published on 7 July 2010 in air

What is the weather like?

On a summer day
On a beautiful, sunny summer day you may plan to spend the day out of town, perhaps near a refreshing river taking along a tasty picnic. The evening before, the weather forecast was favourable, but after a first swim and just before starting to gobble the sandwiches and pieces of white pizza, the weather starts changing : the wind becomes stronger and threatening clouds hover above, and a little later you find yourself wet to the bone under a torrential rainfall. Now who hasn’t found himself in a similar situation?
In fact many have experienced a similar situation because the weather of the atmosphere is a vast phenomenon with fronts and perturbations that can even affect half  a continent or on the contrary, they may be extremely localized, almost “personal” events! For this reason we are concerned and attracted towards knowing what the weather will be like in a particular place at a precise time : this is an almost impossible task for meteorologists who now are able to formulate weather forecasts that are increasingly precise with the help of more and more powerful instruments and computers. In Western Europe the average of exact forecasts in the 24-hour period forecast has reached 98%,  93% in the 48-hour period and 70% in the 72 hour period. However we must not forget that it is very difficult to anticipate every climatic condition because even small atmospheric changes may completely alter meteorological phenomena.

Let us start making forecasts
In the progress of meteorology, the invention of the barometer (1643) was fundamental. With the barometer it became possible to note the relation between changes in the weather and variations in atmospheric pressure. Around 1780 some French experts began to form a European network of meteorological observatories equipped with homogeneous instruments, that could carry out observations of the atmospheric changes at the same time in different places. But for the first “weather charts ” we must wait for 1820, when the mathematician Brandes  was studying a way to represent atmospheric movements on a large scale.  The invention of the telegraph, instead, in 1857, enabled the observatory in Paris to communicate with other 12 observatories in other European capital cities, and to publish the first international meteorological bulletin on a daily basis.
In fact in order to forecast how the weather will be tomorrow, it is indispensable to know what is defined as the “initial state”, the present atmospheric conditions. At present every day centres that collect meteorological data are submerged by a storm of information : in the United States data are forwarded to the nine  NCEP (the National Centre for Environmental Prediction) offices  that are headed by Nws, while in Europe data from the whole world  converge at ECMWF (the European Centre for Medium-range Weather Forecasts) that is situated just outside London. ECMWF in turn provides data to 25 European countries, among which Italy, and is specialized in medium-range weather forecasts.
This European centre has been operative from 1979 and it is in charge of formulating the most accurate forecasts for the European and Mediterranean areas. In fact, the global system for processing meteorological data is based on three main centres : two operating in the Northern Hemisphere, in Washington and in Moscow, and the third in the Southern Hemisphere, in Melbourne. These three centres provide medium range weather forecasts (1-6 days)  on a planetary scale, to the member countries of the World Meteorology Organization.

Weather forecasts
The weather forecast is the expression of the probability that a certain event will take place in the atmosphere, and can be of a very short-range of time (only few hours), a short-range of time (2 days) a medium-range of time (7-10 days) or a long-range of time (over 10 days).
At present computer models are the fundamental instrument for statistical reprocessing of the  initial data and  their analysis, in order to obtain descriptive equations of the dynamics of the atmosphere. The first step for processing the weather forecasts consists in summarizing the values of the different atmospheric variables that are recorded by the various meteorology stations. These data are updated every minute and are represented by imaginary nodes of a three-dimensional network that covers the whole planet. The rapidity in carrying out the calculations and the memory of the computers enable a subsequent calculation of the variations of the initial conditions at each of the nodes of the network, with very short time intervals, with the aim of replacing new values with the previous ones. The process is repeated   till it covers the entire period for which the forecast is made, which is then presented in the form of a weather forecast chart. Just imagine, in order to elaborate a 3-day forecast in the Northern hemisphere, the calculator at the European Centre carries out 500 billion calculations!
One of the greatest problems that must be faced in formulating a weather forecast, based on the charts produced by the calculator is to evaluate the phenomena on a small scale. In fact the computer models record and process the atmospheric conditions in points that are many dozens of kilometres distant from each other, while atmospheric weather  can change drastically within a few kilometres. For this reason phenomena on a small scale, local forecasts, are processed with the help of statistical programmes that evaluate the more probable total effect of all the local atmospheric processes that take place within a space of the imaginary three-dimensional network.

Images from satellites
Observations from the meteorology stations are integrated by satellite images that are particularly useful in short-term forecasts because it is possible to obtain information regarding the extension and the movement of the cloud systems. Furthermore, a comparative study of the images taken at the same time of the visible image and the infrared image, enable a classification of the type of clouds  in the cloudy surface and it is possible to foresee the nature and intensity of the associated precipitations. An important role is carried out by the satellites to control tropical cyclones and to assess the position and follow the evolution of the jet streams.
Satellites also enable the identification of areas of the planet that are more exposed to climatic changes on a regional and continental scale, and support the studies carried out in the polar environment (for navigation in the ice, to control the isotherms and melting ice on the pack).

Wind, clouds and precipitations
The weather is characterized by wind, clouds and precipitations, which are typical phenomena of the atmospheric perturbations that depend on the distribution of the high and low pressure areas. Weather charts indicate closed and concentric lines called isobars: the same isobar unites points having the same atmospheric pressure and their distribution on the map indicates areas of low and high pressure. High pressure areas characterized by isobars with growing pressure values as one moves towards the more internal isobars are known as anticyclones. Anticyclones indicate stable weather without precipitations, without clouds because the air that moves downwards gets heated due to compression and the rise in temperature leads to a decrease in relative humidity.
In the low pressure areas, the values indicated by the isobars decrease towards the centre of the area and are called cyclones. The atmospheric perturbation is associated with low pressure areas where the hot and humid air moves upwards and expands and then cools causing possible precipitations.
Some areas of the Earth are characterized by stable and constant low or high pressures while in other areas cyclones and anticyclones alternate in quite a rapid succession as in the case of the Italian territory, characterized by variable weather all year round.

Disturbance in the middle latitudes
In the middle latitudes, the meteorological conditions are influenced by general circulation in the troposphere and the jet stream of the polar front that is mainly responsible for the formation of cyclone and anticyclone areas.
At lower levels the warm air coming from the Tropics moves toward the North-East and meets the cold air coming from the Polar regions, following a South-Westerly direction around the 60° latitude. The two masses of air meet and a contact surface forms, called the Polar front, characterized by different temperatures and humidity. Along the contact surface between the masses of air gatherings form where the denser cold air slips below the hot air that is lighter and is forced to move upwards. This movement continues and progressively the cold air completely surrounds the hot air which remains isolated and forms a cyclone; at the same time   cold air is isolated in the hot air and forms an anti-cyclone.
In particular, different types of air movement  can lead to the formation of a cyclone :

  • cold air can advance horizontally and force the hot air to rise vertically causing intense and violent precipitations with cumuliform clouds;
  • the hot air flows above the gathering of cold air of the Polar front, provoking light , diffused and persistent precipitations with stratiform clouds;
  • the cold air pushes up the hot air completely and precipitations are diffused and intense.

At the middle latitudes cyclones form in series of 4 or 5 in rapid succession and move from West to East at a speed of 40-50 km/h due to the action of the Western winds, and generally  die in a week. In summer, in Europe , this movement determines the shifting of the Azores cyclone toward the North and transit of the perturbations across central and Northern Europe. In winter the anticyclone moves towards the South and also the movements of the perturbations affect the Mediterranean area. These facts explain why there is a difference in the weather in summer and in winter in the Mediterranean area.

Written by Elisabetta Monistier

With the sponsorship of the Italian Ministry of Education, Universities and Research
Eni S.p.A. - P.IVA 00905811006