On November 13, 1946, General Electric researcher Vincent Schaefer piloted a small aircraft above Mount Greylock in Massachusetts, releasing several pounds of crushed dry ice into a cloud at an altitude of approximately 4,300 meters. The cloud was supercooled to minus 18.5 degrees Celsius, yet it consisted entirely of liquid droplets. According to the company's press release the following day, Schaefer dispersed about 2.7 kilograms of dry ice.1

Irving Langmuir, observing from the control tower, watched as snow showers began to fall from the base of the cloud within two minutes. The snow fell for roughly 900 meters before evaporating in the dry air below, never reaching the ground. Before the seeding, Schaefer had observed the colorful optical rings formed by water droplets around the sun; afterward, he saw the 22-degree halo characteristic of ice crystals.1 Despite the cloud's temperature of minus 18.5 degrees, no rain had fallen until the introduction of ice.

What defines this process?
  • Clouds below freezing can remain liquid, preventing precipitation.
  • Satellite data shows that the vast majority of rain originates from clouds containing ice, with purely liquid clouds contributing minimally.
  • Oceanic clouds can occasionally produce rain without ice, but land-based clouds rarely achieve this.
  • Raindrops are not tear-shaped; large drops are inherently unstable and break apart before hitting the ground.

Why do clouds hold onto their water?

Clouds consist of countless microscopic droplets suspended in the air. According to training materials from the U.S. National Weather Service, a typical cloud droplet has a diameter of 0.02 millimeters, while a typical raindrop measures 2 millimeters. Because volume scales with the cube of the radius, a single raindrop contains as much water as one million cloud droplets.2 For these droplets to form, they require a nucleation point. Water vapor condenses onto minute particles such as dust, sea salt, or soot, which meteorologists call cloud condensation nuclei.2

The terminal velocity of these droplets was measured in 1949 by Ross Gunn and Gilbert Kinzer of the U.S. Weather Bureau. By dropping over 1,500 electrically charged water droplets through stagnant air, they determined that a 0.1-millimeter drop falls at 27 centimeters per second, while a 2-millimeter drop accelerates to 6.5 meters per second. The largest drops they could measure were 5.8 millimeters, reaching 9.17 meters per second, beyond which the droplets become too unstable to remain intact.3 Because cloud droplets are significantly smaller than this threshold, even weak updrafts within a cloud can easily keep them suspended.

How does the introduction of ice trigger precipitation?

Water does not necessarily freeze at zero degrees Celsius. Droplets lacking a suitable surface can remain liquid at much lower temperatures. Schaefer discovered in the laboratory that all clouds colder than minus 40 degrees Celsius spontaneously generate ice crystals.4 Because dry ice is roughly minus 78 degrees, it instantly transformed the Greylock cloud.

When ice crystals mix into such a cloud, the delicate balance is disrupted. At the same temperature, the saturation vapor pressure over ice is lower than over water. At minus 10 degrees Celsius, air that is fully saturated relative to water is 10 percent supersaturated relative to ice. The crystal grows by pulling vapor from the air; as the surrounding air dries, nearby droplets evaporate to compensate, effectively transferring their mass to the ice crystal. The crystal grows, gains weight, and begins to fall, merging with other crystals to form snowflakes. Upon passing the zero-degree isotherm, these flakes melt into rain. This theory was proposed by Swedish meteorologist Tor Bergeron in 1933 and refined by German scientist Walter Findeisen in 1938.5

Researchers at the University of Leipzig, led by Johannes Mülmenstädt, quantified the prevalence of this mechanism. By analyzing over 50 million rain measurements collected by the CloudSat and CALIPSO satellites between 2006 and 2011, the team examined the phase of the cloud tops. They found that for land-based clouds located outside the tropics, 67 percent of rain-producing clouds had tops consisting entirely of ice, and 32 percent were a mixture of ice and water. Precipitation originating from clouds composed strictly of liquid droplets accounted for only 1 percent of occurrences.6

Notes and Sources

  1. General Electric News Bureau (1946). G-E scientists transform cloud into snow over Greylock Mountain. Press Release, Nov 14, 1946.
  2. NOAA National Weather Service. Cloud Development; Cloud and Precipitation Processes.
  3. Gunn, R., Kinzer, G. D. (1949). The terminal velocity of fall for water droplets in stagnant air. Journal of Meteorology, 6(4), 243-248.
  4. Schaefer, V. J. (1981). The serendipitous happenings which led to weather modification. The Journal of Weather Modification, 13.
  5. Wallace, J. M., Hobbs, P. V. (2006). Atmospheric Science: An Introductory Survey, 2nd ed. Academic Press, Chapter 6: Cloud Microphysics.
  6. Mülmenstädt, J., Sourdeval, O., Delanoë, J., Quaas, J. (2015). Frequency of occurrence of rain from liquid-, mixed-, and ice-phase clouds derived from A-Train satellite retrievals. Geophysical Research Letters, 42, 6502-6509.