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| Thunderstorm near
Garajau, Madeira. |
Weather
Weather is the state of the atmosphere, describing for
example the degree to which it is hot or cold, wet or
dry, calm or stormy, clear or cloudy. On Earth, most
weather phenomena occur in the lowest level of the
planet's atmosphere, the troposphere, just below the
stratosphere. Weather refers to day-to-day temperature
and precipitation activity, whereas climate is the term
for the averaging of atmospheric conditions over longer
periods of time. When used without qualification,
"weather" is generally understood to mean the weather of
Earth.
Weather is driven by air pressure, temperature, and
moisture differences between one place and another.
These differences can occur due to the Sun's angle at
any particular spot, which varies with latitude. The
strong temperature contrast between polar and tropical
air gives rise to the largest scale atmospheric
circulations: the Hadley cell, the Ferrel cell, the
polar cell, and the jet stream. Weather systems in the
middle latitudes, such as extratropical cyclones, are
caused by instabilities of the jet streamflow. Because
Earth's axis is tilted relative to its orbital plane
(called the ecliptic), sunlight is incident at different
angles at different times of the year. On Earth's
surface, temperatures usually range ±40 °C (−40 °F to
100 °F) annually. Over thousands of years, changes in
Earth's orbit can affect the amount and distribution of
solar energy received by Earth, thus influencing
long-term climate and global climate change.
Surface temperature differences in turn cause pressure
differences. Higher altitudes are cooler than lower
altitudes, as most atmospheric heating is due to contact
with the Earth's surface while radiative losses to space
are mostly constant. Weather forecasting is the
application of science and technology to predict the
state of the atmosphere for a future time and a given
location. Earth's weather system is a chaotic system; as
a result, small changes to one part of the system can
grow to have large effects on the system as a whole.
Human attempts to control the weather have occurred
throughout history, and there is evidence that human
activities such as agriculture and industry have
modified weather patterns
Studying how the weather works on other planets has been
helpful in understanding how weather works on Earth. A
famous landmark in the Solar System, Jupiter's Great Red
Spot, is an anticyclonic storm known to have existed for
at least 300 years. However, the 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. |
|
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| Cumulus mediocris
cloud surrounded by stratocumulus. |
Causes
On Earth, the common weather phenomena include wind,
cloud, rain, snow, fog and dust storms. Less common
events include natural disasters such as tornadoes,
hurricanes, typhoons 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.
Weather occurs primarily due to air pressure,
temperature and moisture differences between one place
to another. These differences can occur due to the sun
angle at any particular spot, which varies by latitude
from the tropics. In other words, the farther from the
tropics one lies, the lower the sun angle is, which
causes those locations to be cooler due to the spread of
the sunlight over a greater surface. The strong
temperature contrast between polar and tropical air
gives rise to the large scale atmospheric circulation
cells and the jet stream. Weather systems in the
mid-latitudes, such as extratropical cyclones, are
caused by instabilities of the jet stream flow. Weather
systems in the tropics, such as monsoons or organized
thunderstorm systems, are caused by different processes.
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. 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.
The uneven solar heating (the formation of zones of
temperature and moisture gradients, or frontogenesis)
can also be due to the weather itself in the form of
cloudiness and precipitation. Higher altitudes are
typically cooler than lower altitudes, which the result
of higher surface temperature and radiational heating,
which produces the adiabatic lapse rate. In some
situations, the temperature actually increases with
height. This phenomenon is known as an inversion and can
cause mountaintops to be warmer than the valleys below.
Inversions can lead to the formation of fog and often
act as a cap that suppresses thunderstorm development.
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 warms the air above it
causing it to expand and lower the density and the
resulting surface air pressure. The resulting horizontal
pressure gradient moves the air from higher to lower
pressure regions, creating a wind, and the Earth's
rotation then causes deflection of this airflow due to
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 atmosphere is a chaotic system. As a result, small
changes to one part of the system can accumulate and
magnify to cause large effects on the system as a whole.
This atmospheric instability makes weather forecasting
less predictable than tides or eclipses. Although it is
difficult to accurately predict weather more than a few
days in advance, weather forecasters are continually
working to extend this limit through meteorological
research and refining current methodologies in weather
prediction. However, 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. |
|
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| Aurora Borealis. |
Shaping the planet Earth
Weather is one of the fundamental processes that shape
the Earth. The process of weathering breaks down the
rocks and soils into smaller fragments and then into
their constituent substances. During rains
precipitation, the water droplets absorb and dissolve
carbon dioxide from the surrounding air. This causes the
rainwater to be slightly acidic, which aids the erosive
properties of water. The released sediment and chemicals
are then free to take part in chemical reactions that
can affect the surface further (such as acid rain), and
sodium and chloride ions (salt) deposited in the
seas/oceans. The sediment may reform in time and by
geological forces into other rocks and soils. In this
way, weather plays a major role in erosion of the
surface.
Effect on humans
Weather, seen from an anthropological perspective, is
something all humans in the world constantly experience
through their senses, at least while being outside.
There are socially and scientifically constructed
understandings of what weather is, what makes it change,
the effect it has on humans in different situations,
etc. Therefore, weather is something people often
communicate about.
Effects on populations
The 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.
French claims to Florida came to an end in 1565 when a
hurricane destroyed the French fleet, allowing Spain to
conquer Fort Caroline. More recently, Hurricane Katrina
redistributed over one million people from the central
Gulf coast elsewhere across the United States, becoming
the largest diaspora in the history of the United
States.
The Little Ice Age caused crop failures and famines in
Europe. The 1690s saw the worst famine in France since
the Middle Ages. Finland suffered a severe famine in
1696–1697, during which about one-third of the Finnish
population died.
Forecasting
Weather forecasting is the application of science and
technology to predict the state of the atmosphere for a
future time and a given location. Human beings have
attempted to predict the weather informally for
millennia, and formally since at least the nineteenth
century. Weather forecasts are made by collecting
quantitative data about the current state of the
atmosphere and using scientific understanding of
atmospheric processes to project how the atmosphere will
evolve.
Once an all-human endeavor based mainly upon changes in
barometric pressure, current weather conditions, and sky
condition, forecast models are now used to determine
future conditions. On the other hand, human input is
still required to pick the best possible forecast model
to base the forecast upon, which involve many
disciplines such as pattern recognition skills,
teleconnections, knowledge of model performance, and
knowledge of model biases.
The chaotic nature of the atmosphere, the massive
computational power required to solve the equations that
describe the atmosphere, the error involved in measuring
the initial conditions, and an incomplete understanding
of atmospheric processes mean that forecasts become less
accurate as of the difference in current time and the
time for which the forecast is being made (the range of
the forecast) increases. The use of ensembles and model
consensus helps to narrow the error and pick the most
likely outcome.
There are a variety of end users to weather forecasts.
Weather warnings are important forecasts because they
are used to protect life and property. Forecasts based
on temperature and precipitation are important to
agriculture, and therefore to commodity traders within
stock markets. Temperature forecasts are used by utility
companies to estimate demand over coming days.
In some areas, people use weather forecasts to determine
what to wear on a given day. Since outdoor activities
are severely curtailed by heavy rain, snow and the wind
chill, forecasts can be used to plan activities around
these events and to plan ahead to survive through them.
Tropical weather forecasting is different from that at
higher latitudes. The sun shines more directly on the
tropics than on higher latitudes (at least in the
average over a year), which makes the tropics warm
(Stevens 2011). And, the vertical direction (up, as one
stands on the Earth's surface) is perpendicular to the
Earth's axis of rotation at the equator, while the axis
of rotation and the vertical are the same at the pole;
this causes the Earth's rotation to influence the
atmospheric circulation more strongly at high latitudes
than low. Because of these two factors, clouds and
rainstorms in the tropics can occur more spontaneously
compared to those at higher latitudes, where they are
more tightly controlled by larger-scale forces in the
atmosphere. Because of these differences, clouds and
rain are more difficult to forecast in the tropics than
at higher latitudes. On the other hand, the temperature
is easily forecast in the tropics, because it doesn't
change much. |
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Modification
The aspiration 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. A recent
example of weather control was China's preparation for
the 2008 Summer Olympic Games. China shot 1,104 rain
dispersal rockets from 21 sites in the city of Beijing
in an effort to keep rain away from the opening ceremony
of the games on 8 August 2008. Guo Hu, head of the
Beijing Municipal Meteorological Bureau (BMB), confirmed
the success of the operation with 100 millimeters
falling in Baoding City of Hebei Province, to the
southwest and Beijing's Fangshan District recording a
rainfall of 25 millimeters.
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: |
- Acid rain, caused by industrial
emission of sulfur dioxide and nitrogen oxides into
the atmosphere, adversely affects 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.
- Heat, generated by large
metropolitan areas have been shown to minutely
affect nearby weather, even at distances as far as
1,600 kilometres (990 mi).
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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.
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