Water vapor (aqueous vapor) is the gaseous phase of water. It is one stage of the water cycle of our planet. Water vapor is formed from either evaporation or heating of liquid water, or from the sublimation of ice. In nature, water vapor is continuously produced by evaporation and removed by condensation. Along with carbon dioxide and methane gas, water vapor is one of the greenhouse gases.
The release of water molecules from liquid water is considered evaporation. The transition of these molecules from liquid to gas is done so by the absorption or release of kinetic energy which is defined as thermal energy and only occurs when there are variations in the temperatures of the molecules. Water molecules take a little heat with them as they turn to vapor. This process is called evaporative cooling. The amount of water vapor in the atmosphere determines how quickly each molecule returns back to the surface.
Evaporative cooling is limited to atmospheric conditions. The temperature of the atmosphere and the surface of the water determine the balance of vapor pressure. Humidity is the amount of water vapor in the atmosphere. 100% relative humidity occurs when partial water vapor pressure is equal to balanced vapor pressure. This condition is also known as complete saturation. Humidity ranges from 0 grams per cubic meter in dry air to 30 grams per cubic meter in saturated air at 86 degrees Fahrenheit. Sublimation is a form of evaporation that occurs when molecules become gaseous from ice or snow. Sublimation occurs when ice and snow disappears slowly in temperatures too low for melting to occur.
Condensation is what happens to water vapor when it forms on a surface that is cooler than the actual water vapor. When water vapor condenses onto a surface, a net warming occurs on the surface. The water molecules contain a parcel of heat that can make the air temperature near it drop slightly. In the atmosphere, condensation can produce clouds, fog and precipitation. The dew point determines how cool it must be before water vapor can condense. Condensation also occurs when the surface temperature is at or below the dew point temperature of the air around it. Deposition is the formation of ice or frost on a surface from water vapor. Deposition is a type of condensation.
Water vapor is both lighter and less dense than dry air. At the same temperature, a column of dry air will be denser or heavier than a column of saturated air. The buoyancy of water vapor in the atmosphere remains relevant with the balance in temperature. At higher temperatures, water vapor increases, making the buoyancy level greater. Increase in buoyancy can have a significant atmospheric impact. When the air and sea temperatures are at or above 77 degrees Fahrenheit, the water vapor in the atmosphere can create powerful, moisture rich, upward air currents. These air currents can lead to cyclonic and/or anti-cyclonic weather events (tornadoes and hurricanes).
Air pressure increases when the concentration of water vapor in the air increases. As the partial pressure contribution of water vapor increases, other gases in the atmosphere decrease in partial pressure contribution. As its concentration increases, it displaces other components in the air. This can have an effect on respiration in air temperatures greater than 95 degrees Fahrenheit. The proportion of water vapor in the atmosphere can be significant enough to cause stuffiness that is experienced in humid jungles or buildings with poor air circulation.
Water vapor represents an environmentally significant component of the atmosphere. Nearly 100% of it is contained within the troposphere. The condensation of water vapor is responsible for clouds, rain, snow, and other forms of precipitation that are all important elements of what we experience as weather. Latent heat, which is released to the atmosphere when condensation occurs, is also a significant factor in atmospheric conditions. Latent heat released into the atmosphere is directly responsible for powering destructive storms that ravage the planet.
As a greenhouse gas, water vapor plays a major role in climate warming due to increases in carbon dioxide that it produces. It is unclear how this process could affect cloudy conditions in the future. Fog and clouds form around cloud condensation nuclei. Without the nuclei, condensation would only occur at much lower temperatures. With global warming on the rise, this could possibly determine how future weather events shape the world’s atmosphere. Water vapor molecules remain in the troposphere for about 10 days of the water cycle. The water depleted from the troposphere is replenished by evaporation from the oceans, lakes, rivers, transpiration of plants, deposition of ice, and other geological processes.
Because water vapor molecules absorb radio wave frequencies, water in the atmosphere dampens radio signals. Atmospheric water will reflect and refract signals to an extent that depends on whether the water is gas (vapor), liquid (rain), or solid (ice). Radio signals lose strength the further they travel through water. Water vapor also reflects radar. Water vapor plays a key role in lightning production in the atmosphere. Though clouds are the real generators of static charge in the atmosphere, the capability of clouds to hold massive amounts of electrical energy is directly related to how much water vapor is present in the system. The amount of vapor in the system controls the energy of the air. In low humidity, static discharge is quick but light. Higher humidity produces fewer discharges that are more powerful.
Water vapor is not restricted to just Earth. Comet tails are made up mostly of water vapor. As the comet approaches the sun, the ice found on most comets begin to sublimate to vapor, which reflects light from the sun. Brighter tails on cold and distant comets suggests that the tail is formed of carbon monoxide sublimation. Scientists hypothesize that if water moves around on Mars, it does so as water vapor. The northern pole of Mars has ice that sublimates as the sun hits it during the Martian summer. This sublimation may enable the massive seasonal storms that could possibly carry water toward the equator. Water vapor circling an aging, massive star has been discovered by one of NASA’s satellites using an onboard spectrometer. Spectroscopic analysis of an extra-solar planet in the constellation Pegasus, provided the first evidence of water vapor in the atmosphere beyond our Solar System.