Clouds are tiny but visible liquid water droplets and/or ice crystals suspended in the air. They form from water vapor present in the air. Water vapor is the gas phase of water, which is invisible. Before diving into details, explore the physical processes that make clouds with this series of hands-on, inexpensive experiments.
Experiment #1: Squeezing Dry Air
Experiment #2: Squeezing Water
Experiment #3: Squeezing Humid Air
Experiment #4: Squeezing Dry Air and Smoke
Experiment #5: Squeezing Humid Air and Smoke
Summarize what you have observed. What components and processes create clouds?
Experiment #6: Clouds be Free!
How do these results relate to clouds moving in the sky?
Experiment #7: Making a Colored Cloud(?)!
How do these results relate to clouds that form over the ocean? See the Deserts over Oceans section for ideas.
2 clear, dry plastic 2-liter soda bottles with caps.
Matches and/or incense stick
LCD thermometer (suggested 58-88ºF range)
The soda bottle provides a visualization of how meteorologist work with air changing position on Earth: air parcels. This model assumes individual volumes of air are separated from each other by thin, expandable barriers so that the parcel retains its shape but may change volume as it changes temperature and altitude. These barriers stop the mixing of properties and molecules between parcels. One of the air parcel's properties that is not exchanged with the environment is energy. A process that does not exchange energy or mass with the surrounding environment is an adiabatic process. This model doesn’t apply to all processes in the atmosphere, but it does work well for most.
This concept also helps meteorologists model the changes to the gas inside the parcel as it moves rather than capture glimpses of the many parcels moving past a fixed point on Earth.
A simplified version of an air parcel: a cube with evenly spaced air molecules.
Use this equation to calculate an air parcel's temperature and volume as it changes altitude.
Ideal gases involve molecules that have no attraction between each other and that also don't have a volume, both of which don't occur in nature. However, the Ideal Gas Law works well for gases at Earth's atmosphere's temperatures and pressures, at least those within 25 km of the Earth's surface. So for our needs, this equation is very useful. To explore when gases act as non-ideal, see this site.
Remember our parcel's walls act as barriers for mixing, so we will assume that the number of molecules inside the parcel (n) stays constant throughout the movement of the parcel. Since our parcel is expandable and contractable, when it changes altitude, it experiences a change in pressure. If the parcel rises, its volume increases, but at a rate that depends on the length of the parcel's side cubed. This means volume increases at a slower rate than the decrease in pressure, so the term PV decreases. Since n and R are not changing, that means temperature, T, must decrease. Conversely, if the parcel sinks toward the Earth's surface, pressure increases, volume decreases but at a slower rate than pressure, and PV increases. This means temperature must increase.
Working through the math, we find that a rising parcel of air without condensation occurring cools at 9.8ºC/km, and sinking air heats up at the same rate. This is called the dry adiabatic lapse rate. The lapse rate changes once a cloud forms since the release of latent heat of condensation warms the air parcel's molecules. Within a cloud, an ascending parcel's temperature decreases at roughly 5ºC/km. This is called the moist adiabatic lapse rate, and it is roughly half of the dry adiabatic lapse rate. To calculate the actual rate, see this article. When air sinks inside a cloud, the cooling caused by evaporation decreases the warming to the moist adiabatic lapse rate. Once all of the water has evaporated, the sinking air returns to warming at the dry adiabatic lapse rate. Because both are adiabatic processes (no energy or mass is being exchanged with the surrounding environment), the temperature and dew point of the parcel that rose above the ground will be the same when it returns back to the ground, even when condensation occurs.
Go to the Ideal Gas Law tab in the Atmosphere, Clouds, and Precipitation software to explore the connections between the Ideal Gas Law and the formation of clouds.
Squeeze a 2-liter soda bottle of air with an LCD thermometer inside to see the temperature change.
Water vapor is one of the gases in air parcels, and it ranges from nearly zero to up to 3% of the molecules. Due to the complex processes water is involved in, meteorologists use a number of variables to describe how much water vapor is present in the air. We will use three of them for our exploration of cloud formation.
Dew point temperature is the temperature the air needs to be cooled for condensation to begin. Condensation occurs when the air becomes saturated. The greater the concentration of water vapor, the higher the dew point temperature, but it is always less than or equal to the temperature of the air parcel. Because increasing air pressure increases the temperature at which condensations occurs, dew point temperature decreases nearly 2ºC/km for rising air.
Mixing ratio is the mass of the water vapor divided by the mass of the dry air. This value remains constant for rising and sinking unsaturated air.
Relative humidity is a combination of both temperature and dew point temperature, so it is useful for indicating how close the air parcel is to saturation. Air is saturated when RH = 100%.
As an unsaturated air parcel begins to rise, pressure decreases, the parcel expands, temperature decreases at 9.8ºC/km, dew point temperature decreases at 2ºC/km, and relative humidity increases. A cloud first forms when the parcel's temperature equals the dew point - this is the base of the cloud. If the parcel continues to rise, both temperature and dew point decrease at the moist adiabatic lapse rate (roughly 5ºC/km), and more of the parcel's water vapor condenses as the cloud grows upward.
A cloud forms when the air parcel's temperature and dew point become equal.
Hot air balloons rise, cold air balloons sink. Heating the air at the ground so it becomes less dense than its surroundings (positively buoyant) and rises. Afternoon summer-time thunderstorms or convective showers are often a result of surface air rising to the top of the troposphere.
Air can't push a mountain out of the way, so wind pushes air up the mountain on the upwind side. This is called orographic lifting and is one type of mechanical lifting - when less dense air rise over a more dense object. The other common type is frontal lifting, where two bodies of air with different densities are next to each other. The denser air moves under the less air (see a movie of two fluids with different densities next to each other).
Convergence is when air moves toward an area, and divergence when air moves away from an area. If surface air converges, the only place it can go it up. If air diverges at the top of the troposphere, air from below rises. This is part of the global wind circulation cells.
Low pressure systems with warm (red) and cold (blue) fronts involve a combination of buoyancy, surface convergence, upper-level divergence, and frontal lifting. These won't be covered in the Earth Systems arc, but will be in the 'to-be-developed' Meteorology arc.
To the right are two photographs of the same scene of clouds, ocean, and land. The top image is a color photograph, so it is in visible light, and the bottom image is a black and white image taken in the near infrared. In both images, the clouds are reflecting the Sun's incoming visible light and the near infrared. Using the Blackbody Radiation software, the Sun emits about 10% ultraviolet, 45% visible light, and 45% near infrared, so clouds are reflecting a great deal of the incoming Sun's radiation back to space so it can't warm the Earth's surface. So the clouds in these photographs cool day time temperatures.
However, clouds absorb the longer wavelengths of infrared, mid and far infrared, that are emitted from the heated Earth's surface. The warmed clouds emit roughly half back to be absorbed by the Earth's surface, so clouds also have a warming effect, especially at night.
Image of clouds and coastline in visible light. Notice clouds are highly reflective.
Different types of clouds have different impacts on warming and cooling. Clouds that are thick and consist only of liquid water droplets reflect the greatest amount of sunlight away from Earth. Since temperature decreases with height in the troposphere, these clouds are lower in the atmosphere so that temperatures are as low as -15ºC. High thin ice clouds reflect little sunlight but do absorb mid and far infrared emitted from the surface, so these have a warming effect both day and night.
At any given moment, two thirds of the Earth is covered by clouds, so they have considerable impact on Earth's radiation budget. Because of the extensive and complex role of clouds, internationally coordinated research started in 1983, and research continues to improve our understanding of clouds' impact on climate change.
Same image of clouds and coastline in Near Infrared. The clouds are still very reflective.