Humidity is the proportion of water vapour in the atmosphere and occurs when water in for example oceans and lakes warms up and rises. The humidity has a major impact on weather and climate. Water vapour is the most abundant of all greenhouse gases and has the greatest effect on Earth's warming. A warmer earth means that evaporation increases so that the atmosphere becomes more humid, which amplifies the initial warming.

Through evaporation of water from oceans, lakes, vegetation, soil, exhalation, etc., water vapour is added to the atmosphere. Water is present in the atmosphere in three phases; solid (as ice or snow), liquid (as water) and gaseous (as water vapour). Condensation occurs when water vapour is cooled and this can form clouds or mist.

Humidity in our daily lives

We are all affected by humidity in our daily lives, perhaps without even thinking about it.

Low humidity can cause materials to dry out and high humidity can lead to mould or corrosion. If electronic devices are exposed to dry air it can cause static electricity, while high humidity can lead to condensation with flashover as a result. Foodstuffs can dry out or turn mouldy, if stored too dry or wet.

Humidity also significantly affects the formation of ice, for instance on roads, aircrafts, wind turbines and masts.

Our health is affected

Relative humidity is the amount of water vapour in the air relative to the maximum amount at same temperature. When the relative humidity is high and the wind is weak the air might feel sticky and muggy. During these conditions sweat cannot evaporate and in combination with high temperatures this can lead to heatstroke.

If the humidity is low, people can withstand high temperatures as long as perspiration is able to cool the body. However, there is a risk of dehydration and therefore it is important to supply the body with extra fluid.

Humidity in future climate

Water vapour is the greenhouse gas that has the greatest effect on global warming. Carbon dioxide comes in second place although it is more commonly spoken about.

Using mathematical models the future climate is simulated with climate scenarios, which show that Sweden is expected to become warmer. This means that evaporation will increase. Consequently the amount of water vapour (absolute humidity) in the atmosphere is predicted to increase which will amplify the greenhouse effect. There is also a risk that precipitation may be more intense with a greater amount of water vapour in the atmosphere.

Whether, or how, the relative humidity will change in the future is on the other hand more uncertain.