Ever wondered where those little water droplets on your lawn grass on a chilly morning come from, even though there was no rain the night before? Those water droplets are there because of a phenomenon known as condensation. It is a process through which the physical state of matter changes from the gas phase to the liquid phase as a result of temperature drop. You can think of it as a reverse process of vaporization. Molecules, which make everything around us, are in a constant state of motion. Their physical state depends on how fast or slow they are moving. For instance, when we heat the water, its molecules start moving more randomly and space themselves much farther apart. It leads to the change in the physical state of water from liquid water to water vapors. Conversely, when we cool down these water vapors, they become slower and come back closer to each other to form liquid water again. You may wonder that how these water vapors can cool down on their own? Let’s take a few examples of condensation in our daily life to elucidate the phenomenon:
1. Clouds in the Sky
Water covers almost 71% of the earth’s surface. It is present in its three phases (solid, liquid, and gas) above and below the earth’s surface and moves continuously in a cycle to tie together the major parts of the earth’s climate system: air, clouds, the ocean, lakes, vegetation, snowpack, and glaciers. This cycle is known as the water cycle, and condensation plays an important role in it. The water present on the earth’s surface, such as in oceans, rivers, lakes, and other water bodies, evaporates due to the sun’s heat. These water vapors, being lighter, rise in the atmosphere. As they move upwards, they face a significant drop in the temperature. This happens because of the low air pressure at higher altitudes. The lower temperature allows the water vapors to cool (loose heat) and slow down to combine back together as a cloud and condensate themselves into water droplets. These water droplets start to accumulate together and form bigger water droplets. When these water droplets become significantly heavy, they precipitate under the influence of gravity to become rain.
2. Morning Dew
If you wake up early on a cold morning, you may find a landscape transformed by thousands of glittering dewdrops. Dew is nothing but water present in the air. There are water vapor molecules present in the air which we cannot see with our naked eyes. As the sun sets, the air and the surfaces start to lose the heat they have gained through solar radiations during the day. Cooler air can not hold as much water vapor as warmer air; as the temperature drops, the rate of condensation exceeds the rate of evaporation, and water droplets form. The temperature at which this occurs is known as the dew point. Dew settle down on surfaces that are cooler than the surrounding air. Therefore, you find tiny water droplets on your cars, roofs, and lawn grass, on a chilly morning, even though it didn’t rain the night before. Moreover, Dew plays an important role in improving the ecosystem’s health and resilience. For instance, high levels of dew in grasslands could provide the moisture needed for different types of seeds to germinate, perhaps even creating conditions that support the transition from grasslands to forests.
3. Fog in the Air
It is not always necessary for water vapors to go up to higher altitudes to become a cloud; sometimes, given certain conditions, they can condense just above the earth’s surface to form the fog. The fog occurs when air with relatively high humidity comes into contact with a colder surface, often the surface of the Earth, and cools down to the dew point. Additional cooling leads to condensation and low-level cloud formation. The fog that develops when the hot air moves over a colder surface is known as advective fog. Another form of fog, known as radiative fog, develops at night when the surface temperatures are cool. If the air is not moving very fast, the fog layer does not mix easily with the air above it, which promotes the production of shallow ground fog.
4. Breathing Fog in Winters
During winters, you can get an approximation of how cold is there outside just by watching the visible air coming out of your mouth. It is a well-established fact that we inhale oxygen and exhale carbon dioxide. Since both gases are colorless, you may wonder how we can see the air coming out of the mouth? Our body is almost 60% water, and when we exhale out, there are some water vapors present along with carbon dioxide. Water molecules need a lot of energy in order to remain in a gaseous form. When the warm water vapor molecules from your lungs reach colder air, they condense into tiny droplets of liquid water that can reflect the light falling on them in such a way that they become visible even to the naked eye. During a normal day, however, the temperature is high enough to keep the water molecules in their vapor state, and therefore, we cannot see them.
5. Foggy Car Windows
During the winter season or cold weather, you see fog on your windows and windshields while driving that can cause difficulty in steering the vehicle. Windshields fog up because of differences in temperature and moisture in the air (humidity). Window fog is caused by moist, humid air inside the vehicle coming in contact with the cold windshield before it’s warmed up. Your windshield will be colder than the air inside the car as it is in contact with cooler outside air. The humid/moist air comes into contact with the cold surface of the window glass and condenses into what we know as fog. The key to whether we can expect to find fog is the temperature at which the maximum amount of water vapor can be held in the air at a given pressure before condensing into a liquid. This is known as the dew point. It is also important to consider that the air trapped in the car from the previous day will be more humid than the outside humidity level, therefore holding more moisture.
6. Foggy Glasses
If you wear a pair of glasses, you are probably familiar with the struggle of foggy glasses during winters. The difficulty can also be observed when you go from a cold, air-conditioned room to outside on a humid day, the lenses fog up as small water droplets coat the surface via condensation. Condensation is responsible for the water inside the frame on a cold day. The water vapors for condensation may originate from your own mouth or maybe rising from the coffee, soup, or any other hot beverage you are enjoying on a cold day.
7. Sweaty Drink-Cans
In the above-mentioned examples, we have been mostly discussed those in which condensation can only be observed during cold weather; however, you can also recognize the phenomenon on your cold drink that you are enjoying on a hot sunny day. As you take that bottle or can out of the refrigerator, tiny droplets start to appear over the surface of the bottle. It may appear that the bottle is sweating from the heat, while the reality is quite the opposite. The cold surface of the bottle provides a low-temperature atmosphere to the water vapors present around it so that they can lose their energy and slow down in order to condense into water droplets.
Most of us have, at some point in our life, gazed up at the skies, looking for familiar shapes in the clouds. Some of you might have come across a white line of cloud falling behind an airplane. These lines are known as contrails, a shorthand notation for condensation trail. There is a common misconception that contrails are just dirty streams of pollutants billowing out of airplanes as they cross the sky, while in reality, they are mostly ice crystals. Water vapor is already present in the atmosphere, but when the extra vapor from the airplane exhaust rapidly saturates already moist air, the water condenses and freezes into minute ice crystals. At high altitudes, the water vapors are already present in a cold environment. The localized increase in water vapor due to exhaustion can raise the relative humidity of the air past saturation point. The vapors then condense into tiny water droplets that freeze if the temperature is low enough. These millions of tiny water droplets and/or ice crystals form the contrails. The condensation can also take place due to the relative decrease in pressure. As an airplane wing generates lift, it forms a vortex around the wingtip. These wingtip vortices persist in the atmosphere long after the aircraft has passed. The reduction in pressure and temperature across each vortex can cause water to condense and make the cores of the wingtip vortices visible. This effect is more common on humid days.
9. Fog Fence
Although this example is not as common as others, it is a very important application of condensation. A fog fence is an instrument used to collect water from fog. Fog collection refers to the collection of fog water using large pieces of a vertical mesh net, known as a fog barrier, fog collector, or fog net, that allows the fog-droplets to flow down to a trough below. Atmospheric water vapor from the air condenses into droplets of liquid water, known as the dew, on cold surfaces by condensation. The phenomenon is most observable on thin, flat, exposed objects including plant leaves and blades of grass. As the exposed surface of the collection mesh cools by radiating its heat to the sky, on the series of parallel wires, water condenses and collects at the bottom of the net. This process needs no external energy and is naturally facilitated by temperature variations, making it desirable in less developed areas for deployment. The word fog fence comes from its long rectangular form that resembles a fence, but this type of structure is not limited to fog collectors. The fog collector’s efficiency is dependent on the net material, the size of the holes and filament, and the chemical coating. An ideal location is a high-altitude arid area near cold offshore currents, where fog is common, and therefore, the fog collector can produce the highest yield.