Meteorology

Atmospheric sciences
Meteorology
Climatology
Atmospheric physics
Atmospheric chemistry

Meteorology is the 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 comprise the interdiscplinary field of hydrometeorology.

History of meteorology

Early achievements in meteorology

• 350 BC

The term meteorology comes from Aristotle's Meteorology.

Although the term meteorology is used today to describe a subdiscipline of the atmospheric sciences, Aristotle's work is more general. The work touches upon much of what is known as the earth sciences. In his own words:

...all the affections we may call common to air and water, and the kinds and parts of the earth and the affections of its parts.

One of the most impressive achievements in Meteorology is his description of what is now known as the hydrologic cycle:

Now the sun, moving as it does, sets up processes of change and becoming and decay, and by its agency the finest and sweetest water is every day carried up and is dissolved into vapour and rises to the upper region, where it is condensed again by the cold and so returns to the earth.

• 1607

Galileo Galilei constructs a thermoscope. Not only did this device measure temperature, but it represented a paradigm shift. Up to this point, heat and cold were believed to be qualities of Aristotle's elements (fire, water, air, and earth). Note: There is some controversy about who actually built this first thermoscope. There is some evidence for this device being independently built at several different times. This is the era of the first recorded meteorological observations. As there was no standard measurement, they were of little use until the work of Daniel Gabriel Fahrenheit and Anders Celsius in the 18th century.

• 1643

Evangelista Torricelli, a contemporary and one-time assistant of Galileo, creates the first man-made sustained vacuum in 1643, and in the process creates the first barometer. Changes in height of mercury in this Toricelli Tube lead to his discovery that atmospheric pressure changes over time.

• 1648

Blaise Pascal discovers that atmospheric pressure decreases with height, and deduces that there is a vacuum above the atmosphere.

• 1667

Robert Hooke builds an anemometer to measure windspeed.

• 1686

Edmund Halley maps the trade winds, deduces that atmospheric changes are driven by solar heat, and confirms the discoveries of Pascal about atmospheric pressure.

• 1735

George Hadley is the first to take the rotation of the Earth into account to explain the behaviour of the trade winds. Although the mechanism Hadley described was incorrect, predicting trade winds half as strong as the actual winds, the circulating cells that Hadley described later become known as Hadley cells.

• 1743- 1784

Benjamin Franklin observes that weather systems in North America move from west to east, demonstrates that lightning is electricity, publishes the first scientific chart of the Gulf Stream, links a volcanic eruption to weather, and speculates about the effect of deforestation on climate.

• 1780

Horace de Saussure constructs a hair hygrometer to measure humidity.

• 1802- 1803

Luke Howard writes On the Modification of Clouds in which he assigns cloud types Latin names.

• 1806

Francis Beaufort introduces his system for classifying wind speeds.

• 1837

Samuel Morse invents the telegraph.

• 1860

Robert FitzRoy uses the new telegraph system to gather daily observations from across England and develops synoptic charts allowing predictions to be made, at the same time coining the term " weather forecast". The first ever daily weather forecasts were published by him in The Times in 1860, and in the following year a system was introduced of hoisting storm warning cones at principal ports when a gale was expected.

The Coriolis effect

Understanding the kinematics of how exactly the rotation of the Earth affects airflow was partial at first. Late in the 19th century the full extent of the large scale interaction of pressure gradient force and deflecting force that in the end causes air masses to move along isobars was understood. Early in the 20th century this deflecting force was named the Coriolis effect after Gaspard-Gustave Coriolis, who had published in 1835 on the energy yield of machines with rotating parts, such as waterwheels. In 1856, William Ferrel proposed the existence of a circulation cell in the mid-latitudes with air being deflected by the coriolis force to create the prevailing westerly winds.

Numerical weather prediction

Early in the 20th century, advances in the understanding of atmospheric physics led to the foundation of modern numerical weather prediction. In 1922, Lewis Fry Richardson published `Weather prediction by numerical process` which described how small terms in the fluid dynamics equations governing atmospheric flow could be neglected to allow numerical solutions to be found. However, the sheer number of calculations required was too large to be completed before the advent of computers.

At this time in Norway a group of meteorologists led by Vilhelm Bjerknes developed the model that explains the generation, intensification and ultimate decay (the life cycle) of midlatitude cyclones, introducing the idea of fronts, that is, sharply defined boundaries between air masses. The group included Carl-Gustaf Rossby (who was the first to explain the large scale atmospheric flow in terms of fluid dynamics), Tor Bergeron (who first determined the mechanism by which rain forms) and Jacob Bjerknes.

Starting in the 1950s, numerical experiments with computers became feasible. The first weather forecasts derived this way used barotropic (that means, single-vertical-level) models, and could successfully predict the large-scale movement of midlatitude Rossby waves, that is, the pattern of atmospheric lows and highs.

In the 1960s, the chaotic nature of the atmosphere was first understood by Edward Lorenz, founding the field of chaos theory. These advances have led to the current use of ensemble forecasting in most major forecasting centers, to take into account uncertainty arising due to the chaotic nature of the atmosphere.

Satellite observation

In 1960, the launch of TIROS-1, the first successful weather satellite marked the beginning of the age where weather information is available globally. Weather satellites along with more general-purpose Earth-observing satellites circling the earth at various altitudes have become an indispensable tool for studying a wide range of phenomena from forest fires to El Niño.

In recent years, climate models have been developed that feature a resolution comparable to older weather prediction models. These climate models are used to investigate long-term climate shifts, such as what effects might be caused by human emission of greenhouse gases.

Weather forecasting

Meteorologists and weather presenters use several methods to predict what the weather will be like in the future. Most of these methods were used several decades ago (before the 70's) when computers simply did not exist or were unable to perform fast enough for Numerical Weather Prediction. They are used nowadays to estimate the effectiveness of weather forecasting : compared to persistence or to climatology ... Analogs were a nightmare : one could not find a reasonable match among the infinity of possibilities offered by the atmosphere. Frontology is used now as a means to describe objects resulting from NWP. Cold fronts exist, but warm fronts are not fronts as such : the symbol of warm air struggling against cold air does not represent the reality.

• Persistence method

The persistence method assumes that conditions will not change. Often summarised as "Tomorrow equals today". This method works best over short periods of time.

• Trends method

The trends method involves determining the speed and direction of fronts, high and low pressure centers, and areas of clouds and precipitation.

• Climatology method

The climatology method involves using historical weather data collected over long periods of time (years) to predict conditions on a given date.

• Analog method

A complex method that involves finding an "analog" or very similar weather conditions from historical data.

• Numerical forecasting method

The numerical weather prediction or NWP method uses computers to take into account a large number of variables and creates of computer model of the atmosphere. This is the most successful and widely used method.

Meteorology and climatology

With the development of powerful new supercomputers like the Earth Simulator in Japan, mathematical modeling of the atmosphere can reach unprecedented accuracy. This is not only due to the enhanced spatial and temporal resolution of the grids employed, but also because these more powerful machines can model the Earth as an integrated climate system, where atmosphere, ocean, vegetation, and man-made influences depend on each other realistically. The goal in global meteorological modeling can be termed Earth System Modeling, with a growing number of models of various processes coupled to each other. Predictions for global effects like Global Warming and El Niño are expected to benefit substantially from these advancements.

Regional models are attracting more interest as the resolution of global models increases. With regional weather disasters such as the Elbe flooding in 2002 and the European heat wave in 2003, decision makers expect from these models accurate assessments about the possible increase of these natural hazards in a specific region. Countermeasures such as dikes or intentional flooding might be effective in preventing or at least attenuating natural hazards.

For models at all scales, increased model resolution means less reliance on parameterizations, which are empirically derived expressions for processes that cannot be resolved on the model grid. For example, in mesoscale models individual clouds can now be resolved, removing the need for formulations that average over a grid box. In global modeling, atmospheric waves such as gravity waves with short temporal and spatial scales can be represented without resorting to often overly simplified parameterizations.