Air has its own weight, and so you can measure the pressure it exerts on the earth's surface.
Since the weight of air increases with its density and decreases with temperature, it follows that high pressure will indicate fine weather and low pressure will instead correspond to worse weather. If atmospheric conditions are stable, atmospheric pressure decreases by about 9 mBar for every 100 metres of altitude gained.
This equation is what makes it possible to relate the barometer to the altimeter, an instrument hikers generally carry: if, while staying at a certain altitude for a long enough time (e.g. a night in a hut), the altimeter shows a change in altitude, then a change in the weather is certainly under way and the apparent change in altitude is due
in reality to a change in atmospheric pressure. In particular:
- if the altitude has decreased, there is high pressure and the weather will improve;
- if the altitude has increased, there is low pressure and the weather will get worse.
On weather charts the points of equal atmospheric pressure are shown with lines (isobars) running concentrically. If the pressure values at the centre of the isobars are low, the disturbance is called cyclonic, while if at the centre of the isobars there is an increase in atmospheric pressure, the disturbance is anticyclonic.
Bear in mind that a weather chart shows the position of the disturbances at a given moment: to get an idea of how things will evolve, you need to know the position of these disturbances at different times (hour/day), so as to estimate their movement and trend and, therefore, make a guess at how the weather will evolve.
Bibliography:
- Polvere di mare, freeride a Roccaraso - Rotili, Verdecchia
- La neve e le valanghe - R.Cresta Editore Mulatero
- Manuale di meteorologia alpina - Kappenberger, Kerkmann
- Dispense di meteorologia - Società Escursionisti Milanesi
- www.montagnapertutti.it - C. Gozzi