We make a basic distinction between mechanical and electronic altimeters.
However, they both do the same thing - measure the air pressure. This means that they not only act as altimeters but also as barometers. This often enables a more exact regional or local weather forecast to be made than that provided by the official weather services.
An altimeter, map and compass are essential for knowing where you are in difficult terrain.
The air pressure falls as the altitude increases. All altimeters use this phenomenon in one way or another. Thus they are really barometers. The familiar mercury barometer was perfectly good for home use, but impractical when travelling. Fahrenheit developed a boiling-point thermometer in 1724, from which the air pressure could be derived from the temperature at which water boiled. Lucien Vidi constructed an instrument in 1847 which measured the air pressure by means of the spring force of an evacuated, metal can. These aneroid barometers are also known as dry barometers.