6.1: The Water Cycle

Phase Changes

Water is considered one of the most important, if not the most important, ingredients for life as we know it. When NASA scientists and astrobiologists evaluate a planet’s ability to sustain life, one of the main criteria is the presence of water. Here on Earth, temperatures are at the perfect range (approximately -129°F to +134°F based on record low/high temperatures) for water to exist naturally in all three of its phases – Liquid, Solid, and Gas. In fact, not only are those temperatures ideal for water to exist in all three phases, but they are also ideal for water to undergo transitions between each of the phases in processes called Phase Changes. These phase changes are the most important for sustaining life on Earth. Without water’s ability to readily change phases, we’d never get rain/snow, we wouldn’t have rivers/lakes/streams, and any water not present in the oceans would eventually drain into the ocean, leaving us without any fresh water sources. Figure \(\PageIndex{1}\) is an extremely simplistic, but useful diagram of Earth’s water cycle, including important phase changes and processes.

Figure \(\PageIndex{1}\): The Water Cycle on Earth. (Public Domain; NWS Jetstream via NOAA) Alternative description of the image.

The primary driving mechanism of the water cycle is heat, which “pulls” water from Earth's surface into the atmosphere.

2. The main source of heat for various processes on Earth is: (hint: which of these three is shown on Figure \(\PageIndex{1}\)?)
   1. Heat from the Earth's core
   2. Solar radiation from the Sun
   3. Metabolism from plants and animals

Without this mechanism, there would be no water cycle on the Earth. Heat is absorbed by water as it evaporates (changes from liquid to gas) and rises in the atmosphere. As water in the form of water vapor (gas) travels through the atmosphere, it loses some of that heat in the form of sensible heat exchanges (such as conduction or convection) and then condenses (changes from gas to liquid) into liquid cloud droplets (note: if the temperature of the air surrounding the liquid water is below freezing, the water can freeze into ice becoming ice and snow). Once these liquid droplets grow large enough, they fall out of the cloud as precipitation (ice can also fall as precipitation). This precipitation then reaches the Earth's surface, where it either collects into ice caps and glaciers, is absorbed by the ground through infiltration, is consumed by plants and animals, or runs off back to the ocean. Other phase changes, such as sublimation (ice to gas), deposition (gas to ice), and melting (ice to liquid), also regularly occur on Earth, as shown in Figure \(\PageIndex{2}\) and are essential components of the Water Cycle.

Figure \(\PageIndex{2}\): Phase changes of water. Melting, evaporation, and sublimation are considered cooling processes because they take heat from the environment. Condensation, freezing, and deposition are considered warming processes because, during the phase change, energy is released into the environment. (CC BY-SA 3.0; Synkizz via Wikimedia Commons). Alternative description of the image.

Latent Heat

While the transport of water is critical to Earth’s climate, the water cycle's equally important, if not more important, role is the transport of heat. The primary reason for this is what is known as latent heat. Latent Heat is the heat that is exchanged between water and its surroundings as it changes phases. For water to remain in a given phase, it must have a certain amount of energy stored in its chemical and molecular bonds. When it undergoes a phase change, it releases heat from those bonds as water condenses, freezes, or deposits, which is known as “moving down a phase” from a higher-energy to a lower-energy phase. Water absorbs heat into those bonds when it melts, evaporates, or sublimates, which is known as “moving up a phase” from a lower-energy to a higher-energy phase.

Important: Before we move on, let’s review these definitions and their importance:

• Phase Change: When water transitions from one phase to another.
• Order of Phases (from lowest to highest): Solid (Ice), Liquid, Gas (Vapor). Ice has the lowest energy, while gas has the highest.
• Moving “Up” a Phase: When water transitions from a lower phase to a higher phase… melting, evaporation, and sublimation. Water absorbs heat from its surroundings as it changes phase, cooling those surroundings.
• Moving “Down” a Phase: When water transitions from a higher phase to a lower phase… freezing, condensation, and deposition. Water releases heat into its surroundings as it changes phase, warming them.

This is why you feel cold when you step out of the shower. The drier air outside the shower causes water on your skin to evaporate. For water to evaporate, it must absorb heat from its surroundings, including the air and your skin. Thus, the water droplets evaporating from your skin make you feel colder. During hot summer days, your body sweats to cool itself. Heat is absorbed by the beads of sweat on your skin as they evaporate, cooling you down. Let's now use this concept to further understand Figure \(\PageIndex{1}\).

3. Water absorbs heat as it evaporates and rises into the atmosphere. As it condenses, it ________ heat into/from the surrounding atmosphere _______ the atmosphere.
   1. absorbs; cooling
   2. absorbs; heating
   3. releases; cooling
   4. releases; heating

This exchange is crucial in the formation of storm systems, wind patterns, and the overall function of the atmosphere. Table \(\PageIndex{1}\) shows the latent heat (L) exchanged by water as it changes phase at 0°C and Standard Atmospheric Pressure.

Table \(\PageIndex{1}\): Latent heat absorbed or released during phase change at 0 °C.

Phase Change	Latent Heat (Calories/gram)
Fusion (melting/freezing)	80
Vaporization (evaporating/condensing)	597
Sublimation (ice to gas, or gas to ice)	677

4. When one gram of ice melts, it absorbs __________ calories of heat from the atmosphere. When one gram of liquid water freezes, it releases ___________ calories of heat into the atmosphere
   1. absorbs; cooling
   2. absorbs; heating
   3. releases; cooling
   4. releases; heating

Here’s a cool little experiment you can do at home. You need a pot, ice water (preferably 50% ice and 50% liquid water), a stove, and a thermometer. Put the ice water in the pot and stir it with the thermometer. If you measure the water's temperature after stirring, it should be approximately 0°C (32°F), the freezing/melting point of water. Now, put the pot with ice water on the stove and turn on the heat. Set a timer and measure the water temperature every minute. You'll notice that as long as there is ice in the pot, the water's temperature will not rise significantly, remaining near 0°C. This is because any heat added to the ice-liquid water “system” is instantly used to melt the ice. Once all the ice melts, the temperature will rise quickly until the water reaches approximately 100°C (212°F), the boiling point of water. If you continue to apply heat, the water's temperature will not keep rising once it starts boiling. Any additional heat added instantly goes towards evaporation.

5. Based on Table \(\PageIndex{1}\), it takes ______heat to evaporate water than it does to melt water.
   1. significantly more
   2. significantly less
   3. the same amount of

6. Thus, when water in the atmosphere condenses, it releases _______ heat than it absorbed when it evaporated.
   1. more
   2. less
   3. the same amount of

7. Suppose water sublimates, going directly from ice to gas. It would absorb ________ calories per gram, which is ________ the sum of the latent heats of fusion (melting) and vaporization (evaporation).
   1. 80; less than
   2. 597; less than
   3. 677; the same as

Because of latent heat exchanges, water becomes an important mechanism for transporting heat throughout the atmosphere. An example of this is the Rain Shadow Effect, which was briefly covered in Investigation 1. Liquid water absorbs energy from the sun over the ocean, causing it to evaporate. This evaporated water vapor is blown onshore by the wind, forming a relatively cool sea breeze. As this moist air moves over land, it rises up hillsides where it cools and condenses into rain, releasing latent heat in the process. Eventually, the air reaches the top of the hillside and begins to travel down the other side, warming as it sinks. By the time the air reaches the bottom of the hill, it has regained the warmth it lost on the way up, plus the latent heat released by water vapor as it condensed/rained out on the other side of the hill. Table \(\PageIndex{2}\) shows the average July temperature for Santa Cruz, on the windward side (side facing the ocean) of the Santa Cruz Mountains, and San Jose on the leeward side (side facing away from the ocean) of those mountains.

Table \(\PageIndex{2}\): Average July high temperature for Santa Cruz and San Jose, CA.

City	Average July High (°F)
Santa Cruz	74
San Jose	82

8. San Jose, on the leeward side of the Santa Cruz mountains has a _________ average high temperature in July than Santa Cruz. This __________ the idea that as air rises over a hillside, it gains latent heat released by water that condenses on the windward side of the hill.
   1. warmer; confirms
   2. cooler; cannot confirm

Figure \(\PageIndex{3}\): Satellite composite water vapor image. (CC BY-NC 4.0; Space Science and Engineering Center via University of Wisconsin-Madison) Alternative description of the image.

Because of global wind patterns, which will be investigated in a future investigation, there are places on Earth where more water evaporates than precipitates (making those places drier) and other places where more water precipitates than evaporates (making those places wetter). Figure \(\PageIndex{3}\) is a satellite-derived water vapor imagery map, which shows the presence of water vapor in the atmosphere. Black represents an area with dry air; the “milky” grey color represents air with at least some water vapor; and brighter white spots represent areas of clouds and precipitation. Let's answer the following questions based on this information.

9. Based on Figure \(\PageIndex{3}\), water vapor is present, at least to some degree: (i.e., there’s some grey shading):
   1. Across most of the world
   2. Across very little of the world

10. However, clouds and precipitation (bright white regions) are present:
    1. Across only a few parts of the world
    2. Across the entire world

This suggests that while water vapor is a component of the atmosphere almost everywhere on Earth, not every place receives precipitation. As discovered in Investigation 1, there are numerous rainforests (where precipitation is bountiful) and deserts (where precipitation is scarce) on Earth. Global weather patterns, differential heating from the Sun, and surface vegetation all play a role in where precipitation falls/doesn’t fall on Earth.