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The Movement of Energy The "cause" of weather Weather is caused by the movement or transfer of energy. Energy is transferred wherever there is a temperature difference between two objects. There are three main ways energy can be transferred: radiation, conduction and convection. Electromagnetic radiation All bodies emit energy in the form of electromagnetic radiation. Light is a form of electromagnetic radiation. So is infrared heat and ultraviolet radiation. Different types of radiation are shown in the figure below. The type of radiation emitted by a body depends upon its temperature. Hotter objects release more energetic radiation. The Sun, for example, emits light wave and ultraviolet energy. The Earth, being much cooler, emits radiation in the infrared part of the spectrum. Living things, including us, also emit radiation in the infrared part of the spectrum. This energy is known as heat energy. Usually we cannot differentiate between the different wavelengths of sunlight, and the light appears white, but they become visible in a rainbow. In a rainbow we see seven colours. In order of decreasing wavelength these are red, orange, yellow, green, blue, indigo, violet. We cannot see electromagnetic energy that has a longer wavelength than red (infrared) or a shorter wavelength than violet (ultraviolet).

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The Movement of Energy

The "cause" of weather

Weather is caused by the movement or transfer of energy. Energy is transferred wherever there is a temperature difference between two objects. There are three main ways energy can be transferred: radiation, conduction and convection.

Electromagnetic radiation

All bodies emit energy in the form of electromagnetic radiation. Light is a form of electromagnetic radiation. So is infrared heat and ultraviolet radiation. Different types of radiation are shown in the figure below. The type of radiation emitted by a body depends upon its temperature. Hotter objects release more energetic radiation. The Sun, for example, emits light wave and ultraviolet energy. The Earth, being much cooler, emits radiation in the infrared part of the spectrum. Living things, including us, also emit radiation in the infrared part of the spectrum. This energy is known as heat energy.

Usually we cannot differentiate between the different wavelengths of sunlight, and the light appears white, but they become visible in a rainbow. In a rainbow we see seven colours. In order of decreasing wavelength these are red, orange, yellow, green, blue, indigo, violet. We cannot see electromagnetic energy that has a longer wavelength than red (infrared) or a shorter wavelength than violet (ultraviolet).

Electromagnetic radiation

Why is the sky blue?

The Earth's sky is blue because the air molecules (largely nitrogen and oxygen) are much smaller than the wavelength of light. When light encounters particles much smaller than its wavelength, light is scattered in all directions. This scattering is known as Rayleigh scattering. Blue light, however, is much more readily scattered than light of longer wavelength. In fact blue light is scattered some 16 times more strongly than red light. Consequently, the sky appears blue. Rayleigh scattering also provides the explanation for the colour of skies at dusk and dawn. A

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setting or rising Sun appears red because the blue light has been scattered away from the direct sunlight.

Occasionally, other colours of the sky can be seen. These are caused by the scattering of light by particles much bigger than air molecules, sometimes larger than the wavelength of light. Dust particles, from natural or man-made pollution, can be many times larger than air molecules but still small enough to not fall out to the ground. If the dust particles are much larger than the wavelength of light, the scattered light will be neutral in colour, for example white or gray. By the same principle water molecules in clouds are sufficiently large to scatter all wavelengths equally. Therefore the scattered light contains all wavelengths and clouds appear white.

If the dust particles are of approximately the same size as the wavelength of light, all sorts of interesting scattering phenomena may happen. This happens here on Earth from time to time, particularly in desert areas, where the sky may appear white, brown, or some other colour.

Transfer of radiation: absorption

The transfer of radiation may be accomplished by direct absorption by the cooler object of energy emitted by the warmer one. Food placed under a grill absorbs energy radiated from the heating elements. Similarly, the Earth absorbs energy emitted from the Sun. How much the cooler object heats up depends upon the nature of that object. Certain materials like rock require much less energy to raise their temperature than others, such as water. That is to say, water has a larger heat capacity than rock (about 5 times larger).

Transfer of radiation: reflection and scattering

Transfer of radiation can occur in other ways besides absorption. Certain material bodies, for example glass or pure air, are transparent to visible radiation (light), allowing the energy to pass straight through without absorption. This is not the case for infrared radiation, which is trapped (absorbed) by glass and certain greenhouse gases. Other material bodies neither absorb nor allow

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through radiation, but reflect it back. Light surfaces such as snow and ice are good reflectors of visible radiation. Dust in the atmosphere scatters radiation. The blue of the sky is due to the scattering of the blue part of visible light from the Sun by air molecules, the reds and yellows of sunset are due to scattering by dust particles.

Conduction

Transfer of energy can also be made by conduction, if two objects are in contact. A heating element of a kettle heats its water by conduction of energy. Metal is a good conductor, as is water. Air is a poor conductor of energy and therefore acts as a good insulator, as, for example, between the outer and inner skins of a cavity wall house, or between the human body and outside air by air spaces between fibres of clothing.

Convection

Convection of energy involves the movement of the body storing the energy. Clearly, convection can only occur in fluids - liquids and gases. Both air and water act as convecting agents for the transfer of energy. Because air is a poor conductor of energy, convection is a major process of energy movement in the Earth's atmosphere. In the atmosphere, convection occurs when a shallow layer of air in contact with a hot surface warms by conduction, acquires buoyancy (warmer air is less dense than colder air), and rises, taking with it the energy that it stores.

Movement of radiation in the atmosphere

In a weather context, energy radiated from the Sun is distributed at the Earth by absorption, conduction, convection, reflection and scattering. The Sun radiates energy at a rate which, for practical purposes, is unchanging. Most of this radiation is visible light and ultra - violet energy. At the outer limit of the Earth's atmosphere, the radiation received is 1370 watts per square metre. This is called the solar constant. Passing through the atmosphere it is depleted in various ways.

Air in an atmosphere free from clouds and pollution absorbs very little solar radiation, which passes through to the Earth's surface. Here, some of the radiation is absorbed and some is reflected. The absorbed part heats the surface, causing its temperature to rise. The Earth's surface also emits energy, but because it is much cooler than the Sun, it does so in the infrared part of the spectrum. This is heat energy. Often, the Sun's energy is called short-wave radiation, whilst radiation from the Earth's surface is called long-wave radiation. The more energetic the radiation, the shorter its wavelength. Thus hotter objects emit shorter wavelength radiation.

The greenhouse effect

Whilst the air allows short-wave light to pass through to the Earth's surface, certain gases in the atmosphere impede the passage of the long - wave heat energy by absorbing it. Water vapour and carbon dioxide are particularly effective at absorbing the long-wave heat radiation. This warms the air, which in turn radiates some of the energy back to the Earth's surface and some to space.

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Thus, the atmosphere, by trapping some of the outgoing radiation, keeps the Earth's surface warmer than it would otherwise be.

This warming effect is called the "greenhouse effect" because it is the same process as that which occurs in a greenhouse on a sunny day. The glass is transparent to short-wave radiation but absorbs the outgoing long-wave radiation, causing a rise in temperature inside the greenhouse. Water vapour and carbon dioxide are called "greenhouse gases". Other greenhouse gases include methane and CFCs from aerosol cans.

The greenhouse effect

Convection in the atmosphere

Heat energy in the atmosphere is also transferred by convection as described above, with the heating and ascension of surface air. Such convection processes in a large part dominate the world weather, including the production of rain and snow, thunderstorms, hurricanes and frontal systems. How many of these weather phenomena originate will be the topic of the following lessons in this section.

Atmospheric Effects on Incoming Solar Radiation

Three atmospheric processes modify the solar radiation passing through our atmosphere destined to the Earth's surface. These processes act on the radiation when it interacts with gases and suspended particles found in the atmosphere. The process of scattering occurs when small particles and gas molecules diffuse part of the incoming solar radiation in random directions without any alteration to the wavelength of the electromagnetic energy (Figure 7f-1). Scattering does, however, reduce the amount of incoming radiation reaching the Earth's surface. A significant proportion of scattered shortwave solar radiation is redirected back to space. The amount of scattering that takes place is dependent on two factors: wavelength of the

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incoming radiation and the size of the scattering particle or gas molecule. In the Earth's atmosphere, the presence of a large number of particles with a size of about 0.5 microns results in shorter wavelengths being preferentially scattered. This factor also causes our sky to look blue because this color corresponds to those wavelengths that are best diffused. If scattering did not occur in our atmosphere the daylight sky would be black.

Figure 7f-1: The process of atmospheric scattering causes rays of sunlight to be redirected to a new direction after hitting a particle in the atmosphere. In this illustration, we see how three particles send light rays off into three different directions. Scattering does not change the striking light ray's wavelength or intensity.

 

If intercepted, some gases and particles in the atmosphere have the ability to absorb incoming insolation (Figure 7f-2). Absorption is defined as a process in which solar

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radiation is retained by a substance and converted into heat energy. The creation of heat energy also causes the substance to emit its own radiation. In general, the absorption of solar radiation by substances in the Earth's atmosphere results in temperatures that get no higher than 1800° Celsius. According to Wien's Law, bodies with temperatures at this level or lower would emit their radiation in the longwave band. Further, this emission of radiation is in all directions so a sizable proportion of this energy is lost to space.

Figure 7f-2: Atmospheric absorption. In this process, sunlight is absorbed by an atmospheric particle, transferred into heat energy, and then converted into longwave radiation emissions that come from the particle.

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The final process in the atmosphere that modifies incoming solar radiation is reflection (Figure 7f-3). Reflection is a process where sunlight is redirect by 180° after it strikes an atmospheric particle. This redirection causes a 100% loss of the insolation. Most of the reflection in our atmosphere occurs in clouds when light is intercepted by particles of liquid and frozen water. The reflectivity of a cloud can range from 40 to 90%.

Figure 7f-3: Atmospheric reflection. In this process, the solar radiation striking an atmospheric particle is redirected back to space unchanged.

 

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Sunlight reaching the Earth's surface unmodified by any of the above atmospheric processes is termed direct solar radiation. Solar radiation that reaches the Earth's surface after it was altered by the process of scattering is called diffused solar radiation. Not all of the direct and diffused radiation available at the Earth's surface is used to do work (photosynthesis, creation of sensible heat, evaporation, etc.). As in the atmosphere, some of the radiation received at the Earth's surface is redirected back to space by reflection. The following image describes the spatial pattern of surface reflectivity as measured for the year 1987.

Figure 7f-4: Annual (1987) reflectivity of the Earth's surface.

(Image produced by the CoVis Greenhouse Effect Visualizer).

 

The reflectivity or albedo of the Earth's surface varies with the type of material that covers it. For example, fresh snow can reflect up to 95% of the insolation that reaches it surface. Some other surface type reflectivities are:

Dry sand 35 to 45% Broadleaf deciduous forest 5 to 10%

Needleleaf confierous forest 10 to 20%

Grass type vegetation 15 to 25%

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Reflectivity of the surface is often described by the term surface albedo. The Earth's average albedo, reflectance from both the atmosphere and the surface, is about 30%.

Figure 7f-5 describes the modification of solar radiation by atmospheric and surface processes for the whole Earth over a period of one year. Of all the sunlight that passes through the atmosphere annually, only 51% is available at the Earth's surface to do work. This energy is used to heat the Earth's surface and lower atmosphere, melt and evaporate water, and run photosynthesis in plants. Of the other 49%, 4% is reflected back to space by the Earth's surface, 26% is scattered or reflected to space by clouds and atmospheric particles, and 19% is absorbed by atmospheric gases, particles, and clouds.

Figure 7f-5: Global modification of incoming solar radiation by atmospheric and surface processes.

 

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Satellite structure and operation

Satellites are built for a broad variety of missions, so each one is different.However, just as any motor-driven vehicle has a chassis, an engine, fuel tanks and steering systems, all satellites share the same basic structure and organization.

The core element is the platform or bus, which accommodates the mission payload and all the equipment needed to operate it. In particular, the bus provides the systems for acquiring measurements, communicating, sending and receiving commands, and performing management and housekeeping operations. It is usually a very rigid truss structure designed to withstand the accelerations and vibrations generated during launch.

Word watch

What is a satellite? Literally, a satellite is a celestial object orbiting a larger body in periodic fashion as a result of gravitational attraction. Since the beginning of the space adventure, we have made a distinction between natural satellites like the Moon and artificial satellites. However, the term satellite used alone is now commonly used to describe the man-made variety.

Altimetry satellite JASON-1. Credits: CNES

Depending on its mission, a satellite may carry a number of instruments, for example to acquire imagery, record data, and transmit and receive radio signals. It generates its own power from solar

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panels.

An Earth satellite generally orbits at an altitude of between 450 and 36,000 km. Its motion is sustained naturally by gravitational attraction. However, it also has its own propulsion system for orbital manoeuvres, which in some cases may include boosting the satellite to its final orbit.

All these systems are controlled by an onboard computer that manages all the equipment and Radar (Microwave) Remote SensingPage 1 of 3

RADAR stands for "RAdio Detection And Ranging". By virtue of sending out pulses of microwave electromagnetic radiation this type of instrument can be classified as an "active sensor" - it measures the time between pulses and their reflected components to determine distance. Different pulse intervals, different wavelengths, different geometry and polarizations can be combined to roughness characteristics of the earth surface.

Radar wavelengths range between less than 1 millimeter to 1 meter.

All photos can be called images but not all images should be called photos - images that are acquired digitally are usually not referred to as photographs ... there is no such thing as a radar photograph!

Radar uses relative long wavelengths which allows these systems to "see" through clouds, smoke, and some vegetation. Also, being an active system, it can be operated day or night. There are disadvantages, such as the non-unique spectral properties of the returned radar signal. Unlike infrared data that help us to identify different minerals or vegetation types from reflected sunlight, radar only shows the difference in the surface roughness and geometry and moisture content of the ground (the complex dielectric constant). Radar and infrared sensors are complimentary instruments and are often used together to study the same types of earth surfaces.

Color Aerial Photography vs Processed Radar Imagery

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Color Photography vs Radar Derived DEM (Digital Elevation Model)

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Unlike instruments that look straight down (i.e. Aerial Cameras and nadir pointing satellite sensors), radar data are collected looking off to the side. Because radar measures the time that it takes for the signals to go from the antenna to the ground and back, this angled perspective is a necessity because this causes there to be a delay between the parts of the returned pulse that are farthest from the antenna and the parts that are closest. This very small delay is measured with very high precision, these measurements are filtered using complex functions for removing noise and then these time measurements are converted to a distance for every location in the radar swath and viola we have a map of the surface topography. 

Cameras capture reflected visible wavelengths. Radar captures emitted microwave wavelengths that are bounced back to the antenna.

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Plan Position Indicator - the kind of radar used by air traffic controllers is not considered an "imaging radar"

Radar was originally developed in the 1950s, the first airborne system was called SLAR (Side-Looking Airborne Radar) and was used for improving the resolution for military reconnaissance, early airborne radar systems were limited by the physical size of the antenna however. Later, SAR (Synthetic Aperture Radar), was developed and are widely used in many countries for civilian applications.

San Francisco (1950's acquired by an early SLAR system)

Types of Imaging Radar SLAR (Side-Looking Airborne Radar) - develop by guess who in the 1950's - airborne, fixed antenna width, sends one pulse at a time and measures what gets scattered back - resolution determined by wavelength and antenna size (narrow antenna width = higher resolution) SAR (Synthetic Aperture Radar) - also developed by those responsible for SLAR, but this configuration is not dependent on the physical antenna size although to achieve higher resolution the receiving antenna components and transmitter components need to be separated.

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- "synthesizes" a very broad antenna by sending multiple pulses

Radar resolution has two components; the "range" resolution and the "azimuth" resolution. These are determined by, among other factors, the width of the synthesized antenna (which is dictated by the pulse interval) and the wavelength.

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The diagrams below illustrate some of the common terms used to describe the geometry of a radar image. Most important are the "look angle", the angle at which the radar pulse hits the surface, and the interval between pulses.

Radar images show surfaces that bounce back the radar pulse (called "radar backscatter"), the bright areas are strong reflectors (such as buildings in an urban area, or a bolder field in the desert), while dark parts of the image represent surfaces that reflect very little or no energy (such as a frozen lake or oil film on the ocean). The amount of backscatter varies as a function of incidence angle, surface roughness and soil moisture (depending on the wavelength).

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A smooth surface acts like a mirror when the angle is small, but with angles greater than 20 degrees the amount of backscatter sharply declines because the signal bounces off the surface away from the antenna. The opposite is true for a rough surface. At steep angles (incidence angle less than 20 degrees), most of the emitted pulse is scattered in random directions so that the total backscatter measured by the antenna is lower than from a smooth surface at the same angle. By changing the incidence angle and comparing how the backscatter changes with different angles it is possible to map different types of surfaces (such as smooth vs. rough lava).

Backscatter for different incidence angles.

Corner reflectors amplify backscatter, particularly in urban areas,  "double bounce" "triple bounce".

Oil film from natural oil seeps off the coast from UCSB. (shorter wavelength)

Strong response from metal, see the oil rigs? (longer wavelength) Open pit Copper mine.

Angkor, Cambodia Jungle Temple 

Using different combinations of wavelength and incidence angle, the characteristics of the recorded backscatter can be compared and interpreted. Acquiring a detailed dataset with the necessary calibration field work and measurements is an ordeal to plan and execute but with a good dataset radar imagery can reveal characteristics of the landslide that visible and infrared imagery cannot.

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The assumption is that different vegetation types (e.g., desert, grasslands, forests or frozen tundra) will all have different backscatter signatures. In addition, the basic reflectivity of the soil, called the

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"dielectric constant" will change depending on the amount of water and organic matter that the soil contains. Dry soil has a low dielectric constant, so that little radar energy will be reflected. Saturated soil will have a high dielectric and will be a strong reflector. Moist and partially frozen soils will have intermediate values.

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Short wavelength radar (3 cm) will be reflected from the tops of trees. Long wavelength radar (24 cm) data will normally go down to the ground and be reflected back. Intermediate wavelengths (6 cm) will sometimes experience multiple scattering events between the canopy, the branches and the ground. By acquiring multiple wavelength, multi directional radar imagery of a forested area it is possible to discern information about the canopy structure and estimate above ground boimass.

. . SAR transmits pulses of microwaves and measures the strength and the time delay of the energy that is scattered back to the antenna.

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. . SAR concept of "synthetic aperture" comes from processing techniques that can simulate a much larger antenna size. The SAR instrument used by the Shuttle Radar Topography Mission (SRTM) consist of a 197 foot long mast with a 28 ft receiver, the biggest such object ever deployed in space. With this long distance between receiver and transmitter the triangulation of the signals is potentially much more accurate.

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The spaceborne SAR flew aboard two space shuttle flights in April and October, 1994. The unique perspective of space and the advanced capabilities of this Spaceborne Imaging Radar-C and X-Band Synthetic Aperture Radar (SIR-C/X-SAR) Mission have allowed scientists to explore our planet in a way that has not yet been possible. Data from SIR-C/X-SAR and other international SAR systems such as the European Space Agency's Remote Sensing Satellites (ERS-1/2), Japan's Earth Resources Satellite (JERS-1), and Canada's Radarsat are currently being used for studies of ecology, hydrology, oceanography, geology, ice sheets, glaciers, icebergs and for archeological exploration.

Ecological applications of SAR include land cover classification, above-ground woody plant biomass measurement, wetland inundation delineation, crop  identification and monitoring. SAR data have also been used for malaria risk mapping in tropical regions by providing imagery that shows the spatial and temporal distribution of flooding.

Hydrologic applications of SAR include measuring snow condition and snow pack thickness, which is used for estimating runoff and water supply. Oceanographers are using SAR data to study surface and internal waves, wave/current interactions, arctic sea-ice motions and for monitoring shipping lanes for icebergs. SAR data are also being used by geologists in studies of past climates and volcanic/earthquake hazards.

Seasat image of wave patterns (not clouds!) off the coast of Southern California

The longer L-band and P-band radar wavelengths have the useful capability of being able to see beneath the surface. SIR-C/X-SAR obtained data of the Sahara Desert that showed braided channels and paleodrainages beneath the surface, this was a complete surprise to the scientists and engineers involved because although is was theoretically possible they did not anticipate being able to see more than several meters into the surface. The important property of this part of the world is that there is no moisture (very low dielectric), even at depth, so the waves can propagate through the sand, bounce off the solid bedrock and then back out again.

These images are from a region of North Africa's Sahara Desert near Kufra Oasis in southeast Libya.

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Volcano, earthquake, floods, fires, landslides and other natural hazard investigations benefit greatly from a unique capability of SAR called "interferometry" which uses coordinated measurements obtained at slightly different angles acquired in sequence along a flight line. These measurements combined to produce distance and surface heights. Combining one interferometric data pair with a third measurement, obtained at a different time, allows for very precise measurements to be made. This technique, known as differential interferometry, has enormous potential for mapping changes in topography on the order of a centimeter and for mapping subtle shifts in earth's crust in areas of active faulting. High-resolution maps of topography and topographic change generated from SAR interferometry are also extremely valuable for studies of ice sheets and glaciers. Over 75% of the world's fresh water is presently locked-up in ice and snow.

Yellowstone Radar Image

While the general retreat of mountain glaciers globally is believed responsible for approximately ¼-½ of the current 2 millimeter-per-year increase in sea level, the other sources are unknown. Radar provides a means of regionally monitoring the health of the ice sheets which can be used to assess the threat of sea level rise.

San Fransisco sea level rise map, Bangladesh sea level rise projection

Another emerging area for SAR study is archeology. For example, a SIR-C image of the region around the site of the lost city of Ubar in southern Oman, on the Arabian Peninsula. The ancient city was discovered in 1992 with the aid of remote sensing data, it is believed that Ubar was a desert outpost and existed from about 2800 B.C. to about 300 A.D. The actual site of the fortress of the lost city of Uba is too small to be detected in the image, but the linear tracks leading to the site appeared prominently due to the propensity of radar to resolve linear features.

Topography of Nigeria and Venezuela, notice the linears.

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The End | Back

The spatial resolution of a sensor can be defined as the area of ground imaged by one pixel. High spatial resolution means each pixel represents a small square of ground, so it is easy to identify tiny features in an image. Spectral resolution is the width of the regions of the electromagnetic spectrum that a sensor will detect. High spectral resolution allows material identification through a characterization of its spectrum. spatial Resolution depends on space while temporal resolution depends on time.