Evaporation Rates, Condensation Rates, and Relative Humidity

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In the preceding section, I made what perhaps can be considered a somewhat surprising assertion: evaporation and condensation are ongoing processes that happen constantly in your surroundings! However, these processes often go unnoticed because they occur at the molecular level, making their results imperceptible to the naked eye. Observable phase changes occur under specific conditions when there is either a “net” condensation or a “net” evaporation, provided there is an initial presence of liquid water. 

Net Condensation and Net Evaporation

“Net” condensation refers to a situation where the rate of condensation surpasses that of evaporation, resulting in the formation of liquid water droplets. Remember our cold drink bottle from a warm day? Conversely, when starting with liquid water, “net” evaporation occurs when the evaporation rate exceeds the condensation rate – water rising off a wet roadway after our summer thunderstorm leaves and is replaced by sunny skies. This leads to the shrinking or complete disappearance of liquid water droplets and the drying up of puddles on the ground. 

The concepts of “net” evaporation and “net” condensation hold significant importance for climate scientists, particularly in their implications for cloud and precipitation formation, as well as the evaporation of precipitation, which contributes to evaporational cooling and the movement of water in the climate system. To better understand these processes, let’s explore what controls the evaporation rate (how many water molecules become vapor) and the condensation rate (how many vapor molecules turn back into liquid).

To begin, it's essential to understand that the bonds holding water molecules together in the liquid phase are relatively weak in the grand scheme of things – that is why water can so easily change shape, whether in a glass or poured out onto a counter. Consequently, occasional natural vibrations of water molecules can break these bonds, resulting in evaporation. As you may already know, temperature directly influences molecular vibrations: the higher the temperature, the greater  amount of energy contained within vibrating molecules. Consequently, liquid water molecules are more likely to break free from their neighboring molecules and transition into water vapor at higher temperatures. Therefore, water temperature plays a pivotal role in regulating the rate of evaporation. Lower water temperatures lead to diminished evaporation rates, whereas higher water temperatures result in increased evaporation rates. Cool water = slow evaporation, warm water = fast evaporation!

Condensation Rates

Now, let's turn our attention to the factors affecting the condensation rate. Let's conduct a simple experiment to explore what determines condensation rate. Begin with a sealed, empty container containing dry air (in other words, no water vapor molecules). Now, let's pour some liquid water into the container and observe the ensuing processes. Over time, water molecules break their molecular bonds with neighboring water molecules and evaporate into the airspace above the water, gradually increasing the concentration of water vapor molecules in that space. As time progresses and more water molecules enter the vapor phase above the water surface, some water vapor molecules will randomly come into contact with the interface between the liquid water and the air above, causing them to condense back into a liquid state.

An experiment that begins with a container free of water molecules (left). In the second step of the experiment, water is added to the container, and the water begins to evaporate. At the same time, water molecules in the gas phase are free to condense back into the liquid. At first, the evaporation rate far exceeds the condensation rate.

Credit: David Babb© Penn State University is licensed under CC BY-NC-SA 4.0

Initially, the condensation rate is low because only a few water vapor molecules are present, and the probability that any of them will come in contact with the interface between air and water is low. In fact, the evaporation rate far exceeds the condensation rate early on (net evaporation occurs). But, as time goes on and net evaporation continues, the air above the water contains an increasing number of water vapor molecules. As the number of water vapor molecules increases, the chance of a water vapor molecule contacting the interface between air and water and condensing back into liquid also increases, which translates to an increase in the condensation rate.

So, as the number of water vapor molecules increases in the air above the water, the condensation rate increases, too. The condensation rate will continue to increase until it matches the evaporation rate. This is a state called equilibrium, meaning the condensation rate equals the evaporation rate. At equilibrium, the temperature of the remaining water is lower than that at the start of the experiment. That's because That's because kinetic energy was consumed in moving water molecules from the liquid to gas phase during evaporation, thereby lowering the average kinetic energy (in other words, the temperature) of the water left behind. Moreover, the temperature of the remaining water equals the temperature of the “air” above the water. This means that the energy exchange between the water and the air has balanced out. This state of equilibrium, where the condensation rate equals the evaporation rate, is depicted on the left below.

In the second phase of the experiment, a container at equilibrium (left) is heated. When water temperature increases (right), the evaporation rate also increases. In turn, the amount of water vapor in the “airspace” above the water increases. Eventually, the condensation rate increases and balances the increased evaporation rate, reaching a new equilibrium.

Credit: David Babb© Penn State University is licensed under CC BY-NC-SA 4.0

What happens if we take our container in equilibrium and increase the temperature (depicted on the right above)? The increase in water temperature causes the evaporation rate to increase, and, for a time, net evaporation occurs. But, with increased evaporation, more water molecules exist in the air above the water, increasing the condensation rate! The condensation rate again increases until it equals the evaporation rate, and a new equilibrium is achieved (with greater evaporation rates and condensation rates than the original equilibrium, shown above on the right).

$$\mathrm{RH} = \frac{\text{ condensation rate }}{\text{ evaporation rate }}\times 100 \mathrm{\%}$$

Relative humidity (RH) is equal to the condensation rate divided by the evaporation rate, multiplied by 100 percent.

How do evaporation rates and condensation rates relate to climate?

Well, they're the basis for a variable that perhaps you are familiar with -- relative humidity. Although you may have heard the term “relative humidity” before, you may not know what it's really telling you. For starters, relative humidity is the rate of condensation divided by the rate of evaporation, multiplied by 100 percent (shown on the right). Relative humidity usually ranges from just a few percent (when the evaporation rate is much larger than the condensation rate) to 100 percent, which occurs at equilibrium. Note: 100 percent is not the absolute upper limit of relative humidity because, in reality, the condensation rate sometimes exceeds the evaporation rate slightly (that's how water droplets grow). But that's just a fun fact for you.

What does relative humidity tell us? It tells us how close the condensation rate is to the evaporation rate. As relative humidity nears 100 percent, the condensation rate nears the evaporation rate. Low relative humidity values mean that the evaporation rate greatly exceeds the condensation rate. But, because relative humidity depends on the evaporation rate, which depends on temperature, relative humidity doesn't tell us how much water vapor is present in the air. For example, the relative humidity is 100 percent in both stages of our experiment above in which the condensation rate equals the evaporation rate (equilibrium), but more water vapor molecules are present in the state of equilibrium after we've increased the temperature. By itself, relative humidity is also not a good indicator of how muggy or humid the air feels to most humans. You can have 95% relative humidity in Alaska and 95% relative humidity in Hawaii and feel much (MUCH) different!

In practice, we can't calculate relative humidity using the equation above because we can't easily determine the evaporation and condensation rates at any given time. However, we can relate evaporation and condensation rates to weather variables we can measure easily. Since we know that the condensation rate is controlled by the amount of water vapor present, and we use dew points to assess the amount of water vapor present, it stands to reason that condensation rates are connected to dew points. Indeed, higher dew points yield higher condensation rates. Meanwhile, temperature controls evaporation rates (higher temperatures yield higher evaporation rates), so relative humidity depends on dew point (which reflects the amount of water vapor present) and temperature. I should point out, however, that we can't just substitute dew point and temperature into the equation for relative humidity above and do a simple calculation. The mathematical connections between condensation rates and dew point, as well as evaporation rates and temperature, are too complex for that and are beyond the scope of this course. Still, understanding the basic connections between temperature and evaporation rates, and dew point and condensation rates leads us to the following important lesson learned:

Important Applications

This lesson has many important applications in understanding broader climate processes. For example, evaporational cooling plays a crucial role in regulating surface and atmospheric temperatures, particularly in regions with high evaporation rates, such as coastal areas and deserts. When there's a large difference between temperature and dew point, the rate of evaporation greatly exceeds the rate of condensation, resulting in significant cooling. Over time, this cooling affects local climate patterns, influencing everything from vegetation types to the intensity of heat waves and drought conditions.

These concepts are also vital for understanding long-term cloud formation patterns and the role of clouds in the climate system (net condensation). Later in the course, we’ll examine how changes in global evaporation and condensation rates, driven by increasing temperatures, influence cloud coverage and precipitation patterns over decades. You may have heard the misconception that “clouds form because cold air can't hold as much water vapor as warm air,” but in reality, it all comes down to the balance between evaporation and condensation rates, which are impacted by the warming climate.