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The Weather Portal

Weather is an all-encompassing term used to describe all of the many and varied phenomena that occur in the atmosphere of a planet at a given time. The term usually refers to the activity of these phenomena over short periods of 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.

Weather most often results from temperature differences from one place to another, caused by the Sun heating areas near the equator more than the poles, or by different areas of the Earth absorbing varying amounts of heat, due to differences in albedo, moisture, and cloud cover. 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 pressure gradient accelerates the air from high to low pressure, creating wind, and Earth's rotation causes curvature of the flow via the Coriolis effect. These simple systems can interact, producing more complex systems, and thus other weather phenomena.

The strong temperature contrast between polar and tropical air gives rise to the jet stream. Most weather phenomena in the mid-latitudes are caused by instabilities of the jet stream flow (see baroclinity) or by weather fronts. 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, while in December it is tilted away, causing yearly changes in the weather known as seasons. In the mid-latitudes, winter weather often includes snow and sleet, while in both the mid-latitudes and most of the tropics, tropical cyclones form in the summer and autumn. Almost all weather phenomena can occur year-round on different parts of the planet, including snow, rain, lightning, and, more rarely, hail and tornadoes.

Related portals: Earth sciences (Atmosphere  · Atmospheric Sciences)  · Tropical cyclones Featured article  · Disasters  · Water

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The sun, street light and Parallax edit.jpg

Parallax is the phenomenon of the change in the apparent position of an object when viewed from different angles. It is demonstrated nicely in this foggy scene, where the reflection of the moon appears in a different position with respect to the street lamp when viewed in the reflection in the water.

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Three-day prediction of the path of Hurricane Rita

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. Human input is still required to pick the best possible forecast model to base the forecast upon, which involves 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, error involved in measuring the initial conditions, and an incomplete understanding of atmospheric processes mean that forecasts become less accurate as 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 help narrow the error and pick the most likely outcome.

There are a variety of end uses 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 traders within commodity markets. Temperature forecasts are used by utility companies to estimate demand over coming days. On an everyday basis, 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 and survive them.

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Did you know...

...that Hurricane Debbie is the only known tropical cyclone ever to strike Ireland?

...that the Tempest Prognosticator, one of the earliest attempts at a weather prediction device, employed live leeches in its operation?

...that eyewall replacement cycles are among the biggest challenges in forecasting tropical cyclone intensity?

...that the Braer Storm of January 1993 is the strongest extratropical cyclone ever recorded in the north Atlantic Ocean?

...that in medieval lore, Tempestarii are magicians with the power to control the weather?

...that the omega equation is essential to numerical weather prediction?

Recent and ongoing weather


This week in weather history...

October 17

1091: One of the earliest recorded tornadoes killed two people in London.

1944: A major hurricane struck Isla de la Juventud, Cuba, killing around 300 people.

1998: Heavy rainfall, which would eventually cause severe flooding, began in central Texas, resulting in almost $1 billion (USD) in damage.

October 18

1985: Typhoon Dot, one of the most severe tropical cyclones ever to strike the area, struck the Bicol Region of the Philippines, killing 101 people.

October 19

1924: The earliest known category 5 hurricane struck the Pinar del Río Province of Cuba, killing around 90 people.

2005: Hurricane Wilma produced the lowest surface atmospheric pressure ever recorded in the Western Hemisphere while over the Caribbean Sea, at 882 millibars (26.05 inHg).

October 20

1991: A severe wildfire fueled by Santa Ana winds became a firestorm in the northern suburbs of Oakland, California, destroying almost 4,000 structures and killing 25 people.

October 21

1638: The Great Thunderstorm produced an occurrence of ball lightning in a church in Widecombe-in-the-Moor, Dartmoor, England, killing four people.

1972: Cyclone Bebe struck Funafuti atoll in the Ellice Islands (now Tuvalu), completely flooding the island and killing 28 people.

1998: Typhoon Babs struck the Philippines just days after the destructive landfall of Typhoon Zeb, killing more than 200 people.

October 22

1943: The crew of a German U-boat covertly planted Weather Station Kurt in northern Newfoundland and Labrador, attempting to gain an advantage in wartime weather forecasting.

October 23

1091: The London Tornado of 1091, the earliest and possibly strongest tornado in the history of Britain, hit downtown London, killing two people.

1878: A hurricane struck North Carolina with winds of at least 100 miles per hour (160 km/h).

1920: Ted Fujita, arguably the most influential severe weather researcher of all time, was born in Japan.

Selected biography

Portrait of William Ferrel

William Ferrel (1817 – 1891) was an American meteorologist who developed theories which explained the general mid-latitude atmospheric circulation in detail, now known as the Ferrel cell in his honor. Ferrel improved upon the concept of the Hadley cell by compensating for the Coriolis effect.

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Weather: Meteorology | Atmosphere | Basic meteorological concepts and phenomena | Climate | Clouds | Cyclones | Floods | Precipitation| Seasons | Severe weather and convection | Snow | Storms | Tornadoes | Tropical cyclones | Weather events | Weather lore | Weather hazards | Weather modification | Weather prediction | Weather warnings and advisories| Winds


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