The Science

Water - the main reason for life on Earth - continuously travels through one of Earth’s most powerful systems, the water cycle. Water moves endlessly between the oceans, atmosphere and land. Precipitation in the form of rain, snow, sleet or hail, for example, is a vital component of the water cycle and affects everyone on Earth. 

Animation of the water cycle.

The effects of rain, hail, sleet and snow are felt at local scales, but understanding the role of Earth’s water cycle requires a global view. Water is distributed throughout the atmosphere as water vapor, liquid droplets in clouds or rain, and ice at the top of clouds or in snowflakes. How water moves between these states and where it moves across the planet is a powerful vehicle for rearranging Earth's energy. The water cycle dominates the behavior of the planet’s weather, climate and other environmental systems.
Energy, in the form of latent heat, is absorbed or released when water changes its state. For example, when water vapor in the atmosphere condenses into clouds, latent heat is released—warming the atmosphere. Conversely, when liquid water evaporates, from oceans in the tropics for example, latent heat is absorbed—cooling the atmosphere. Latent heat plays a major role in cloud formation and storm development. It also drives wind patterns and the movement of air masses around the globe. 
In turn, these clouds and storms produce precipitation over land and oceans, showing up in daily weather forecasts and influencing our lives on a daily basis. From drizzle or snow on a morning commute, to flash floods caused by thunderstorms, rain enters local watersheds and contributes to the water supply. From there, it becomes available for fresh drinking water and agricultural needs. 
Scientists study moisture, rain, snow, and other forms of precipitation in the atmosphere. They study how they interact and how they behave in bigger weather and climate systems. Water, in all its forms, is difficult to measure consistently around the globe. 
Water vapor, rain, snow and other precipitation types, such as hail and sleet, vary greatly over land and oceans. This variation makes it difficult to make reliable measurements from the ground. Monitoring instruments, like rain gauges for example, are often located far apart. Over oceans, the gaps between islands and passing ships are even bigger.
Satellite observations from space, however, cover broad areas. The view from space also gives more frequent and accurate measurements that offer insight into when, where, and how much it rains or snows worldwide. Earth-observing satellites carry a number of instruments. Each instrument is designed to observe specific things in the atmosphere, such as water droplets and ice particles. These observations are detailed enough to allow scientists to distinguish between rain, snow and other precipitation types, as well as observe the structure, intensity and dynamics of storms. These data are then fed into the weather forecast models that meteorologists use to issue weather warnings, which can improve the health and safety of people on the ground.

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