Vesihöyrynä

Vesihöyrynä (water vapor) is the gaseous state of water. It forms when water molecules gain enough energy to overcome intermolecular forces and enter the gas phase, through evaporation or boiling. In ordinary weather vesihöyrynä is invisible; it becomes visible only when it condenses into tiny droplets as dew, fog, or clouds, or when steam is produced by rapid boiling.

In the atmosphere, vesihöyrynä constitutes a major part of humidity. Its amount is described by partial pressure or specific humidity and by relative humidity relative to the temperature. The saturation vapor pressure increases with temperature; at 20°C it is about 2.3 kPa, and at 100°C it equals the surrounding air pressure (~101 kPa). When the air reaches saturation, further cooling causes condensation and cloud formation; the temperature at which this happens is the dew point.

Physically, vesihöyrynä has a high latent heat of vaporization; energy is required to convert liquid water to gas, which is released when it condenses. It also acts as a powerful greenhouse gas: water vapor traps infrared radiation and contributes to the natural greenhouse effect, though it is governed by the climate system through feedbacks with temperature and clouds.

Industrial and everyday uses rely on vesihöyrynä: steam heating, sterilization, cooking, laundry, and power generation via steam turbines. Measuring its amount is central to meteorology and engineering; common quantities include relative humidity, dew point, and mixing ratio. Safety and comfort considerations in buildings and agriculture depend on controlling humidity levels.