Air

Isobars e isotherms

In order to indicate the pressures and temperatures of the atmosphere at ground level and at high altitudes in various parts of the Earth, special charts are created.
The Isobar chart or Pressure chart, shows the pressure distribution. From the Greek words isos, equal and baros, weight, having an equal pressure,  isobars are lines that join together points having an equal atmospheric pressure, analogously to the isohypse contour lines (to indicate equal altitudes) that are used to show the mountains on a topographical map. Since the pressure of an air mass depends on its altitude and the temperature, in order to compare the pressure values in different zones at different altitudes and different temperatures, it is necessary to make the data “uniform” before indicating them on a chart. With opportune conversion tables, the pressures are brought to sea level and to the same temperature, which has been agreed to be 0° C and only after this fundamental operation they are marked on the charts. Isobar charts are a basic instrument for meteorology, because they enable the identification of zones with greater or lesser pressure, which are very important in the determination of atmospheric circulation.
In order to represent the trend of the pressures at the higher altitudes better,  isohypse charts are used  instead. These charts show the trend at the high altitudes, compared to sea level, of the surface of a given pressure value (usually 500 mb), in a manner that is analogous to the topographical contours, where the isohypses refer to the Earth’s surface.
In the same way it is possible to draw isotherm charts, i.e. the lines joining points of equal temperature. Also in this case, before drawing the chart it is necessary to eliminate the effect of the altitude and convert the data to sea level values.
It is very useful, in order to analyze the climate and to make weather forecasts, to compare the isobar charts, (on ground level and at high altitudes) and the isotherms recorded at different hours of the day and in different periods of the year.