 1440 km/h / 900 mi/h)
have been measured on the planet Neptune. This has created a puzzle for
planetary scientists. The weather is ultimately created by solar energy and
the amount of energy received by Neptune is only about 1/900th of that
received by Earth, yet the intensity of weather phenomena on Neptune is far
greater than on Earth.[7] The strongest planetary winds discovered so far
are on the extrasolar planet HD 189733b, which is thought to have easterly
winds moving at more than 9,600 kilometers per hour.
Extra-planetary weather
Weather is not limited to planetary bodies. A star's corona is constantly
being lost to space, creating what is essentially a very thin atmosphere
throughout the solar system. The movement of mass ejected from the Sun is
known as the solar wind.
Inconsistencies in this wind and larger events on the surface of the star,
such as coronal mass ejections, form a system that has features analogous to
conventional weather systems (such as pressure and wind) and is generally
known as space weather. The activity of this system can affect planetary
atmospheres and occasionally surfaces. The interaction of the solar wind
with the terrestrial atmosphere can produce spectacular aurorae, and can
play havoc with electrically sensitive systems such as electricity grids and
radio signals.
References
1. ^ O'Carroll, Cynthia M. (2001-10-18). Weather Forecasters May Look
Sky-high For Answers. Goddard Space Flight Center (NASA).
2. ^ American Meteorological Society
3. ^ American Meteorological Society
4. ^ Intergovernmental Panel on Climate Change
5. ^ Intergovernmental Panel on Climate Change
6. ^ Britt, Robert Roy (2001-03-06). The Worst Weather in the Solar System.
Space.com.
7. ^ Sromovsky, Lawrence A. (1998-10-14). Hubble Provides a Moving Look at
Neptune's Stormy Disposition. HubbleSite.
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