The Hydrologic Cycle; Meteorological Satellites (General) - Remote Sensing Application -
The Hydrologic Cycle; Meteorological Satellites (General) Part-1

In the early days of the space program, particle and field physics, communications, and weather satellites were the principal applications of the new technology that grew out of Sputnik. Exploration of the planets, using cameras on space probes, soon followed. Most agree that the highpoint of the first 25 years was the landings on the Moon. Earth-oriented monitoring of the environment and human activities on the surface began in earnest during the 1970s. Astronomical observatories were placed in orbit in the 1990s as launch vehicles became more powerful.

These many uses define the values of space exploration. As far as the U.S. public (the taxpayer) is concerned, however, the first visible payoff from the space program was the images of clouds and weather systems that began to appear on television news in the late '60s. Thus, aspects of the science of meteorology, which experienced a quantum leap of capabilities, were brought home to the proverbial "average man on the streat".

We start this Section that deals with meteorological applications by summarizing the distribution of water, both saline and fresh, above, on, and in the outer Earth:

Partition of water in the outer Earth.

What is particularly striking about this chart's content is the large amount of fresh water locked up in snow and ice, the size of the groundwater reserve, and the fact that rivers and lakes, which are obvious bodies of water familiar to us, actually contain only a tiny fraction of the Earth's supply of fresh water.

Oceans and large freshwater bodies cover more than 70% of the Earth's surface. At any moment, around 50% of that surface, land and sea, is hidden from satellite 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:

Hydrologic Cycle 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 not only hold the bulk of the planet's water but are the source of most of the precipitation that constantly recycles water. Water in transit appears as circulating (wind-driven) 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 ice, mostly in the Antarctic and Greenland, snow (much being seasonally ephemeral), and freshwater lakes.

Most remote sensing observations tend to be two-dimensional, that is, the sensors look at the surface or very near surface. Geophysical remote sensing is, in part, 3-dimensional as the instruments used can provide information about the supracrustal rocks (e.g., sedimentary sections) and the crystalline basement crust and some techniques provide information about the mantle and even the Earth's core. Meteorological remote sensing is primarily directed towards making observations of the atmospheric profile - the column of air above the ground which varies in temperature, pressure, and composition. The atmosphere is layered or zoned, with general subdivisions (based on altitude and on composition and physical properties) named as follows (a more detailed version is on page 14-1a):

The subdivisions of the atmosphere, as a function of altitude

An astronaut photo shows most of this thick atmosphere, appearing from outer space as a blue band which diminishes in color outward:

Astronaut photo of the blue layers in the outer atmosphere; note the Moon beyond.

This more recent photo taken near sunset shows the lower layers to better advantage:

The layers of the atmosphere visualized near sunset.

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

Common sense tells one that satellites or spacecraft hundreds to thousands of kilometers above the Earth looking down towards its surface will see large areas at a time. Clouds and other weather phenomena scanned over wide vistas would give meteorologists a much better handle on moving weather systems. This became possible after Sputnik; some of the early efforts to utilize space for meteorological studies are briefly described on page 14-4. We show an image that surely kindled excitement among the weather people - indicating the value of the panorama approach - even before orbiting satellites went on line. It is a mosaic of photos taken from an Aerobee rocket that reached great heights during a 1954 mission from its launch site at White Sands Proving Grounds, New Mexico. A significant part of the northern hemisphere, with its various cloud systems, was viewed in this composite.

A mosaic of photos covering part of the Northern Hemisphere, taken by a camera riding on an Aerobee sounding rocket launched in 1954.

The first ever United States satellite in Earth orbit designed specifically to image and monitor conditions on and above the surface was the meteorological satellite called TIROS-1, launched on April 1, 1960 soon after NASA came into existence. It looked like this.

Artist�s sketch of TIROS-1 in space.

It was powered by solar energy collected from the 9260 solar cell plates on its exterior. Being small (42 inches or 106 cm) in diameter, it carried two TV cameras, one with low the other with high resolution. Here is the first image taken by the "Adam" of meteorological satellites.

The first TIROS image of clouds as seen from space; April 1, 1960.

Below is a series of image frames centered on the Mediterranean Sea that constitute the first set of meteorological imagery taken by Tiros, the first metsat:

Sequence of TV pictures showing the Mediterranean region as seen by Tiros-1

During its short three-month lifetime, Tiros-1 took 23,000 images of Earth. The TIROS program provided the first accurate weather forecasts based on data from space, demonstrating that it was possible to use satellites to observe weather. The satellite has a long legacy. TIROS-1 led to nine more TIROS satellites, seven Nimbus-series meteorological research satellites, 14 Geostationary Operational Environmental Satellites, 19 NOAA Polar Orbiting Satellites, and many more meteorological satellites maintained by the Department of Defense and other nations.

From 1959 through 2009, many countries launched satellites (approximately 330 - but some failed), such as the United States, the former 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 and magnetosphere), solar radiation, and related aspects of the environment. In contrast, so far, they dedicated fewer satellites to land observations. Astronauts conducted meteorological experiments during Mercury, Gemini, Apollo, Shuttle, and Mir flights. The International Space Station is also a good observation platform.

Clearly, the most widespread applications to date of remote sensors operating from space platforms have been to image water - either in oceans, lakes, and streams or in the air as water vapor - in its main functions in the Earth System. But, as the sensors improved, the ability to measure temperatures, and indirectly pressures (highs and lows), in the atmosphere as well as wind speeds and the rates of movements of air masses became possible in a quantitative way. But, the single most obvious thing that meteorological satellites could observe from the beginning are clouds. Most cloud formations (cumulus, stratus, nimbus, and combinations thereof) are straightforward and obvious as to types, but some unusual atmospheric effects expressed by clouds warrant special attention. Two good websites that treat the general characteristics of clouds are at Wikipedia and the University of Illinois. For reference to the various cloud types discussed in the next paragraphs, consider this diagram:

Pictorial classification of clouds, by shape and height; ..nimbus refers to rainmakers.

Let us now display over much of the rest of this page images taken by satellites of cloud systems and types. Often entire regional systems can be imaged in a single broad view satellite view. We will begin this coverage of clouds by considering moving air systems at regional scales.

The atmosphere on Earth is in constant motion, being driven by differences in pressure (pressure gradient). The changing pressures are associated with differences in temperature (as described in the tutorial beginning on the next page). Locally, atmospheric movement is recognized as wind. The wind may seem to move in a linear pattern but can change direction and even swirl (shifting or spinning).

Clouds result when water vapor is cooled enough to condense. Clouds can move in circulating atmospheric patterns. These are distinctive in shape, approaching spiral forms (gyres). These circulation patterns are the result of the Coriolis effect(Wikipedia website). When the air is clear, satellites may not "see" the pattern. Clouds make the pattern visible. The circulating clouds when visualized over time (either from geostationary satellites or multiple satellite passes) seem to be moving like wheels. In the northern hemisphere, those cloud systems that appear to rotate counterclockwise (hands on clock would move "backwards") are termed 'cyclones'; those rotating clockwise are called 'anticyclones. Both modes of circulation are evident in this image.

Clouds with an anticyclone at the top and a cyclone near bottom.

One of the most striking, and common, cloud patterns is the cyclone. In the northern hemisphere, this cloud system has a counterclockwise (ccw) spiral swirl that is often associated with a major low that delivers rain and even stronger storms, such as hurricanes. Wide field views can encompass the full systems of clouds comprising such lows.

Here are two images: the first shows a single cyclonic low and the second two cyclonic cloud banks off the coast of Iceland:

A broad low pressure system near Iceland.
MODIS image of cyclonic clouds off Iceland in the North Atlantic.

Atmospheric highs are often associated with fair weather systems; hence, the clouds may be few and a high is hard to recognize. This MODIS image shows a high with cumulus clouds.

Part of a fair weather high in the eastern U.S.

Multiple highs are evident in this next image:

Multiple high pressure vortices.

In the southern hemisphere, again owing to the directions imposed by the Coriolis effect, the direction of rotation is reversed, with highs moving counterclockwise and lows clockwise.

A southern hemisphere cyclonic low circulation pattern.
Low in the southern hemisphere.

The appearance of highs and lows is affected by the angle at which they are imaged. This view of an incoming storm low approaching the coast of California was made by the SeaWIFS instrument on the OrbView-2 satellite. The low has been compressed into an ellipse because this view actually looks to the horizon, causing the storm clouds to appear distorted owing to the curvature of the Earth.

SeaWIFS view of a spiral low approaching the California coastline.

The spiral pattern can develop over land or water bodies less than ocean-sized. A ccw spiral assemblage of clouds has formed over the Black Sea that touches northern Turkey, southern Ukraine and other neighboring countries. The adjacent land has few clouds, mostly independent of the cloud cover over the Black Sea as heat has evaporated water and condensed it into this forming low. This MODIS image covers 700 km (430 miles) on a side:

Cloud bank generating a counterclockwise spiral low over the Black Sea.

The cloud type most familiar to most of us is "cumulus", sometimes known as "fair weather clouds". Here are cumulus clouds over the Amazon jungle:

Photo from the International Space Station of cumulus clouds in the Amazon.

Similar to cumulus clouds are these puffy clouds, often called "cell" clouds to denote their origin by advection from the ocean surface:

Cumuluslike cell clouds.
Open and closed cloud cell patterns.

Cumulonimbus clouds can rise well up into the atmosphere and may be quite wide. These thunderhead clouds show as broad tops when seen from above, either from space or from high flying aircraft/

Cumulonimbus cloud tops.

Cumulonimbus clouds can be dispersed laterally by wind currents, producing what has been termed an "anvil cloud". Here is a view from the International Space Station.

ISS photo of an anvil cumulonimbus cloud.

Cumulonimbus clouds can coalesce to form a huge continuous cloud cell, as seen in this meteosat image of the Balkans:

A very large cumulonimbus cloud bank in the Balkans.

At the other extreme in altitude is fog. Fog is simply a cloud bank so low that it pervades the environment on the ground, often creating conditions of poor visibility. One of the most famed of fogs is that which can blanket the London, England area, producing "spooky" conditions that seem to be favored in movies featuring that city. In mid-December, 2006 several nights of thick fog covered London, making travel difficult as the airports were closed. Here is this fog as seen by MODIS:

A fog bank covering the London area around December 20, 2006.

We saw an example of valley fog in Section 6, on the page that includes San Francisco. Here is another satellite image that shows this type of fog in the San Joaquim Valley:

Valley fog in the central valley of California; MODIS image.

Both snow and fog appear in this California scene. How can one tell them apart? Using different bands to produce a color composite, the two can be separated as seen in this MODIS image of the northern Rocky Mountains in which snow is shown in red, fog in peach, and vegetation in green:

Snow in red, fog in peach.

Smog is a type of fog in which harmful chemicals may comprise the bulk of the cloud or may mix with natural fog. Smog is human-induced, resulting from the burning of fossil fuels (in autos) and power and industrial plants. Here is a mix of fog and smog in northern India, and a second image in the Po Valley of Italy:

Smog in the Ganges valley of India.
Smog in the Po Valley.

Still another dark cloud maker is volcanic ash and steam, producing what is called "vog". This example shows the blackish vog over the island of Hawaii, with the three sources of volcanic material labeled in the image:

Vog over Hawaii.

Volcanic eruptions produce distinctive clouds both from the vantage of the ground and from space. These are examples:

Mt. Redoubt in Alaska in eruption
Eruption of the submarine volcano in the Tonga islands.
The Sarychev Peak eruption in the Kuril islands as seen from the International Space Station.

The scale of Landsat images, covering 180 km on a side, is especially suited to showing clouds in some detail but over an area in which their context is well displayed. This Landsat image shows a pattern of stratocumulus cloud cells, each about 7-10 miles (10-15 km) over the Pacific Ocean. As warmer moist air rises in convection cells over the ocean and cools, condensing the water vapor into cloud droplets (which usually coalesce to form clouds), cold air then sinks around the sides of the cells.

Stratocumulus cloud cells.

Stratocumulus clouds can often arrange themselves in waves, much like ripple marks on sand dunes, as evidenced in this Landsat image taken over the Barents Sea, near the Kola Peninsula:

Stratocumulus clouds organized in waves; Barents Sea off eastern Siberia.

Stratocumulus clouds are common above the oceans, as seen here in this MODIS image of the west coast of the United States. Of special interest is the crosslink between these clouds and low fog along the coast and especially in Puget Sound (state of Washington) and the San Francisco Bay (California).

Stratocumulus clouds above the Pacific Ocean off the U.S. west coast; note fog emplaced in the Puget Sound and San Francisco Bay.

Note in the above image that the large areas east of the Sierra Nevada and Cascade mountains show up as browns in this color version. Nevada, Oregon, and Washington there consist of arid (desertlike) country owing to the orographic effect, which results when moisture-laden air is forced upwards by mountains, cooling the air and causing significant precipitation, leaving drier air to proceed past the mountains. This effect is evident in this MODIS image of the eastern Ural Mountains of Russia, with thin linear cloud formations over the plains of western Siberia as air moves eastward and downward off the high terrain.

Cloud streaks caused by the orographic effect.

The next visible image, taken on an afternoon over the Bering Straits off Alaska, shows a series of cumulus clouds aligned in cloud streets. The cumulus clouds result from radiative heating over land, which forces buoyant bubbles (thermals) up. The cumuli are the visible tops of these thermals. They are aligned by wind shear.

Cloud streets.