Looking east on a balmy August afternoon, we are struck by the towering clouds rising into the atmosphere. We are all familiar with the persistent marine layer clouds of early summer. Winter brings wet storm clouds from the ocean.Everyone has looked into the sky and projected a common figure to a puffy white mass of moisture.
“Look at that dragon, there is the tail …”
“Can you see Africa up there?”
But where do clouds come from? How do they form and what can they tell us about the weather?
Clouds are masses of water droplets, ice crystals and dust that are visible above the surface of the Earth. They exist in many different sizes, shapes and compositions. Each different composition represents differing conditions in the atmosphere.
Earth’s atmosphere is separated into layers based on altitude. The layer that we are most familiar with begins at our feet and reaches an altitude of about 40,000 feet. This is the troposphere and it contains about 80 percent of the mass of the entire atmosphere and almost all of the clouds.
The formation of clouds is generally caused when air containing invisible water vapor is forced upwards. Low pressure in the atmosphere, convective heating from the Earth’s surface and landforms like mountains can force air masses higher into the atmosphere.
As the moist air rises, it cools and reaches its dew point, the temperature at which the moisture condenses from invisible water vapor into visible liquid water droplets. The droplets are attracted to dust, salt and other particulates in the atmosphere known as cloud seeds.
The nomenclature or naming of clouds is a seemingly confusing system. We’ll start with three basic categories: cirrus, cumulus and stratus. The naming system then combines these (cirrostratus, stratocumulus, etc.) and adds a few affixes to describe clouds in finer detail.
Cirrus (Latin meaning curling lock of hair) clouds are high-altitude clouds characterized by their white, wispy, transparent appearance. Their shape is often representative of the winds in the upper troposphere. Although cirrus clouds do not bring precipitation, they may represent a changing atmosphere preceding a weather front. Because of their high altitude, cirrus clouds are mostly made of ice crystals.
Cumulus (Latin for heap or pile) clouds are the puffy, cotton ball clouds with defined edges that often spark our imagination. Cumulus clouds are sometimes benign, such as cumulus humilis (humble), the small, puffy clouds that do not rise into the atmosphere. However, when cumulus clouds begin to rise high into the troposphere, low pressure is present and inclement weather might be imminent.
Cumulonimbus (nimbus is Latin for dark clouds) clouds tower high into the atmosphere. They have very dark, gray bases caused by light scattering through the mass of moisture above. These are the clouds most often associated with bad weather.
Stratus (Latin meaning stretch or extend) clouds are the low, featureless, gray clouds that extend across most or all of the sky. Our coastal “June gloom” marine layer is an example of stratus clouds. Stratus clouds are often the result of a process called inversion. When a mass of warmer air rises over the top of cooler air, it traps the cool air beneath it. Stratus clouds can create fog and drizzle and if they become nimbostratus, will bring precipitation.
The Earth’s atmosphere is a dynamic place with perpetually changing conditions. Variables including moisture, temperature, convection, wind, altitude, particulates and pressure cause the dynamism. Clouds give us a window of understanding into the current state of the atmosphere.