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Weather:
Weather is the set of all extant phenomena in a given atmosphere at a given time. It also includes interactions with the hydrosphere. 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 Mechanisms:
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 weather comes from, the Latin word for dark and scary 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 are 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.
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. Chaos theory says that the slightest variation in the motion of the ground can 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 Earth:
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. 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.
History of Weather:
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, Louisiana 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 immune system is affected in extreme heat or cold. Mood 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:
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.
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 Station, 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 Station, Antarctica. The coldest average annual temperature in a permanently inhabited location is at Resolute, Nunavut, in Canada
Climate:
The average and variations of weather in a region over long periods of time. Climate zones can be defined using parameters such as temperature and rainfall. Paleoclimatology focuses on ancient climate information derived from sediment found in lake beds, ice cores, as well as various fauna and flora including tree rings and coral. Climate models can be used to determine the amount of climate change anticipated in the future.
Meteorology:
The interdisciplinary scientific study of the atmosphere that focuses on weather processes and forecasting. Meteorological phenomena are observable weather events which illuminate and are explained by the science of meteorology. Those events are bound by the variables that exist in Earth's atmosphere. They are temperature, pressure, water vapor, and the gradients and interactions of each variable, and how they change in time. The majority of Earth's observed weather is located in the troposphere.
Meteorology, climatology, atmospheric physics, and atmospheric chemistry are sub-disciplines of the atmospheric sciences. Meteorology and hydrology compose the interdisciplinary field of hydrometeorology.
Interactions between Earth's atmosphere and the oceans are part of coupled ocean-atmosphere studies. Meteorology has application in many diverse fields such as the military, energy production, farming, shipping and construction.
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