An Overview of Climate Processes
Climate and the Water Balance

Bountiful, liquid water is what makes planet Earth unique in this solar system and the known universe. Most of the water is held in the oceans (97%), with another 2% being frozen up in icecaps and glaciers, with less than 1% being on land surfaces or in the atmosphere.

The "residence time," or time it takes for a volume of water to cycle-through the system, can vary from a few days in the atmosphere, to a few weeks or months in rivers to 10,000 years or more in icecaps. The time it takes water to cycle through groundwater can vary from weeks and months (shallow aquifers) to hundreds of thousands of years in the case of deep fossil water.

Hydrologic variability, particularly precipitation, is another key component of climate variability, and a complicated one at that. At a local and regional level, precipitation can vary widely depending on geography and numerous other factors. Precipitation also doesn't necessarily correlate with temperature. There can be hot-dry periods, hot-wet periods, cold-dry periods and cold-wet periods, and everything in between. See Looking at the Water Budget for more on hydrologic variability over the course of a year.

There have been times in Earth's history when more of the water was in the oceans (Climate Science Beyond) or periods when there was more ice (Resources Beyond).

While only a small percentage of all of the Earth's water is in the form of clouds at any given time, clouds play a extremely important role in the global climate system. Not only do they serve as the vehicle for water to be conveyed around the world through the atmosphere, but in addition they play an integral role in the warming or cooling of the climate. Many climate researchers regard clouds and water vapor as the "wild card" in terms of current climate research since they can cool the surface by their shading effect or warm the planet by absorbing sun's radiation.

Many research teams, such as the NOAA's Environmental Technology Lab's Clouds, Radiation, and Surface Processes Division study the effects of clouds on climate.

Below is a figure showing the effects of clouds on Earth's radiation.
Next- Time Scales of Climate Change