Weather forecasting is the application of science and technology to predict the state of the atmosphere The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night. Dry air contains roughly (by volume) 78% nitrogen, 21% 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 The term data refers to groups of information that represent the qualitative or quantitative attributes of a variable or set of variables. Data are typically the results of measurements and can be the basis of graphs, images, or observations of a set of variables. Data are often viewed as the lowest level of abstraction from which information and about the current state of the atmosphere and using scientific understanding of atmospheric processes Meteorology is the interdisciplinary scientific study of the atmosphere that focuses on weather processes and short term forecasting . Studies in the field stretch back millennia, though significant progress in meteorology did not occur until the eighteenth century. The nineteenth century saw breakthroughs occur after observing networks developed to project how the atmosphere will evolve.

Once an all-human endeavor based mainly upon changes in barometric pressure Atmospheric pressure is the force per unit area exerted against a surface by the weight of air above that surface in the Earth's atmosphere. In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low pressure areas have less atmospheric mass above, current weather conditions, and sky condition, forecast models Numerical weather prediction uses current weather conditions as input into mathematical models of the atmosphere to predict the weather. Although the first efforts to accomplish this were done in the 1920s, it wasn't until the advent of the computer and computer simulation that it was feasible to do in real-time. Manipulating the huge datasets and 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 Teleconnection in atmospheric science refers to climate anomalies being related to each other at large distances . The most emblematic teleconnection is that linking sea-level pressure at Tahiti and Darwin, Australia, which defines the Southern Oscillation, knowledge of model performance, and knowledge of model biases. The chaotic Chaos theory is a field of study in mathematics, physics, economics and philosophy studying the behavior of dynamical systems that are highly sensitive to initial conditions. This sensitivity is popularly referred to as the butterfly effect. Small differences in initial conditions yield widely diverging outcomes for chaotic systems, rendering long- 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 Historically, two equivalent concepts of temperature have developed, the thermodynamic description and a microscopic explanation based on statistical physics. Since thermodynamics deals entirely with macroscopic measurements, the thermodynamic definition of temperature, first stated by Lord Kelvin, is stated entirely in empirical, measurable and precipitation In meteorology, precipitation is any product of the condensation of atmospheric water vapor that is pulled down by gravity and deposited on the Earth's surface. The main forms of precipitation include rain, snow, ice pellets, and graupel. It occurs when the atmosphere, a large gaseous solution, becomes saturated with water vapour and the water are important to agriculture Agriculture is the production of food and goods through farming. Agriculture was the key development that led to the rise of human civilization, with the husbandry of domesticated animals and plants creating food surpluses that enabled the development of more densely populated and stratified societies. The study of agriculture is known as, 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 Wind chill is the felt air temperature on exposed skin due to wind. It measures the impact of wind on air temperature. The wind chill temperature is usually lower than the air temperature, since the air temperature is usually lower than the human body temperature. In contrast, humidity on the skin can result in a higher felt air temperature, and, forecasts can be used to plan activities around these events, and to plan ahead and survive them.

History

For millennia people have tried to forecast the weather. In 650 BC, the Babylonians Babylonia was an ancient cultural region in central-southern Mesopotamia , with Babylon as its capital. Babylonia emerged when Hammurabi (fl. ca. 1696 – 1654 BC, short chronology) created an empire out of the territories of the former Akkadian Empire. Babylonia adopted the written Semitic Akkadian language for official use, and retained the predicted the weather from cloud patterns as well as astrology Astrology is a group of systems, traditions, and beliefs which hold that the relative positions of celestial bodies and related details can provide information about personality, human affairs and other "earthly" matters. A practitioner of astrology is called an astrologer. Astrologers believe that the movements and positions of.^[1] In about 340 BC, Aristotle Aristotle (384 BC – 322 BC) was a Greek philosopher, a student of Plato and teacher of Alexander the Great. His writings cover many subjects, including physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology. Together with Plato and Socrates (Plato's teacher), Aristotle is one of the most described weather patterns in Meteorologica.^[2] Later, Theophrastus Theophrastus , a Greek native of Eresos in Lesbos, was the successor of Aristotle in the Peripatetic school. He came to Athens at a young age, and initially studied in Plato's school. After Plato's death he attached himself to Aristotle. Aristotle bequeathed to Theophrastus his writings, and designated him as his successor at the Lyceum compiled a book on weather forecasting, called the Book of Signs.^[3] Chinese China is seen variously as an ancient civilization extending over a large area in East Asia, a nation and/or a multinational entity weather prediction lore extends at least as far back as 300 BC,^[4] which was also around the same time ancient Indian astronomers Indian astronomy—the earliest textual mention of which is given in the religious literature of India —became an established tradition by the 1st millennium BCE, when Jyotiṣa Vedānga and other ancillary branches of learning called Vedangas began to take shape. During the following centuries a number of Indian astronomers studied various developed weather-prediction methods.^[5] In 904 AD, Ibn Wahshiyya Ibn Wahshiyah (Arabic: أبو بكر أحمد بن وحشية Abu Bakr Ahmed ibn 'Ali ibn Qays al-Wahshiyah al-Kasdani al-Qusayni al-Nabati al-Sufi‎) was a Nabataean Arab writer, alchemist, agriculturalist, Egyptologist and historian born at Qusayn near Kufa in Iraq. He was known in early modern Europe as Ahmad Bin Abubekr Bin Wahishih's Nabatean Agriculture The Arab Agricultural Revolution is a term coined by the historian Andrew Watson in his influential 1974 paper postulating a fundamental transformation in agriculture from the 8th century to the 13th century in the Muslim lands. This was an extension of an earlier hypothesis of an agricultural revolution in Islamic Spain proposed much earlier in 18 discussed the weather forecasting of atmospheric changes and signs from the planetary astral alterations; signs of rain based on observation of the lunar phases A lunar phase or phase of the moon refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. The lunar phases vary cyclically as the Moon orbits the Earth, according to the changing relative positions of the Earth, Moon and Sun. One half of the lunar surface is always illuminated by the Sun , and; and weather forecasts based on the movement of winds.^[6]

Ancient weather forecasting methods usually relied on observed patterns of events, also termed pattern recognition. For example, it might be observed that if the sunset was particularly red, the following day often brought fair weather. This experience accumulated over the generations to produce weather lore Weather lore is the body of informal folklore related to the prediction of the weather. However, not all of these predictions prove reliable, and many of them have since been found not to stand up to rigorous statistical testing.^[7]

It was not until the invention of the electric telegraph An electrical telegraph is a telegraph that uses electrical signals, usually conveyed via telecommunication lines or radio. The electromagnetic telegraph is a device for human-to-human transmission of coded text messages in 1835 that the modern age of weather forecasting began.^[8] Before this time, it had not been possible to transport information about the current state of the weather any faster than a steam train. The telegraph allowed reports of weather conditions from a wide area to be received almost instantaneously by the late 1840s.^[9] This allowed forecasts to be made by knowing what the weather conditions were like further upwind Windward is the direction from which the wind is blowing at the time in question. The side of a ship that is towards the windward is the weather side. If the vessel is heeling under the pressure of the wind, this will be the "higher side". The two men most credited with the birth of forecasting as a science were Francis Beaufort Rear-Admiral Sir Francis Beaufort, FRS, FRGS was an Irish born hydrographer and officer in Britain's Royal Navy. Beaufort was the creator of the Beaufort scale for indicating wind force (remembered chiefly for the Beaufort scale The Beaufort Scale is an empirical measure for describing wind speed based mainly on observed sea conditions. Its full name is the Beaufort Wind Force Scale) and his protégé Robert Fitzroy Vice-Admiral Robert FitzRoy RN achieved lasting fame as the captain of HMS Beagle during Charles Darwin's famous voyage, and as a pioneering meteorologist who made accurate weather forecasting a reality. He was an able surveyor and hydrographer and served as Governor of New Zealand from 1843 to 1845 (developer of the Fitzroy barometer A barometer is a scientific instrument used to measure atmospheric pressure. It can measure the pressure exerted by the atmosphere by using water, air, or mercury. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high). Both were influential men in British The United Kingdom of Great Britain and Ireland was the formal name of the United Kingdom from 1 January 1801 until 12 April 1927. It was formed by the merger of the Kingdom of Great Britain and the Kingdom of Ireland, with Ireland being governed directly from Westminster through its Dublin Castle administration naval and governmental circles, and though ridiculed in the press at the time, their work gained scientific credence, was accepted by the Royal Navy The Royal Navy of the United Kingdom is the oldest of HM Armed Forces . From the beginning of the 18th century until well into the 20th century it was the most powerful navy in the world, playing a key part in establishing the British Empire as the dominant world power from 1815 until the early 1940s. In World War II the Royal Navy operated almost, and formed the basis for all of today's weather forecasting knowledge.^[10] To convey information accurately, it became necessary to have a standard vocabulary describing clouds; this was achieved by means of a series of classifications and, in the 1890s, by pictorial cloud atlases Cloud Atlas is a 2004 novel, the third book by British author David Mitchell. It won the British Book Awards Literary Fiction Award and the Richard & Judy Book of the Year award, and was short-listed for the 2004 Booker Prize, Nebula Award, Arthur C. Clarke Award, and other awards placing it among the most-honored works of fiction in recent.

Great progress was made in the science of meteorology during the 20th century. The possibility of numerical weather prediction was proposed by Lewis Fry Richardson in 1922,^[11] though computers did not exist to complete the vast number of calculations required to produce a forecast before the event had occurred. Practical use of numerical weather prediction began in 1955,^[12] spurred by the development of programmable electronic computers A computer is a programmable machine that receives input, stores and manipulates data//information, and provides output in a useful format.

How models create forecasts

Data collection

Surface weather observations Surface weather observations are the fundamental data used for safety as well as climatological reasons to forecast weather and issue warnings worldwide. They can be taken manually, by a weather observer, by computer through the use of automated weather stations, or in a hybrid scheme using weather observers to augment the otherwise automated of atmospheric pressure, temperature, wind speed and direction, humidity Humidity is the amount of water vapor in the air. Relative humidity is defined as the ratio of the partial pressure of water vapor in a parcel of air to the saturated vapor pressure of water vapor at a prescribed temperature. Humidity may also be expressed as specific humidity. Relative humidity is an important metric used in forecasting weather, precipitation are made near the Earth's surface by trained observers, automatic weather stations An automatic weather station is an automated version of the traditional weather station, either to save human labour or to enable measurements from remote areas. An AWS will typically consist of a weather-proof enclosure containing the data logger, rechargeable battery, telemetry (optional) and the meteorological sensors with an attached Solar or buoys Weather buoys are instruments which collect weather and ocean data within the world's oceans. They measure parameters such as air temperature at the ocean surface, water temperature, wave height, dominant wave period, barometric pressure, wind speed , and wind direction. The buoys transmit this data via satellite communications to meteorological. The World Meteorological Organization The World Meteorological Organization is an intergovernmental organization with a membership of 188 Member States and Territories. It originated from the International Meteorological Organization (IMO), which was founded in 1873. Established in 1950, WMO became the specialised agency of the United Nations for meteorology (weather and climate), acts to standardize the instrumentation, observing practices and timing of these observations worldwide. Stations either report hourly in METAR METAR is a format for reporting weather information. A METAR weather report is predominantly used by pilots in fulfillment of a part of a pre-flight weather briefing, and by meteorologists, who use aggregated METAR information to assist in weather forecasting reports,^[13] or every six hours in SYNOP reports.^[14]

Measurements of temperature, humidity and wind above the surface are found by launching radiosondes A radiosonde is a unit for use in weather balloons that measures various atmospheric parameters and transmits them to a fixed receiver. Radiosondes may operate at a radio frequency of 403 MHz or 1680 MHz and both types may be adjusted slightly higher or lower as required. A rawinsonde is a radiosonde that is designed to also measure wind speed and on weather balloons A weather or sounding balloon is a balloon which carries instruments aloft to send back information on atmospheric pressure, temperature, and humidity by means of a small, expendable measuring device called a radiosonde. To obtain wind data, they can be tracked by radar, radio direction finding, or navigation systems (such as the satellite based.^[15] Data are usually obtained from near the surface to the middle of the stratosphere The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler higher up and warmer farther down. The border of the, about 21 km (13 mi).^[16] In recent years, data transmitted from commercial airplanes through the Aircraft Meteorological Data Relay Aircraft Meteorological Data Relay is a program initiated by the World Meteorological Organization. AMDAR is used to collect meteorological data worldwide by using commercial aircraft (AMDAR) system has also been incorporated into upper air observation, primarily in numerical models.^[17]

Increasingly, data from weather satellites A weather satellite is a type of satellite that is primarily used to monitor the weather and climate of the Earth. Satellites can be either polar orbiting, seeing the same swath of the Earth every 12 hours, or geostationary, hovering over the same spot on Earth by orbiting over the equator while moving at the speed of the Earth's rotation. These are being used because of their almost global coverage.^[18] Although their visible light images are very useful for forecasters to see development of clouds, little of this information can be used by numerical weather prediction models. The infrared Infrared light is electromagnetic radiation with a wavelength between 0.7 and 300 micrometres, which equates to a frequency range between approximately 1 and 430 THz (IR) data however can be used as it gives information on the temperature at the surface and cloud tops.^[19] Individual clouds can also be tracked from one time to the next to provide information on wind direction and strength at the clouds steering level. Both polar orbiting A polar orbit is an orbit in which a satellite passes above or nearly above both poles of the body being orbited on each revolution. It therefore has an inclination of (or very close to) 90 degrees to the equator. Except in the special case of a polar geosynchronous orbit, a satellite in a polar orbit will pass over the equator at a different and geostationary satellites A geosynchronous Satellites is a satellite whose orbital track on the Earth repeats regularly over points on the Earth over time. If such a satellite's orbit lies over the equator and the orbit is circular, it is called a geostationary satellite. The orbits of the satellites are known as the geosynchronous orbit and geostationary orbit. Another provide soundings Sounding generally refers to a mechanism of probing the environment by sending out some kind of stimulus. The term derives from the ancient practice of determining the depth of water by feeding out a line with a weight at the end of temperature and moisture throughout the depth of the atmosphere.^[20] Compared with similar data from radiosondes, the satellite data has the advantage of global coverage, however at a lower accuracy and resolution.^[21]

Meteorological radar A weather radar, or weather surveillance radar , is a type of radar used to locate precipitation, calculate its motion, estimate its type (rain, snow, hail, etc.), and forecast its future position and intensity provide information on precipitation location and intensity, which can be used to estimate precipitation accumulations over time.^[22] Additionally, if a Pulse Doppler weather radar is used then wind speed and direction can be determined.^[23]

Data assimilation and analysis

During the data assimilation process, information gained from the observations is used in conjunction with a numerical model's most recent forecast for the time that observations were made, since this contains information from previous observations. This is used to produce a three-dimensional representation of the temperature, moisture and wind called a meteorological analysis. This is the model's estimate of the current state of the atmosphere.^[24]

Numerical weather prediction

Numerical weather prediction models are computer simulations of the atmosphere. They take the analysis as the starting point and evolve the state of the atmosphere forward in time using physics and fluid dynamics. The complicated equations which govern how the state of a fluid changes with time require supercomputers to solve them. The output from the model provides the basis of the weather forecast.^[25]

Model output post processing

The raw output is often modified before being presented as the forecast. This can be in the form of statistical techniques to remove known biases in the model, or of adjustment to take into account consensus among other numerical weather forecasts.^[26] MOS or model output statistics is a technique used to interpret numerical model output and produce site-specific guidance. This guidance is presented in coded numerical form, and can be obtained for nearly all National Weather Service reporting stations in the United States.

Techniques

Persistence

The simplest method of forecasting the weather, persistence, relies upon today's conditions to forecast the conditions tomorrow. This can be a valid way of forecasting the weather when it is steady state, such as during the summer season in the tropics. This method of forecasting strongly depends upon the presence of a stagnant weather pattern. It can be useful in both short range forecasts and long range forecasts.^[27]

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