God’s Control Systems: The Water Cycle

My first job out of graduate school involved process control systems. From complex semiconductor chips for computers and cell phones to simple injection-molded plastic parts, many items we use every day must be manufactured at precise temperatures. If too hot or too cold, these items couldn’t be made—or at best wouldn’t work.

Precise temperature control utilizes what is referred to as a “feedback loop.” First, a sensor precisely measures the critical temperature and feeds it back to an electronic controller. The controller compares that measurement to the proper temperature and notes any deviation (+ or -). If the temperature isn’t correct, the controller signals the heating system to add or reduce the heat by just the right amount to bring the temperature to where it should be.

A home thermostat is a simple example of a control system. It measures the room temperature and turns the furnace or air conditioner on or off to make our home warmer or colder.

All such control systems are designed by an intelligent human agent. Hence when similar systems are found in nature, design by an intelligent agent is inferred. We believe this suggests an intelligent Creator-God—as does evidence of design in the universe and in the laws of physics.1

We can refer to such natural controls as “God’s control systems,” and we find evidence of them in the water and carbon cycles. This post will discuss the water cycle; a subsequent post will address the carbon cycle.

The Water Cycle

Water (H2O) exists in three phases: solid (ice), liquid (water), and gas (water vapor). Ice becomes water (and vice versa) at a melting point temperature of 32oF (or 0oC), and water becomes water vapor at a boiling point temperature of 212oF (or 100oC).

We experience ice naturally when the temperature falls below 32oF. We also experience water vapor in the atmosphere, known as humidity. But Earth hasn’t been as hot as 212oF for millions of years, so how does water vapor get into the atmosphere?

This process is explained by physics:

Gasses and liquids consist of molecules moving at high speed. Some liquid molecules evaporate into gas because of their high velocity, and some gas molecules with a lower velocity condense into liquid.

This introduces the water cycle as a natural control system which moderates Earth’s seasonal temperature variations. Summer heat and winter cold occur because the axis of Earth is tilted as it rotates around the Sun. The northern hemisphere experiences the intense warming of direct sunlight in summer, but less warming in winter due to indirect sunlight. The reverse occurs in the southern hemisphere.

Step 1: Evaporation

As this graphic illustrates, the first step in the water cycle is the evaporation of surface water. This process has the effect of cooling Earth’s surface—as explained by the physics:

Water doesn’t become water vapor just by reaching 212oF; additional heat—called “latent heat”—must be added. If water is boiled on the stove, the latent heat comes from the burner; if the water is evaporated naturally, latent heat is removed from the surroundings, cooling the surroundings. Moreover, more molecules evaporate from a hot liquid than from a cold liquid; this removes more latent heat and provides more cooling of the surroundings.

This concept may be easier to understand in the ice-to-water transition, which works the same way. The ice in a beverage gets smaller as it liquefies; in so doing, it removes latent heat from the beverage, keeping the beverage cool.

Step 2: Condensation

The second step in the water cycle is also based on physics:

Hot gas rises in the atmosphere.

The evaporated water vapor rises in the “troposphere,” the lowest level of our atmosphere, which is up to 130 (70) colder than Earth’s surface. The cold temperature causes the water vapor to condense, and clouds to form.

Step 3: Precipitation

The clouds produce rain. This is the third and final step in the water cycle, and it involves more physics:

When a cold object is in contact with a hot object, heat is transferred from the hot object to the cold one; the hot object becomes cooler, and the cold object becomes warmer.

The rain is cold, coming down from the troposphere. As cold rain touches a warm Earth, heat is transferred from Earth to the rainwater, cooling Earth as the rainwater is warmed.

Thus the water cycle moderates Earth’s temperature with two cooling events:

  1. Evaporation of surface water cools Earth by removing latent heat.
  2. Cold rain cools a warm Earth.

Saturation: Keeping Humidity in Check

In addition to the water cycle itself, there is an associated natural control system: saturation. Saturation occurs when evaporating liquid molecules equal condensing gas molecules; this limits the amount of water vapor in the air. Humidity refers to water vapor in the atmosphere, and “100% relative humidity” refers to water vapor saturation. The saturation effect assures us that even though it can be really uncomfortable on a hot day with 100% relative humidity, it can’t get any worse! Can you imagine how oppressive humidity could be without this natural control system?

How Much Cooling Is Involved?

How significant are the cooling effects of the water cycle? A final bit of physics helps us understand:

In the heat transfer from a hot object to a cold one, different substances require greater or lesser amounts of heat to warm (or cool) them by a particular number of degrees. The term “specific heat capacity” indicates the relative amount of heat that must be added (or given up) to raise (or lower) temperature. The higher the specific heat capacity of a substance, the more heat is needed (or given up) for each degree of temperature change.

This allows us to assess the cooling effect of the water cycle:

The One Who Is in Control

We still experience hot, humid days in summer and cold days in winter. God’s control system of the water cycle does not prevent that—it just moderates the effect. Without the water cycle and the saturation effect, hot, humid days could be hotter and more humid and cold winter days could be colder. These natural control systems reinforce the biblical idea that God is in control and prepared Earth as a habitable planet for humans, his ultimate creation.


  1. Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids, MI: Baker, 2016). See also articles on the topic of design.