Condensation is the process of gaseous water (water vapor) turning into liquid water. It is important for the water cycle, and is responsible for ground-level fog, moisture on cold glass, and the dripping water you see on a can of cold drink.
Condensation happens when warm air comes into contact with a cold surface or when the air becomes saturated with water vapor, and molecules become closer together.
Clouds
Condensation is one of the most important steps in the water cycle because it turns water vapor into liquid water. This process helps to form clouds and precipitation, which replenishes Earth’s water sources, such as rivers, lakes, and groundwater, enabling the redistribution of water around the globe.
Air can only turn back into liquid water if it has something to “cling” to; that’s where cloud condensation nuclei come in. These can be anything from specks of dust or pollen to particles from smoke, ocean spray, or salt, as well as air masses associated with cold fronts. These nuclei are hygroscopic, meaning that they attract water molecules. The hygroscopic particles must also be small, about a million times smaller than the water droplets they cling to, and at a particular altitude known as the cloud base or lifting condensation level.
As warm air rises, it cools because atmospheric pressure decreases with height. This cooling causes the water vapor in the air to condense on its surface. As it does, the water vapor forms billions of tiny droplets that become clouds.
Dew
The glistening droplets of dew that you find on grass and metal surfaces in the morning are formed by condensation. However, it is important to note that while the physics behind condensation and dew are very similar, they are two different things. Dew is a natural form of water that forms when air cools to its dew point temperature, which varies by region and time of day.
The dew point is defined as the lowest temperature at which water vapor in an atmosphere will condense and become liquid, or saturation level. It is possible to have a dew point below the current air temperature and still not have rain, but this would not be a normal situation. If it rained in such an environment, the extra moisture that is not already condensed would be in the form of ice crystals or fog instead of precipitation.
Dew is most commonly seen in the mornings, and it can appear on nearly any surface that is not conducting heat from the ground. The best conditions for dew are clear skies, light wind and decent soil moisture that allows the surface to cool to its dew point. A typical dew night will see the downward diffusion of the water vapor from the atmosphere, although it may also occur where the underlying soil surface is the major source of the vapor (in which case this is known as distillation). The amount of dew that is produced in a typical dew night can range from very little to up to 0.5mm.
Fog
Like clouds, fog is a visible aerosol made up of tiny droplets or ice crystals suspended close to the Earth’s surface. But unlike overhead clouds, which form from a specific weather situation (like when warm air meets cooler airflows), fog occurs on a much more global scale and can be caused by a wide range of factors.
Fog forms when water vapor cools enough to cling to dust, salt and other particles or to become a liquid or solid. This happens when the temperature of the water vapor drops below its saturation point, which is the temperature at which the water will start condensing.
The same process that causes fog can also make it thicker and stickier. Fog can appear and disappear very quickly as conditions change. For example, a slight breeze can disrupt the formation of fog by mixing the moist ground air with the dry upper atmosphere.
One of the most common types of fog is called radiation fog, which typically appears in the winter. This type of fog develops at night when the ground is cooled by radiant heat loss, which can be more efficient when skies are clear and winds are light or calm. This is especially true if it has been raining the previous day, as this will raise the dew-point temperature and make it easier for the air to reach saturation.
Rain
Condensation is one of the major steps in the water cycle. It happens when moisture-packed warm air comes into contact with a cold surface, such as the water droplets on a window on a humid day. It also happens when glasses fog up as the air cools to the dew point temperature.
Condensations are also responsible for the moisture that forms on plants and other living things, causing it to form droplets or crystals that can fall to the ground as rain. It is also why the water on a plant’s leaves evaporates as it moves through its stomata.
Water vapor can condense into liquid water only when it reaches saturation humidity, meaning it has reached its maximum state of saturation at its present temperature. The water vapor can become saturated either by adding more water to it through evaporation, or by cooling the air to its dew point. Cooling the air is the most common method of creating condensation.
In order for condensation to result in precipitation (rain, snow, sleet, hail) the water droplets must combine and grow large enough to be too heavy to stay afloat and fall to the ground. This is called coalescence or the Bergeron process. In tropics and other warm climates, this is usually the dominant mechanism of precipitation formation. In cold climates, however, it is more likely that ice crystals and water droplets coexist side by side. In this case, ice crystals form around condensation nuclei, which are microscopic bits of clay mineral, salt, solid pollutants such as ash from smoke, or even microscopic bacteria.