17.1: Water and Humidity

Phase Changes of Water

In order to discuss weather, we first need to understand one of the essential elements of weather: water. Water is an amazing substance! It is made of the two most abundant elements in the universe, two hydrogen atoms and an oxygen atom, forming H₂O. More than 70% of the Earth’s surface is covered with it. We are constantly surrounded with water, in its gaseous form, in the air around us. It can also be found in its liquid form in the ground under our feet as soil moisture and groundwater. Serve yourself a glass of ice water (Figure \(\PageIndex{1}\)). You’ll have the only substance on Earth found in all three phases (solid, liquid, and gas) simultaneously.

Figure \(\PageIndex{1}\): Water Can be Found in Three Phases Simultaneously. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

Water is the only substance that is less dense in its solid state (ice) than in its liquid state. Ice floats in water. Because of this, life can continue to thrive in our lakes, rivers, and streams that freeze over each winter. Water is a universal solvent. Given enough time, everything will dissolve in water. This not only allows our blood to carry nutrients throughout our body, but water also functions in nature, bringing nutrients to the leaves of plants, transporting minerals across the landscape, and acting as a major erosional process. Water is the basis of life on Earth.

Further, water conserves heat energy much more efficiently than other natural substances such as soil. It requires a much greater amount of energy to change the temperature of water. This is referred to as the specific heat capacity. With our world being 70% covered with water, our oceans have an extraordinary capacity to store and distribute energy.

Water releases a huge amount of energy when it condenses or freezes. Inversely, water absorbs that same, huge amount of energy when it evaporates and melts. This is referred to as latent heat. Water is found naturally in three different phases. You are probably already familiar with most of the six phase changes of water:

• Freezing occurs when liquid water transforms into ice.
• Melting occurs when ice transforms into liquid water.
• Evaporation occurs when liquid water transforms into water vapor (gas).
• Condensation occurs when water vapor transforms into liquid water.
• Deposition occurs when water vapor transforms into ice.
• Sublimation occurs when ice transforms into water vapor.

Figure \(\PageIndex{2}\): The phase changes of water. Red arrows indicate absorption of energy which cools the environment, while blue arrows indicate the release of energy which warms the environment.

What is required to change the phase of water? For each of these steps, there is a significant transfer of energy between the water and the surrounding environment. Let’s think of a few examples:

• In the morning you take a warm shower. Some of the warm water evaporated in the bathroom. When you get out of the shower, you find that the cold glass of the mirror has small water droplets all over it. The cold glass absorbed some energy causing water to condense on the surface. Condensation releases energy and warms the environment (in this case, the glass).
• Later that day, you injure your knee in a soccer accident. Your coach pulls out a plastic bag, fills it with ice and tells you to sit down and “ice” it. What happens? As the ice begins to melt (changing from a solid to a liquid) it absorbs heat. It makes your knee feel cold because that energy is being absorbed from your knee. So, the process of melting absorbs energy and cools the environment.
• After practice, you head to the nearest river, lake, or beach to cool off. It’s several degrees cooler near the water than the soccer field. Evaporation absorbs energy and cools the environment.

While these are some common examples, this absorption and release of heat energy by the phase changes of water help regulate temperature and distribute energy from equatorial areas to polar regions. This transfer of energy between water and the surrounding environment is referred to as latent heat. Each phase change of water will either absorb or release latent heat energy. Figure \(\PageIndex{3}\) outlines the amount of energy required to change the phase of one gram (1 gram of water is equivalent to 1 milliliter or 1 cubic centimeter) of water. Between ice and liquid water, 80 calories are absorbed during melting or released during freezing at 0°C. It takes 100 calories to raise or lower the temperature of water between 0°C and 100°C. Between liquid water and water vapor, 540 calories are absorbed during evaporation or released during condensation.

Figure \(\PageIndex{3}\): Latent Heat Related to Phase Changes of Water. Figure by Scott Crosier is licensed under CC BY-NC-SA 4.0

Consider the huge amount of energy absorbed and released through evaporation and condensation! This is a major means of storing and transferring energy across our Earth. At the equator, water evaporates and then condenses as it rises. Further, the condensation that takes place in some storms actually causes an additional uplift in the storm. This is particularly true in tropical storms like hurricanes.

Atmospheric Humidity

Imagine an air parcel as a small box of the atmosphere that is distinct from the atmosphere surrounding it in terms of temperature and the water vapor it contains. Humidity is the amount of water vapor in the air in a particular parcel of air. We usually use the term to mean relative humidity, the percentage of water vapor a certain volume of air is holding relative to the maximum amount it can contain. If the humidity today is 80%, it means that the air contains 80% of the total amount of water it can hold at that temperature.

Relative humidity can only reach a maximum of 100%. This is called the saturation point. What will happen if the humidity increases to more than 100%? The excess water condenses and forms precipitation. This is a simplistic look at this topic, because depending on the temperature of the air, the capacity of water content per kilogram of air changes. Warm air can hold more water vapor than cool air, so raising or lowering the temperature can change the air’s relative humidity. The temperature at which saturated air can condense is called the dew point temperature or dew point. This term makes sense, because the water condenses from the air as dew. The dew point temperature depends on the amount of moisture in the air. The more moisture in the air, the higher the dew point temperature.

A smaller scale example of this would be a cup full of ice water. Depending on the temperature and humidity levels for the day, if the contents in the cup are cooler than the surrounding air, the glass will cause the moisture in the air around the cup to condense along the glass surface.

Figure \(\PageIndex{4}\): A glass of ice water. The glass is covered in condensation since the ice water is cooler than the surrounding air, causing the water vapor in the air to condense on the surface of the glass.

The following graph shows the relationship between relative humidity, dew point and overall air temperature.

Figure \(\PageIndex{5}\): Graph of the amount of water in air (in grams per kilogram of air) for 100% and 50% relative humidity across a range of air temperatures (in °C). For example, at 40°C, 1 kg of air at 50% relative humidity contains 25 g of water, while 1 kg of 100% relative humidity contains 50 g of water. Note that 100% relative humidity is the dew point.

References

• Phase changes of water. Authored by: LuckySoul. Located at: stock.adobe.com. License: education license
• Glass of ice water. Generated by: Gemini AI by N. Ikeda. License: Public Domain
• Relative humidity. Modified from: GregBenson by N. Ikeda. Located at: https://commons.wikimedia.org/wiki/File:Relative_Humidity-ru.svg. License: Public Domain