Rainfall is one of the most salient aspects of climate and it is vital for nearly all life forms and social structures on Earth. Large-scale and long-term patterns of rainfall define natural biomes across the globe from the tropics to temperate and arid regions and give the cadence for the seasons. Significant changes in these patterns can lead to migrations and extinctions and can have important socio-economic consequences.


Rainfall depends primarily on atmospheric circulation that creates the conditions for air and vapor to rise and form clouds. But rainfall also depends on the amount and properties of aerosol particles that are in the atmosphere. These particles help water vapor to condense into tiny liquid droplets and to coalesce into larger droplets that are heavy enough to fall as rain. Many of the particles in the atmosphere are mineral particles. But some of them have biological sources such as the microorganisms that live on plants. Atmospheric particles come from natural sources (sand, emissions from forest fires, and pollen and microorganisms from plants, for example) and from human activities (emissions from smoke stacks, from cars and from agricultural activities, for example).

After a rainfall, microorganisms on plants can multiply and become more abundant in the atmosphere. Some of these microorganisms have properties that can influence the behavior of clouds. In light of these observations, scientists have wondered if rainfall can create conditions that are favorable for more rainfall - at least on a regional scale and over a short period of a few weeks. If there is such feedback, then it suggests that changes in the abundance and types of aerosol particles in the atmosphere could alter local and short term rainfall patterns.

This website provides maps to explore where rainfall feedback could be occurring. We have produced these maps to encourage research on mechanisms involved in feedback and that could lead to the discovery of means to favor positive rainfall feedback, in other words to favor situations where a rainfall event will increase the likelihood of subsequent rainfalls. You will find details about how the maps were made and how to use them by clicking on DESCRIPTION, HOW TO USE, APPLICATIONS, and MAKE MORE MAPS in the navigation bar on the top of the page. You will find the maps by clicking on JAN-DEC, APR-SEPT, and OCT-MAR. Data used to make these maps can be downloaded by clicking on DATA.