Air is a colourless, odourless and tasteless gas mixture. It consists of many gases, but primarily oxygen and nitrogen. Air can be considered a perfect gas mixture in most calculation contexts. The composition is relatively constant, from seal level and up to an altitude of 25 kilometres. Air is always more or less contaminated with solid particles, for example, dust, sand, soot and salt crystals. The degree of contamination is higher in populated areas, less in the countryside and at higher altitudes. Air is not a chemical substance, but a mechanically mixed substance. This is why it can be separated into its constituent elements, for example, by cooling.




Air can be considered as a mixture of dry air and water vapour. Air that contains water vapour is called moist air, but the air?s humidity can vary within broad limits. Extremities are completely dry air and air saturated with moisture. The maximum water vapour pressure that air can hold increases with rising temperatures. A maximum water vapour pressure corresponds to each temperature. Air usually does not contain so much water vapour that maximum pressure is reached. Relative vapour pressure (also known as relative humidity) is a state between the actual partial vapour pressure and the saturated pressure at the same temperature. The dew point is the temperature when air is saturated with water vapour. Thereafter with a fall in temperature the condensation of water takes place. Atmospheric dew point is the temperature at which water vapour starts to condense at atmospheric pressure. Pressure dew point is the equivalent temperature with increased pressure.