Rain rates up to 400 millimeters per hour have been measured in particularly strong tropical cyclones. However, even within the strongest storms, such high instantaneous rates are rarely sustained for periods longer than a minute, although a point on the ground can accumulate as much as 1800 millimeters per 24 hours during the passage of an exceptionally strong rainstorm.

Availability of water vapor and intensity of updrafts within a cloud determine the size of a raindrop. Larger drops tend to result from the vigorous updrafts within a thunderstorm and fall faster than smaller drops. Mist or drizzle produce smaller drops that fall at lower speeds.

Humans are directly impacted by changes in precipitation on a range of scales. An increase in rainfall can cause flooding or landslides that affect individual homes, cities, or even entire countries. Drought conditions can impact a region’s susceptibility to wildfire or diminish crop yields for local farmers—both of which can have cascading effects on the local to global economy.

Precipitation forms when cloud droplets or ice particles in clouds grow and combine to become so large that the updrafts (e.g. upward moving air) in the clouds can no longer support them, and they fall to the ground.

By providing more accurate and timely precipitation observations, GPM will lead to improved weather forecasting, allowing for more accurate monitoring of transportation hazards and timelier travel advisories.