Why do we measure rain from space? Isn't a rain gauge good enough?

Water is everywhere on Earth and is the only known substance that can naturally exist as a gas, liquid, and solid within the relatively small range of air temperatures and pressures found at the Earth's surface. In all, the Earth's water content is about 1.39 billion cubic kilometers (331 million cubic miles) and the vast bulk of it-about 96.5%-is in the global oceans. Approximately 1.7% is stored in the polar icecaps, glaciers, and permanent snow, and another 1.7% is stored in groundwater, lakes, rivers, streams, and soil. Finally, a thousandth of 1% exists as water vapor in the Earth's atmosphere.

The water cycle or hydrologic cycle describes the journey of water, as water molecules make their way from the Earth's surface to the atmosphere, and back again. This large system, powered by energy from the sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land. The water cycle is the key circuit moving water through the Earth's system.. This large system, powered by energy from the sun, is a continuous exchange of moisture between the oceans, the atmosphere, and the land.

Even though the amount of water in the atmosphere is only 12,900 cubic kilometers (a minute fraction of Earth's total water supply that, if completely rained out, would cover the Earth's surface to a depth of only 2.5 centimeters), some 495,000 cubic kilometers of water are cycled through the atmosphere every year-enough to uniformly cover the Earth's surface to a depth of 97 centimeters. Because water continually evaporates, condenses, and precipitates, with evaporation on a global basis approximately equaling global precipitation, the total amount of water vapor in the atmosphere remains approximately the same over time. However, over the continents, precipitation routinely exceeds evaporation, and conversely, over the oceans, evaporation exceeds precipitation.

Precipitation (including rain, snow, sleet, freezing rain, and hail), is the primary mechanism for transporting water from the atmosphere back to the Earth's surface and is the key physical process that links aspects of climate, weather, and the global water cycle. It has been well recognized that natural and/or human-induced climate variability manifests most significantly in the global water cycle. If the Earth's climate is changing, i.e. if global temperatures are increasing as suggested by some observations, resulting higher evaporation and precipitation rates would result in overall change of global water and energy cycle. Understanding the Earth's climate and how water cycle processes respond to climate changes relies on what is known about how atmospheric moisture, clouds, latent heating ("hidden energy), and the atmospheric circulation vary with changing climate conditions. Moreover, precipitation is the parameter that has direct and most significant influence on the quality of human lives in terms of availability of drinking water and agriculture. Therefore, high quality precipitation measurements with global, long-term coverage and frequent sampling are considered to be crucial in understanding and prediction of Earth's climate, weather, and water cycle processes, and their consequences on life on earth.

Precipitation can vary greatly in time and space. For example, it may be raining where you are standing but not across the street, or it may be raining very hard where you are standing but only drizzling one mile away. Ground weather radars and rain gauges can be used to measure rainfall fairly well, but most global weather, climate, flood, and water resource applications require global rainfall measurements. But, Roughly 70.9% of the Earth's surface (361,740, 000 km2) is water. This implies that only about 29% of the Earth's surface could theoretically be equipped with rain gauges or ground-based weather radars. However, not all of the land is easily accessible (e.g. mountainous terrain, polar ice caps, desert, jungle). Even if rain gauges or ground-based radar made perfect measurements, they can only be placed on less than 25% of the Earth's surface. Also, new microwave instruments on spacecraft can make "cat-scans" or x-rays of rain clouds and thunderstorms to provide more information on their four-dimensional (space and time) life cycle.

For rainfall measurement, a global perspective provided by satellites is required. See http://trmm.gsfc.nasa.gov or http://gpm.gsfc.nasa.gov for more information.

This week's question is provided by Dr. J. Marshall Shepherd. Dr. Shepherd is a research meteorologist in the Laboratory for Atmospheres at NASA Goddard Space Flight Center. He specializes in mesoscale weather systems (e.g. rainfall systems, hurricanes, and thunderstorms) and remote sensing meteorology. He also serves as the Deputy Project Scientist for the forthcoming Global Precipitation Measurement (GPM) mission.