In this blog post, we’ll explore why the base of a cumulus cloud—which is commonly seen on clear summer days—appears flat, by examining the processes of air ascent and cooling, the dew point, and condensation.
If asked to picture the sky in their mind, most people would likely envision a deep blue sky covered by white formations. These white formations are clouds. Clouds are collections of tiny water droplets or ice crystals floating in the sky. When you look up at the sky, you can observe clouds in different shapes every day. There are cumulus clouds, feathery clouds, thin clouds covering the sky, clouds seen during the rainy season, and dark storm clouds that pour down showers—as you can see, the shapes of clouds are truly diverse. Among these, the cloud we will discuss in this article is the cumulus cloud, which is easily seen on clear summer days. Their formal name is cumulus. The World Meteorological Organization (WMO) classifies basic cloud forms into 10 cloud types, and cumulus is one of them—belonging to the category of clouds characterized by prominent vertical development. While you may have wondered what animal these versatile-looking clouds resemble, few people have likely noticed that their bases are flat. It may sound like a strange claim, but the base of a cumulus cloud that has just begun to form is usually flat.
Their official name is cumulus. The World Meteorological Organization (WMO) classifies basic cloud forms into 10 cloud types; cumulus is one of these and belongs to the category of vertically developing clouds.
Before we explore why this is the case, let’s first take a step-by-step look at how water droplets form in the air. There is a fixed amount of water vapor that dry air can hold, and this is determined by temperature. Warmer air can hold more water vapor, while cooler air can hold less. When air contains the maximum amount of water vapor it can hold, it is said to be saturated, and the amount of water vapor at that point is called the saturation vapor pressure.
When does air become saturated? Let’s draw an invisible boundary in the air and consider only the air within that boundary. Since the atmosphere is continuous and vast in scale, it is difficult to grasp its characteristics. Therefore, when analyzing the atmosphere, this method of defining boundaries is commonly used; such a mass of air is called an “air mass.” That air mass will contain a specific amount of water vapor depending on the path it has traveled and its surrounding conditions. There are two main situations in which air becomes saturated: one is when water vapor continuously flows into the air. However, this situation is rarely observed in nature. Air saturation mostly occurs when the air cools.
Since the atmosphere is continuous and vast in scale, it is difficult to understand its characteristics. Therefore, when analyzing the atmosphere, this method of defining boundaries is commonly used; such masses of air are called “air masses.”
Now, let’s assume that this air mass is cooling for some reason. As the temperature drops, the amount of water vapor the air mass can hold—that is, the saturation vapor pressure—decreases. As the temperature continues to drop, a point will be reached where the saturation vapor pressure equals the amount of water vapor the air already contains. The temperature at this point is called the dew point. As the name “dew point” suggests, this is the temperature at which dew begins to form. The dew point is determined by the amount of water vapor in the air. If the air temperature drops below the dew point, some of the water vapor it contains will turn into liquid water—a process called condensation. When condensation occurs near the ground, if there are blades of grass nearby, the water vapor will form dew on the grass. Condensation occurs more easily when there is a surface for the water vapor to adhere to. The water droplets that make up clouds also form around small condensation nuclei, such as fine dust particles. However, even if condensation nuclei are present, cloud droplets are unlikely to form unless the air is saturated. In very clear upper atmospheres, there may be a shortage of condensation nuclei, so a higher degree of supersaturation may be required. When condensation occurs in this way, tiny water droplets form in the air.
When condensation occurs near the ground, if there are blades of grass nearby, the water vapor will condense into dew on the grass. Condensation occurs easily when there is something for the water vapor to attach to. The water droplets that make up clouds are also formed around small condensation nuclei, such as fine dust particles. However, in very clear upper air where nothing is present, condensation may not occur easily; in such situations, the air may need to cool to a lower temperature or reach a slight state of supersaturation in order to reach saturation.
Now, let’s take a closer look at how clouds are actually formed. In the upper atmosphere, it is difficult to assume a situation where water vapor is constantly being supplied. Therefore, for the tiny water droplets that make up clouds to form, the air must cool. Why does the air cool?
The reason is that the air loses heat as it rises. Let’s assume that the air mass mentioned earlier is now rising for some reason. Cases include rising along a frontal boundary, rising due to localized heating causing the air to expand and become less dense, rising along a mountain slope, and rising due to atmospheric circulation. As this air mass moves away from the Earth’s surface, it senses that the surrounding pressure is gradually decreasing. A decrease in external pressure means that the compressive force has diminished, causing the air to expand. According to thermodynamics, when air expands in this way, its internal energy—that is, its temperature—decreases. The change in internal energy can be thought of as the amount of thermal energy gained by the air minus the amount of work done on the external environment. As the air expands, it displaces surrounding air masses, which constitutes work done physically. Consequently, as the air rises, its temperature decreases. For dry air, the temperature drops by approximately 9.8°C for every 1 km of ascent; the ratio of this temperature decrease to the altitude gained is called the “dry adiabatic lapse rate.” The dry adiabatic lapse rate can be derived from the First Law of Thermodynamics and the equations of hydrostatic equilibrium. It is not simply because temperatures are lower at higher altitudes that the temperature of an air mass decreases. Air does not conduct heat quickly, and the rate at which it cools through the aforementioned thermodynamic processes is fast enough to make heat exchange with the surrounding air negligible. The rate at which the surrounding air removes heat depends on the temperature difference between the surrounding air and the rising air, while the extent of cooling due to adiabatic expansion is determined by the process of the air’s ascent and expansion. Since the variables involved differ in these two situations, it is not easy to analyze the rates quantitatively. However, in general, it can be said that the cooling resulting from the expansion of a rising air mass occurs faster than heat exchange with the surroundings. Therefore, we refer to this thermodynamic process as adiabatic expansion. “Adiabatic” means that heat transfer to and from the outside is blocked.
Examples include air rising along a power line, air rising due to localized heating causing volume expansion and a decrease in density, air rising along a mountain slope, and air rising due to atmospheric circulation.
The change in the internal energy of air is determined by the amount of heat energy gained and the amount of work done on the external environment. As the volume of air expands, it displaces surrounding air masses, which constitutes work done from a physical standpoint.
This dry adiabatic lapse rate can be derived from the First Law of Thermodynamics and the equations of hydrostatic equilibrium. The dry adiabatic lapse rate is typically treated as approximately 9.8°C/km under constant conditions.
The rate at which surrounding air removes heat is influenced by the temperature difference between the surrounding air and the rising air, and the extent to which the air cools during adiabatic expansion is related to its ascent process. Since the variables involved differ in these two situations, it is not easy to analyze the rates quantitatively. However, in general, the adiabatic cooling of a rising air mass occurs faster than heat exchange with the surrounding air.
“Adiabatic” means that heat transfer to and from the outside is blocked.
It is not only the temperature of the rising air that decreases; the dew point also decreases as altitude increases. As mentioned earlier, air expands in volume as it rises; this expansion reduces the amount of water vapor per unit volume. Therefore, for condensation to occur, the air must cool to a lower temperature. Consequently, the dew point decreases with altitude, though the rate of this decrease varies depending on the air’s moisture content and meteorological conditions. Rather than assuming, as suggested in the original text, that the dew point decreases at a constant rate of 2°C per kilometer with altitude, it is more appropriate to understand that it varies depending on the extent to which the water vapor content of the rising air mass decreases.
As this rising air mass undergoes adiabatic expansion, its temperature decreases, and at the same time, its dew point also decreases. However, since the dry adiabatic lapse rate is generally greater than the rate of decrease in the dew point, as the air rises, its temperature will eventually reach the dew point. This point can be expressed in terms of altitude rather than temperature. This altitude is called the lifting condensation level (LCL); at this point, water vapor begins to condense into tiny water droplets, which appear as white masses when viewed from a distance. We call this a cloud. As long as the air continues to rise, water vapor will continue to condense through this process; at the same time, the cloud’s shape constantly changes as dry air from the surroundings flows in or as water droplets evaporate.
Let’s summarize what we’ve covered so far. As a mass of air rises, both its temperature and dew point change, and these changes are determined by the altitude reached and the amount of moisture in the air. Generally, the point at which the air mass’s temperature reaches the dew point occurs at a specific altitude, and from this point onward, condensation occurs and clouds begin to form. In other words, cloud formation begins at a specific altitude. In other words, when air masses with similar conditions rise, the altitude at which clouds begin to form may be similar; put another way, this is why the base of cumulus clouds appears flat. Of course, as time passes, water vapor and air—which constantly repeat random movements—will drift far from their original point of formation, and by the time you see the cloud then, the idea of a flat base might sound like a pipe dream. However, I hope you won’t forget why the base of these cumulus clouds appears flat when they first begin to develop. When summer comes again, cumulus clouds will form in clusters. I’ll conclude this post by hoping that you’ll take a closer look not only at the fluffy parts of the clouds but also at their flat bases.
