Condensation is a change from the gaseous state to a liquid or solid. It happens when warm, moisture-heavy air comes into contact with a cool surface and is deposited as drops of water.
This process is a key part of the water cycle, and it also forms the basis for fog and clouds. It’s why warmer climates often have higher humidity levels than colder ones.
Water vapor
Water vapor is one of the most important constituents of the atmosphere, both as the raw material for clouds, fog and precipitation, and as a conveyor of energy (latent heat) and a regulator of planetary temperatures through absorption and emission of radiation, most importantly in the thermal infrared. Its amount varies widely in space and time because of the enormous variety of “sources” of evaporation and “sinks” of condensation.
Condensation happens whenever moisture-packed warm air contacts a cold surface, such as the ground, oceans or ice. The water molecules release their latent heat, raising the air temperature to the dew point (or its equivalent), and then releasing it again as the air cools. The cycle repeats itself, causing the temperature of the air to rise and fall on a regular basis.
As it rises in the atmosphere, water vapor transports heat from warmer to cooler regions and from lower to higher altitudes, especially over land areas and oceans. It also regulates planetary temperatures through its capacity for both absorption and radiation feedback (the greenhouse effect).
Water vapor measurements have historically been sparse, but significant improvements are being made. Radiosonde observations have good resolution in the lower troposphere over populated areas, but are less reliable in remote regions and at high altitudes. Satellite remote-sensing methods, including microwave and infrared sensors, are also being used to measure total column water vapor. These data, along with data assimilation systems that combine observations and output from atmospheric models, are being used to improve climatological analysis of long-term trends in global and regional climate.
Clouds
The air we breathe is chock full of water vapor. It’s what makes a summer day feel humid and muggy, and it is what we see when clouds move by overhead. Clouds are actually collections of tiny water droplets or ice crystals that are light enough to float. When the water vapor in the clouds cools and condenses back into liquid form, the water droplets or crystals come together to form the billowing shapes we see.
For condensation to take place, the water vapor must have something to “stick” to. This is called a condensation nucleus and is a particle that attracts water molecules. It could be a microscopic bit of clay, salt, or solid pollutant (like ash from smoke) or it could be just the bare surface of a tree limb. As the water molecules attach to these nuclei, the cloud grows and raindrops begin to form.
As the cloud grows, it may also be mixed with sand and dust particles. This is a type of cloud called an anvil cloud.
The atmosphere is constantly in motion. As a block of air rises, it expands and cools, and its temperature changes very little due to the lower pressure higher in the atmosphere. This is adiabatic cooling. As the block of air cools, more moisture may condense to produce more and larger cloud droplets. If there is more evaporation than condensation, the cloud dissipates.
Dew
The water droplets that appear on leaves or grass in the morning are called dew. Dew is water that has condensed from the air during the night due to cooling. Dew can be a beautiful sight and kids enjoy playing with it when it is on the ground.
Dew is also a source of moisture for plants and soil. It is especially important in arid regions where it reduces the amount of water that is lost through evaporation and transpiration.
For dew to form, the air must be cooled to a temperature known as the dew point. This temperature is different for each location and can vary throughout the day. It is determined by using a wet-bulb thermometer, a humidity slide-rule or a hygrometer. Once the air reaches the dew point, any surface will experience condensation. The moisture can be in the form of dew, fog, mist or frost.
The best conditions for forming dew are clear skies, light wind and adequate soil moisture. Moisture in the ground can contribute to dew formation by being radiated into the air by longwave radiation or by the ground’s surface cooling as it heats up during the day. The more cooling an object experiences, the lower its dew point will be and it will become more susceptible to condensation. Dew is most common on the ground, but can also form on other surfaces such as trees, cars and railings.
Rain
Teachers can use Twinkl’s range of resources to help children learn about condensation as part of the water cycle. They can find lesson plans, classroom activities and teaching materials to support students in KS2 science, geography or weather lessons.
Condensation occurs when warm air rises, cools and the water vapor it contains drops back to Earth’s surface as fog or dew. Then, it can fall again as rain. Water vapor can also change phase from liquid to gaseous form by cooling more quickly, which gives rise to steam.
As the water droplets form, they stick together, forming clouds of various shapes and sizes. Some of these clouds are very large and fluffy, like the ones we see in the sky. Other clouds are smaller and dense, forming heavy precipitation like rain. To get to the point where rain forms, clouds must contain many of these tiny water droplets that are constantly bumping into each other and growing larger until they are too big to evaporate any more.
To begin with, the water droplets must have something to rest on, a small particle known as a cloud condensation nucleus. There are trillions of these particles in the atmosphere. They’re dirt from the ground, bits of pollution, pollen, spores, plant fragments and even bacteria. Some of them are hygroscopic, meaning they attract moisture, and serve as the condensation nuclei that can initiate cloud formation.