Clouds are collections of tiny water droplets, ice crystals or both suspended in the atmosphere. They are an important part of the water cycle and influence precipitation, sunlight and Earth's energy balance.

How clouds form

Water evaporates from oceans, lakes, soil and other surfaces. As moist air rises and cools, water vapor can condense onto tiny airborne particles. When conditions are suitable, this process produces cloud droplets or ice crystals.

Whether a cloud produces rain or snow depends on factors including temperature, moisture, air movement and the size and concentration of its droplets or ice particles.

What is cloud seeding?

Cloud seeding is a form of weather modification that introduces particles into suitable existing clouds with the aim of changing cloud microphysics and, in some circumstances, influencing precipitation.

Cloud seeding cannot create a suitable cloud system from a clear sky. Its results depend strongly on the type and condition of the cloud and the surrounding atmosphere.

How cloud seeding is performed

Seeding material may be released from aircraft or ground-based systems. Different methods target different cloud conditions.

Glaciogenic seeding can use materials such as silver iodide to encourage ice formation in clouds containing supercooled liquid water. Hygroscopic seeding uses particles intended to influence the growth and interaction of liquid droplets.

Does cloud seeding work?

Scientific evidence shows that certain cloud microphysical processes can be modified under appropriate conditions. Research on some wintertime orographic clouds has provided evidence of precipitation enhancement, but results cannot automatically be transferred to every cloud type or region.

Natural variability makes the effect difficult to measure in many situations. Cloud seeding therefore should not be described as a guaranteed way to produce rain or snow.

Safety and limitations

Claims that ordinary cloud-seeding operations can create earthquakes, famines or other major natural disasters are not supported by established evidence. Weather systems contain enormous amounts of energy, and cloud seeding cannot control large-scale atmospheric systems or severe weather at will.

Research and monitoring remain important because possible environmental and downwind effects should be evaluated scientifically. Weather-modification programs should use appropriate oversight, measurement and evidence-based assessment.