Oceans and large freshwater bodies cover more than 70% of the Earth's surface. At any given moment, around 50% of that surface, both land and sea, is hidden from view by clouds. Over smaller areas, but still significant, rain descending from these clouds impacts on the surface to run off and then coalesce into streams and rivers. This great system of interconnected water circulation comprises the hydrologic cycle, as summarized in this diagram:

(Christopherson, R.W., GEOSYSTEMS: An Introduction to Physical Geography, 2nd Ed. © 1994. Reproduced by permission of Prentice Hall, Upper Saddle River, New Jersey)

The numbers associated with this chart clearly demonstrate that the oceans hold not only the bulk of the planet's water but are the source of most of the precipitation that constantly recycles water introduced to the atmosphere by evaporation. Water in transit shows up as circulating (wind-driven) condensates of visible clouds and invisible water vapor or as water mobilized in fluvial systems. In addition to the ocean bulk, which accounts for nearly 98% of the volume of water at or near the Earth's surface, most of the remaining fraction is stored as ice, mostly in the Antarctic and Greenland, snow (much being seasonally ephemeral), and freshwater lakes.

The advent of satellites after Sputnik (in 1957) opened up large regions in sweeping vistas for direct observation of weather systems, oceanographic conditions, and water runoff on continents and islands. A series of adjacent scenes acquired during short time periods could be easily combined in mosaics to give global coverage on a daily basis. In time, satellites placed in geosynchronous orbit afforded near instantaneous coverage of hemispheres of the Earth that could present rapidly updated views of cloud decks and circulation patterns over almost any part of the world. Ironically, the very thing that compromises observations of the land or open ocean - clouds - is the prime target of meteorological satellites (metsats). As more versatile sensors evolved, various other atmospheric or oceanographic properties, such as the stratosphere, tropospheric temperatures, earth radiation budget, air chemistry (e.g., ozone, CO₂, sulphur compounds, aerosols), wind and sea current movements, sea-ice, and marine biotic nutrients, could be monitored quantitatively.

From 1959 through 1994, approximately 270 satellites have been launched (a few failed) by the United States, the Soviet Union/Russia, Japan, China, India, Italy, France, and the European Space Agency primarily to provide current timely data for weather system monitoring and forecasting but also to conduct scientific studies to better understand the atmosphere, the oceans, the Earth's force fields (ionosphere; magnetosphere), solar radiation, and related aspects of the environment. In contrast, so far only 25 other satellites have been dedicated largely to land observations. Meteorological experiments have also been conducted by human observers during Mercury, Gemini, Apollo, Shuttle (STS), and Mir flights. Clearly, the most widespread applications to date of remote sensors operating from space platforms have been concentrated in fields related to water in its several main functions in the Earth System.

To most of the general public, including many in the technical fields, the one incursion of earth-observing satellites into everyday life comes during the Weather segment of the TV news. We all are familiar with synoptic views of clouds over our home region as well as panoramas across the continent in which we live. These are usually produced in visible and thermal IR bands on sensors mounted in geostationary satellites. Even more commonly presented are images made by ground-based Doppler radar systems that scan through a sweeping circular pattern; radio signals are bounced off (scattered by) particulates such as raindrops or ice, yielding estimates of precipitation amounts and wind speeds (using the Doppler principle [see Section 8] in which frequencies increase [decrease] as the reflecting particles move towards [away from] the radar site). Currently, in the U.S. most weather radars are operated by NOAA's National Weather Service; the NEXRAD (Next Generation Radar) network consists of S-band radars at 164 stations across the country. This system is especially geared towards detection and early warning of severe storms, tornadoes, and flood-generating heavy rains. Here are two examples, cloud patterns on the top and precipitation on the bottom, for the 48 states downloaded from the Accuweather site (http://www.accuweather.com). on the day this paragraph was written.

By arranging frequent observations into a time-lapse sequence (usually over an interval of the last 6 to 24 hours), a " movie" of advancing weather systems from local to continental scales is displayed on TV to the viewer.

Section 14 will review the history and accomplishments of this use of satellites to monitor the daily changes of the Earth's weather systems, oceans, rivers, and snow/ice and to conduct long-term research into the interactions of the atmosphere and hydrosphere that control the meteorological state of the planet. This subject is vast - worthy of its own Web Site (and there are many now, as you can ascertain by doing a Search). Only a digest of the types of observations - mainly through presentation of images and a few graphs - can be accommodated in the brief, simplistic, and generalized treatment we will allot to the topic. Emphasis is placed on meteorological applications, with an abbreviated summary of selected oceanographic (surface temperatures; seastate; currents; phytoplankton distribution) and hydrologic (flooding; water storage; drainage regime) uses. Many working through this Tutorial will likely lack a background in meteorology, and even more so, in oceanography and hydrology. For those who seek a broad overview, or wish to delve into greater technical and scientific details, we suggest perusal of a good introductory Meteorology or Oceanography text or, better yet, the relevant chapters in a Physical Geography text, such as:

• Satellite Meteorology: An Introduction, S.Q. Kidder and T.H. Vonder Haar, 1995, Academic Press, 466 p.
• Images in Weather Forecasting: A Practical Guide to Interpreting Satellite and Radar Imagery, 1994, Cambridge University Press
• Meteorology Today: An Introduction to Weather, Climate, and the Environment, 1995, C.D. Ahrens, West Publishing Co.
• The Atmosphere, F.K. Lutgens and E.J. Tarbuck, 1993, Prentice-Hall
• Introductory Oceanography, H.V. Thurman, 1988, MacMillan Publ. Co.
• Introducing Physical Geography, A. Strahler and A. Strahler,1994, J. Wiley & Sons
• Geosystems: An Introduction to Physical Geography, R. W. Christopherson, 1992, MacMillan Publ. Co.

In addition, a Website course on Satellite Meteorology has been prepared by EuroMET. To access this site, you must first get a User name and password through this URL address; click on Satellite Meteorology.

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