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— CH. 1 · FOUNDATIONS OF CLOUD PHYSICS —

Cloud physics

~4 min read · Ch. 1 of 6
6 sections

  • Modern cloud physics began in the 19th century and was described in several publications. Otto von Guericke originated the idea that clouds were composed of water bubbles in 1358. Augustus Waller used spider web to examine droplets under the microscope in 1847. These observations were confirmed by William Henry Dines in 1880 and Richard Assmann in 1884. Scientists study microscopic droplets of liquid water or tiny crystals of ice within atmospheric layers called the troposphere, stratosphere, and mesosphere. Condensation nuclei such as dust and salt particles are necessary for cloud droplet formation because of the Kelvin effect. The Kelvin effect describes the change in saturation vapor pressure due to a curved surface at small radii. Raoult's law describes how the vapor pressure is dependent on the amount of solute in a solution. At high concentrations when the cloud droplets are small, the supersaturation required is smaller than without the presence of a nucleus.

  • Adiabatic cooling drives the process when moist air rises and expands in a process that expends energy and causes the air to cool. Atmospheric pressure decreases with altitude so rising air expands and cools which makes water vapor condense into cloud. Water vapor in saturated air is normally attracted to condensation nuclei such as dust and salt particles that are small enough to be held aloft by normal circulation of the air. The water droplets in a cloud have a normal radius of about 0.002 mm (0.00008 in). Droplets suspended in the air will interact with each other either by colliding and bouncing off each other or by combining to form a larger droplet. In warm clouds larger cloud droplets fall at a higher terminal velocity because drag force per unit of droplet weight on smaller droplets is larger than on large droplets. When drops become large enough their downward velocity relative to surrounding air exceeds upward velocity of surrounding air and they fall as precipitation.

  • Clouds in the troposphere are classified according to height and shape across atmospheric layers. Cirriform clouds are high thin wispy forms seen most extensively along leading edges of organized weather disturbances. Stratiform clouds appear as extensive sheet-like layers ranging from thin to very thick with considerable vertical development. Unstable free-convective cumuliform clouds form mostly into localized heaps. Stratocumuliform clouds show a mix of cumuliform and stratiform characteristics appearing in rolls or ripples. Highly convective cumulonimbiform clouds have complex structures often including cirriform tops and stratocumuliform accessory clouds. High-level clouds form at altitudes of 5 to 12 kilometers while mid-level clouds occupy ranges between 2 to 7 kilometers. Low level clouds exist around 2 kilometres or lower without height-related prefixes. The cumulus genus includes four species that indicate vertical size and structure. Towering cumulus species congestus and cumulonimbus may form anywhere from near surface to intermediate heights of around 3 kilometres.

  • The primary mechanism for formation of ice clouds was discovered by Tor Bergeron. The Bergeron process notes saturation vapor pressure of water depends on what vapor interacts with. Saturation vapor pressure with respect to ice is lower than saturation vapor pressure with respect to water. Water vapor interacting with water droplet may be saturated at 100% relative humidity but same amount would be supersaturated when interacting with ice particle. Extra water vapor condenses into ice on surface of particle forming larger ice crystals. This process happens only at temperatures between -40°C and 0°C. Below -40°C liquid water will spontaneously nucleate and freeze. Supercooled liquid water exists down to about -48°C at which point spontaneous freezing occurs. Riming occurs when supercooled liquid drop collides with solid snowflake. Aggregation happens when two solid snowflakes collide and combine. Hailstones have been found with diameters of up to 15 cm.

  • Satellites gather data about cloud properties using instruments such as MODIS POLDER CALIPSO or ATSR. Instruments measure radiances from which relevant parameters can be retrieved through inverse theory. Clouds tend to appear brighter and colder than land surface making detection difficult above bright surfaces like oceans and ice. Global Energy and Water Cycle Experiment uses quantities including cloud cover values between 0 and 1. Cloud temperature at top ranges from 150 to 340 K while pressure tops range from 1013 to 100 hPa. Cloud height measured above sea level ranges from 0 to 20 km. Visible optical depth varies within a range of 4 and 10. Effective particle size for both liquid and ice ranges from 0 to 200 μm. The method relies on comparing data quality from different satellites to establish reliable quantification of cloud properties.

  • Two main model schemes represent cloud physics in scientific research. Bulk microphysics models use mean values to describe cloud properties such as rain water content and ice content. These properties can represent only first order concentration or also second order mass. Bin microphysics scheme keeps moments like mass or concentration in different bins for different sizes of particles. Bulk microphysics models are much faster than bin models but less accurate. Scientists choose between speed and precision depending on simulation needs. Models help predict how individual droplets behave within complex atmospheric systems. Research continues into improving accuracy without sacrificing computational efficiency for global weather forecasting applications.

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Common questions

When did modern cloud physics begin and who originated the idea that clouds were composed of water bubbles?

Modern cloud physics began in the 19th century with Otto von Guericke originating the idea that clouds were composed of water bubbles in 1358. Augustus Waller used spider web to examine droplets under the microscope in 1847.

What is the normal radius of water droplets found within atmospheric clouds?

The water droplets in a cloud have a normal radius of about 0.002 mm (0.00008 in). Droplets suspended in the air will interact with each other either by colliding and bouncing off each other or by combining to form a larger droplet.

At what altitudes do high-level, mid-level, and low level clouds exist respectively?

High-level clouds form at altitudes of 5 to 12 kilometers while mid-level clouds occupy ranges between 2 to 7 kilometers. Low level clouds exist around 2 kilometres or lower without height-related prefixes.

How does the Bergeron process explain the formation of ice clouds and at what temperatures does it occur?

Saturation vapor pressure with respect to ice is lower than saturation vapor pressure with respect to water which causes extra water vapor to condense into ice on surface of particle forming larger ice crystals. This process happens only at temperatures between -40°C and 0°C.

Which satellites gather data about cloud properties using instruments such as MODIS POLDER CALIPSO or ATSR?

Satellites gather data about cloud properties using instruments such as MODIS POLDER CALIPSO or ATSR. Instruments measure radiances from which relevant parameters can be retrieved through inverse theory.

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