Glacier Power – Teacher Guide Glossary

Glacier Power Glossary

Ablation, Ablation zone
Processes (especially melting) by which a glacier loses ice and snow: melting, evaporation, calving, and erosion. The area of a glacier where ablation occurs.

Accumulation, Accumulation zone
Process (especially snowfall and compression) by which a glacier gains snow and ice. The area of a glacier where accumulation occurs.

Sharp, narrow ridges formed on a mountain by a glacier.

Basal Slip
The sliding of a glacier over its bed.

The solid rock under a glacier or ice sheet.

Also called, “erratic boulder.” Large boulders are sometimes left behind when a glacier recedes or retreats.

An Erratic Boulder. Illustration by Amy Stubsjoen.

Glaciers calve icebergs, which are chunks of ice that break off glaciers and fall into water.

A bowl or basin carved out of a mountain by a young glacier.

A Cirque Glacier. Photo by Hambrey.

An open crack in the glacier surface. (See also Types of Crevasses).

A crevasse is a crack in the surface of a glacier. Photo by Kristina Ahlnas.

To make more compact, press together into less volume or space.

Geology: An accumulation of relatively large rock fragments: glacial debris.

To let go; to empty out; to pour forth.

Smooth rounded mounds of glacial till (rock, dirt, and debris) deposited under a glacier.

European Remote Sensing Satellite-1.

Rounded and compact snow that is older than one year.

Deep U-shaped coastal valleys, originally eroded by a glacier but now filled by the sea. Some fjords still contain a glacier.

Individual layers or bands of ice crystals produced during the metamorphic process that changes snow to glacier ice.

Folia/Foliations are individual layers or bands of ice crystals produced during the metamorphic process and compression that changes snow to glacier ice. The layers have distinctive crystal sizes and usually alternate between bubbly and clear ice or between fine-grained and coarse-grained ice. Photo by Hambrey.

A large accumulation of many years of snow, ice, rock, sediment and water that originates on land and moves down slope under the influence of its own weight and gravity.

Root Glacier. Photo by the Alaska Geographic Society.

Glacier Flour
The very finely ground particles of rock, silt, or clay created by a glacier when its rock-filled ice scrapes over bedrock and which flow out from beneath a glacier in the meltwater.

Glacier flour. Photo by Dr. Bruce Molnia, Alaska Geographic Society.

Glacier Table
A boulder sitting on a pedestal of ice. The boulder protects the ice from melting during sunny weather.

Hanging Glacier
A glacier that spills out from a high level cirque or clings to a steep mountainside.

Steep-sided peaks, shaped like pyramids, formed when cirque glaciers erode on three or more sides of a mountain.

Ice Ages
A time when large sheets of ice covered a large amount of the earth because of cooler temperatures.

Ice Apron
A steep mass of ice that clings to steep rock at the summits of high peaks. (A common source of ice avalanches.)

See “Calving“.

A large iceberg floating in Glacier Bay, Alaska. Photo by James Roush.

Ice Cap
A steep mass of ice that clings to high peaks.

Ice Fall
Jumbled and broken ice which forms when a glacier flows over a steep drop-off.

Ice Sizzle
A crackling or sizzling sound, similar to soda or Rice Krispies, that is produced by an iceberg while it is melting. The sound is caused by the release of air bubbles that were trapped in the glacier ice during its formation under high pressure. This is also called “bergy seltzer.”

Ice Stream
A stream of ice flowing down-valley.

Ice Tongue
A floating extension of an ice stream or valley glacier projecting into the sea.

Ice Worm
Ice worms are the only earthworms known to inhabit snow and ice. They thrive in temperatures just above freezing.

Scanning Electron Micrograph of an actual ice worm. W.P. Wergin & E.F. Erbe, ARS, U.S.D.A.

The relatively warmer periods between each ice age when the ice sheets retreat.

Isostatic rebound
Land that was once pushed down by the heavy weight of glacial ice may experience actual rising at the small rate of about a centimeter per year after the ice melts away.

(Outburst Flood) A tremendous release of water that was trapped behind a glacier or underneath a glacier.

Steep-sided hills of sand and gravel deposited by glacial streams or in crevasses.

The result of a very large block of ice being left behind as a glacier recedes. The melting forms potholes which are sometimes filled with water in a glacier, till, or outwash plain. Vegetation may grow up around kettles.

A Kettle. Photo by James Roush.
Vegetation growing around a kettle filled with water. Photo by McMillan.

Lateral Moraine
A large mass of glacial material on the sides of a glacier.

Mass Balance
The difference between the amount of material that a glacier accumulates and the amount lost during ablation is called its mass balance. The equilibrium line moves down or up a glacier as the mass balance changes.

Medial Moraine
A central moraine formed where two glaciers join to form a large glacier.

Water produced by melting glacier ice, firn, and surface snow. Meltwater flows down the bed of the glacier and emerges from the end as a stream often colored gray-green by the rock flour it contains.

Something that undergoes changes in structure or composition, texture, or internal structure by (for glaciers) heat or pressure.

A glacial deposit of rocks and debris that forms through direct action or contact with glacier ice.

Moraines at Glacier Bay. Photo by Stephen J. Kraseman.

A hole or tube in a glacier into which water flows. Moulin is French for mill, so called because of the loud, roaring noise made by the falling water.

The End of the Same Moulin. Photo by James Roush.

The area of a glacier covered with snow throughout the year.

Staging area on a glacier's névé, the area covered with snow throughout the year. The term also refers to the accumulation area at the upper end of a glacier. Glaciologists land their planes in this area, as it is relatively safe from crevassing. Photo by James Roush.

Mountain peaks or ridges which poke through the ice, forming islands within the glacier.

Nunatak. Photo by Alaska Geographic Society.

Repeated curved bands created within glaciers at the base of ice falls.

Outwash Plain
A broad, gentle plain composed of sand and gravel washed out of a glacier and deposited by meltwater streams.

Piedmont Glaciers
The parts of glaciers, fed by mountain glaciers, that have spread out over broad lowlands.

Rock Flour
Same as “Glacier Flour.”

Synthetic Aperture Radar can “see” through clouds and can “see” in the dark like a bat! SAR is a microwave radar used to penetrate through clouds or darkness to obtain images of the earth.

Frozen precipitation in the form of white or translucent six-sided ice crystals that fall in soft, white flakes.

Snow crystals.

A single flake or crystal of snow.

Scanning Electron Micrograph of a Snow Crystal. W.P. Wergin & E.F. Erbe, ARS, U.S.D.A.

A short period when a glacier can go as much as one hundred times faster that it normally goes. This happens when a glacier slides downstream on water trapped beneath it.

Surge Front
The leading edge of the wave or bulge of moving ice which represents the surge before it reaches the terminus of the glacier.

A small lake filling a hollow which was eroded out by ice or dammed by a moraine. Frequently found with cirques.

The snout, end, or leading edge of a glacier.

Tidewater Glacier
A glacier that ends in the sea.

Hubbard Glacier, Alaska, During a Surge. 1986 Photo by Hambrey.

The rock, dirt, and debris deposited beneath a glacier.

Valley Glacier
A glacier which flows from a high mountain, from an ice cap, or from an ice sheet into a valley.

A Glacier Valley. Illustration by Erica Herbert.

The region draining into a river, river system, or other body of water.

Glacier Power – Teacher Answer Key Segment Review

Segment Review Questions Teacher's Guide

What is a Glacier?

How does the glacier move down it’s slope? Gravity

Are all glaciers the same? No

Where are the world’s largest ice sheets found? Antarctica and Greenland

Do ice shelves float? Yes

Which are bigger, ice shelves or ice caps? Ice Shelves

From what sources might you see a valley glacier flowing? Cirques, ice caps, highland ice fields, or ice sheets

What does SAR stand for? Synthetic Aperture Radar

In what season is meltwater abundant, summer or winter? Summer

Do cold glaciers erode a lot? No 

Brain Challenge!
When rivers freeze, do they turn into glaciers?

Where Have Glaciers Been?

What three continents were covered in ice sheets for 2 million years? North America, Europe, and Asia

What made the glaciers melt two million years ago? Rising Temperatures

Why do geologists and glaciologists study the Ice Age? To find out what happened in the past and what may happen in the future

Do glaciers leave clues behind? Yes or No? Yes  

Name two clues that glaciers leave. Scraped rock, huge boulders, u-shaped valleys, silt, fjords

What two things have to be just right for a glacier to exist? Temperature and precipitation

What does “deposition” mean? The laying down of matter by a natural process

What does “erosion” mean? Process by which material is worn away from the Earth’s surface

What is the difference between a U-shaped valley and a V-shaped valley? U-shaped valleys are formed by glaciers and V-shaped valleys are formed by rivers   

Brain Challenge!
Have glaciers been around where you live? How can you tell?

Anatomy and Other Diagrams of a Glacier

Is the accumulation zone near the upper or lower region of a glacier? Upper

Is the ablation zone near the upper or lower region of a glacier? Lower

The process of calving stops, balances, or increases (pick one) the flow of ice from behind? Balances

True or False: Meltwater flows along the top of glaciers. False

Explain what process is occurring in #3 on the glacier anatomy image. What does it tell you? The equilibrium line divides the accumulation zone and the ablation zone. 

What is the fastest a glacier can surge? 5 times faster than it normally goes or 100 times faster than it normally goes? 100 times faster than it normally goes

In a surge, can a glacier swallow its own valley? Yes

In the diagram in this section that shows what happens when a glacier surges, look at letter B and find out what it’s about. The glacier surging, sliding downhill.

Draw a diagram of shearing at the side of a glacier. (Hint: it’s in the diagram of “types of crevasses”) Located also in the “What is a Crevasse” segment   

Brain Challenge!
Do you think a surging glacier could knock down Denali? 

Strange Glacier Phenomena

Can glaciers make sounds? Yes. such as ice sizzles, ice quakes (a traveling/cracking sound), and the roar of moulins

What are small black wingless springtail bugs that live in firn on glaciers? Glacier fleas

What do ice worms eat? Algae and pollen

There is an image of a fossil in glacial till in this section. What is the fossil? A log or a stump

What are sudden glacial outburst floods of water that can be catastrophic? Jokulhlaups

Do ice quakes sound like earthquakes (rumbling sound) or do they make a hissing and crackling sound? Hissing and crackling sound

Are moulins holes in a glacier or the steel spikes you put on your boots to hike on a glacier? Holes in a glacier   

Brain Challenge!
Would you ever want to be an ice worm? Why or why not?

What are Crevasses?

What can cause a crevasse? Stress in the ice, by ice flowing over bumps or steps in the bedrock

Why don’t crevasses reach to the very bottom of the glacier? Most are squeezed shut by the pressure of the ice below about 30 meters (100 feet)

Where is the most common place on a glacier to find longitudinal or splay crevasses? Near the terminus

What causes marginal crevasses? Shear between the valley wall and the glacier

In a deep crevasse, can scientists see the ice crystals of a glacier? Yes or No? Yes

Why do scientists go down into crevasses: to observe the layers of snow or to test their bravery? To observe the layers of snow

Transverse crevasses form across a glacier where the speed is (pick one) increasing or decreasing? Increasing

Radial crevasses form where a glacier turns a corner. True or False. True

In which zone of a glacier are transverse crevasses most common? Accumulation zone   

Brain Challenge!
What would you do if you ever fell into a crevasse while climbing on a glacier?

Danger and Safety

What is a tower of ice surrounded on all sides by crevasses? A serac

What causes a block of ice to break off and fall? Stress

What do snow bridges cover on a glacier? Crevasses

Does wind drifting cause mechanical hardening in the snow? Yes

Should you walk over a snow bridge? No

What do glacier travelers wear on their boots so they don’t slide on the ice? Steel spikes called crampons

Name two correct ways to travel on a glacier. Travel in a team, use ropes to tie team members together for safety, have an experienced glacier climber with you, and use proper equipment.

Name a piece of proper clothing to wear when traveling on glaciers. Boots, warm jacket, warm pants, and gloves

If they go fast enough, snow machines can cross a snowbridge safely. True or False? False   

Brain Challenge!
What one thing you would like to do on a glacier?

How do Glaciers Form?

The formation of a huge glacier begins with a single, small _____________? Snowflake

What types of summer temperatures need to occur for a glacier to form? Cooler temperatures so snow stays on the mountain

How does over-lying weight affect the snow? It makes snow grains beneath become coarser and larger

What is wetted snow that has survived one summer without being transformed to ice? Firn

How long does it take for firn to form? About one year

When does firn become glacial ice? When the interconnecting air passages between the grains are sealed off

What is the line that separates bare ice from snow at the end of the ablation season? The firn line

What is the difference between a perennial snow patch and a glacier? A glacier flows

What causes a glacier to move downhill? Gravity   

Brain Challenge!
What would Alaska look like if all of the glaciers melted?

How do Glaciers Move?

What causes the glacier to be in motion? Gravity

True or False: Glaciers slide on their beds and this enables them to move faster. True

True or False: Glaciers can’t flow down to sea level or carve fjords. False

What is the zone where a glacier gains snow and ice? The accumulation zone

What is the zone where a glacier loses ice through melting and calving? The ablation zone

What is the difference between the amount of material that a glacier accumulates and the amount it loses during ablation? Mass balance

If the glacier gains more than it loses, will the glacier have a positive or negative mass balance? A positive mass balance

True or False: The snout is another name for the terminus on a glacier. True

Name one type of moraine. Terminal, lateral, or medial   

Brain Challenge!
When climbing a glacier, if you could only bring one other thing with you besides warm clothes, boots, and a camera, what would you bring?


Do glaciers have cows? No

What are chunks of ice that break off of glaciers and fall into the water? Icebergs

List the three rules starting with the letter “L” that you should do at Child’s Glacier. Look, Listen and Leave

Do you move to higher or lower ground when you see a glacier calving? Higher ground

One of the rules is to “Look” at Child’s Glacier. What can you probably see in the woods? Large rocks

There are photographs taken of Child’s Glacier calving. How high did the wave reach? 12 feet

What town in Alaska is Child’s Glacier close to? Cordova

If an iceberg has lots of bubbles inside, what color might it be? White

What does a darkly-striped iceberg consist of? Frozen water and moraine debris   

Brain Challenge!
What would it be like if you were in a boat and a glacier started to calve? Do you think it would rock your boat if you were right next to the glacier?

Why is Glacier Ice Blue?

Glacier ice is so blue because the dense ice of a glacier absorbs/reflects (circle one) every other color of the spectrum except blue/yellow (circle one). Absorbs and blue

Glacial ice is different than regular ice. True or False? True

Are there rocks in glacial ice? Why or Why not? Yes, because glaciers move through rock and soil as they carve their way down slope

What could happen to your glass of water if you dumped glacial ice in it? Your glass could explode!

What is the stuff called that is either alive now or was alive in the past (it may be trapped in glacier ice)? Organic matter

True or False? Glacial ice is just like the water in your freezer. False

What would be in your glass of water if the glacial ice melted? dirt, gravel, and organic matter (living stuff)

Glaciers are just frozen compacted snow. True or False? False

The ice on a glacier has been there for a long time and has been compacted down. True or False? True   

Brain Challenge!
If all the glaciers in the world melted, what would happen? (Use your imagination!)

Why do Scientists Study Glaciers?

Name one thing you can find out by studying glaciers. How the atmosphere was a long time ago

As the ice compacts, is there more/less pore space? Less pore space

Glaciers sometimes make popping sounds. Why? Pressurized air escapes from the ice

What is another big word for “warm spells”? Interglacials

How long was each interglacial period? 10,000 years

18,000 years ago, there was an ice age. How much (%) of the world’s land surface was covered under thick ice? 30%

Dr. Craig Lingle likes to study glaciers. What kind of scientist is he? A glaciologist

What is the largest glacier in North America? Bering Glacier

What did James Roush and Dr. Craig Lingle use to look at glaciers from space? SAR or Synthetic Aperture Radar satellite imagery

Brain Challenge!
Do you think there will be an Ice Age or Global Warming in the next 100 years? (Don’t worry, there is no wrong answer!)

Glacier Power – Teacher Answer Key Exercises Answers

Exercises - Teacher's Guide

Where Have Glaciers Been?

Answers: Connect the Related Words

U-Shaped Valley
V-Shaped Valley
Grand Canyon

Glacier Anatomy

Answers: Matching

accumulation zone – B
ablation zone – D
tributary – A
moraine – C
terminus – E

Strange Glacial Phenomena

Answers: Crossword Puzzle

Down Across
1. Jokulhlaups 1. moulins
2. ice sizzle 2. fossils
3. ice worms

What are Crevasses?

Answers: Word Scramble – Types of Crevasses

  1. bergschrund
  2. radial
  3. transverse
  4. longitudinal

Glacier Danger and Safety

Answers : Dress Your Friend for His/Her Hiking Adventure

The right items are:

experienced traveler

How Do Glaciers Form?

Answers: Circle the Facts

1. Firn: A, C, D
2. Snowflake(s): A, C, D
3. Snow on a glacier: B, C

How Do Glaciers Move?

Answers: Connect the Words with Definitions


Answers: Circle the Calving Glacier

A, C, D

Why is Glacier Ice Blue?

Answers: Blue Ice

1. A, C

2. How is glacier ice different from the ice in your freezer?

It is compacted

Its structure is different

It’s not frozen water or snow

It contains dirt, gravel, and even organic matter

Its air bubbles are pressurized

Why Do Scientists Study Glaciers?

Answers: Why Study?

A, B, D, F

Answer: Why or when might the Earth undergo global warming?

If human influence causes the greenhouse effect, that may delay the natural planetary cycle of ice ages and cause a period of global warming.

Answers: Glacier Study: True or False

A. True
B. False
C. True
D. False
E. False

Answer: Why, for the first time, were scientists able to make regular repeated measurements of the surge of Bering Glacier by using SAR satellite imagery?

Because Synthetic Aperture Radar (SAR) can “see” through clouds and darkness which usually obscure the Alaskan coastal mountains.

Glacier Power – Credits


With special thanks to…
Dr. Frank Carsey, 1995-1997 Chief Scientist of the Alaska SAR Facility, for direction and encouragement, and
Fairbanks North Star Borough School District, officials and teachers for cooperation, direction, and participation in both formal and informal assessment and review, and
UAF School of Education, for beginning and completing Glacier Power with us.

Lead Author and Editor-in-Chief

Donna Sandberg, ASF Education Outreach Team Leader
Alaska Satellite Facility (ASF)

Artists, Writers, Research and Assistant Writing

Animations, Computer Art

Ben Barton, Computer Art Student
University of Alaska Fairbanks

Cartoons, Legend of Miner Ed, and Glacier Anatomy

Donna Redhead
Freelance Author/Illustrator
Fairbanks, Alaska

Glacier Illustrations

Amy Stubsjoen
Freelance Artist
Fairbanks, Alaska

Erica Herbert
Freelance Artist
Fairbanks, Alaska

Author of Lesson Plans

Claude (Chip) McMillan III, Professor
School of Education, UAF

Vocabulary Exercises

Amy Bethune
Student Assistant
ASF Science Division
Geophysical Institute, UAF

Glacier Detectives

Shelly Worley
Graduate Student Assistant
ASF Science Division
Geophysical Institute, UAF

Dave Sanches

Research Technician
ASF Operations Division
Geophysical Institute, UAF

Advisors and Contributors

Science Content Specialist

Dr. Craig Lingle, Glaciologist
ASF Science Division
Geophysical Institute, UAF

Science Advisors

Dr. William Harrison,
Prof. of Glaciology and Geophysics
Geophysical Institute, UAF

Dr. Keith Echelmeyer,
Prof. of Glaciology and Geophysics
Geophysical Institute, UAF

Dr. Nettie La Belle-Hamer, ASF Director
Alaska Satellite Facility
Geophysical Institute, UAF

Education Advisors

Dennis Schall, Professor
School of Education, UAF

Claude (Chip) McMillan III, Professor
School of Education, UAF

Neal Brown, Professor
Geophysical Institute, UAF

Tom George, ASF Planning Manager
Geophysical Institute, UAF

Other Advisors

Terry P. Dickey, Education Coordinator
University of Alaska Museum

Geophysical Institute/UAF Contributors

Kristina Ahlnas, Remote Sensing Specialist
ASF Science Division

Dennis R. Fatland, Ph.D. Candidate
Geophysical Institute, UAF

Rick Guritz, ASF Technology
ASF Science Division

Bob Huebert, Viz Systems Analyst
Alaska Region Supercomputing Center

Dr. Craig Lingle, Glaciologist
ASF Science Division

Matt Nolan, Ph.D. Candidate
Geophysical Institute, UAF

James Roush, Graduate Research Assistant

Mike Shindle, Graduate Student Assistant
ASF Science Division

Other Contributors

Austin Post Collection, USGS
Dr. Juerg Alean
Harvey Bowers
Dr. Sue A. Ferguson
Dr. Dorothy Hall
Dr. Michael Hambrey
Rod March, USGS
Dr. Bruce Molnia
Ron Rose
Gordin Q. Robin
Robert P. Sharp

Implementation Team

Implementation Team Leader

Allison Kipple, World Wide Web Designer
Alaska SAR Facility
Geophysical Institute, UAF

Implementation Team

Dan LaSota, Microcomputer Specialist
Geophysical Institute, UAF

Matt Barkdull, Microcomputer Specialist
Geophysical Institute, UAF

Jeff Beiderbeck, Microcomputer Specialist
Geophysical Institute, UAF

Video and Sound Engineering

Jim Desrochers, Video Engineer
Geophysical Institute, UAF

Rick Guritz, ASF Technology
Geophysical Institute, UAF

Joe Riley, TV Producer, Director, Sound Engineer
KUAC Public TV and Radio
University of Alaska Fairbanks

Clerical Support

Monica Court, Desktop Publisher
Geophysical Institute, UAF

Cheryl Stahl, Word Processor
Geophysical Institute, UAF

Cheryl Katje, ASF User Services Consultant
Geophysical Institute, UAF

Glacier Power Lessons based on the Glacier Power software product

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Glacier Power – What is Glacial Calving?


Cows have calves, glaciers calve icebergs, which are chunks of ice that break off glaciers and fall into water.

Calving is when chunks of ice break off at the terminus, or end, of a glacier. Ice breaks because the forward motion of a glacier makes the terminus unstable. We call these resulting chunks of ice “icebergs.”

Icebergs can be BIG. At least one has been seen that’s as big as the state of Rhode Island!

Icebergs come in various colors. An iceberg’s color can tell us a lot about what it’s made of or where it came from.
For example:

  • White icebergs have lots of bubbles inside
  • Blue icebergs are very dense
  • Greenish-black icebergs may have calved off the bottom of a glacier
  • Darkly-striped icebergs carry moraine debris from the glacier

Icebergs in Glacier Bay

A large iceberg floating in Glacier Bay, Alaska. Photo by James Roush.
Many icebergs in Glacier Bay, Alaska. Photo by Kristina Ahlnas.

Did you know that 90 percent of an iceberg is underneath the water’s surface? Icebergs can be dangerous to ships. In April of 1912 a ship called the Titanic collided with a huge iceberg. The iceberg ripped a 90 meter hole (approximately the size of a football field) in the ship, causing it to sink. More than 1,500 people died in the accident.

Icebergs calved from tidewater glaciers are still a danger today. An iceberg over 80 kilometers long and 40 kilometers wide recently broke off from the Larsen Sea Shelf in Antarctica. Many ships sail in this area, so satellites monitor the area for any icebergs that may be in a ship’s path.

The Titanic

In the spring of 1912, the luxury passenger liner Titanic, advertised as the first iceberg-proof ship, set off on its maiden voyage, full of passengers.

On April 12 seven messages were dispatched to the Titanic, warning the ship’s crew of a huge iceberg in their path. The Titanic tried to maneuver around the iceberg, but couldn’t escape the gigantic chunk of ice.

All the passengers on the ship were alerted, and many scrambled for the lifeboats. However, there was such a panic that many boats were lowered into the sea only half full! Some boats weren’t even used at all.

Many people didn’t realize the Titanic was sinking. Some passengers were sleeping, and the band even kept playing until the boat sank. In all, 1,503 people died in the accident.

Never underestimate the power of an iceberg. If you’re ever on a boat in iceberg territory, always have a plan of escape. Be a good shipmate!

Child's Glacier Wave Warning

Seeing is believing! Look at the large rocks in the woods and picture how they arrived there.

The thunder of falling ice is a warning signal from Child’s Glacier. Heed this warning thunder — it signals an approaching glacial wave that can rise up to 20 feet when it breaks over the beach, sweeping boulders before it.

Be prepared to move to higher ground!



moraine debris

Review Questions
(some of the answers may come from the vocabulary list)

  1. Do glaciers have cows? 
  2. What are chunks of ice that break off glaciers and fall into the water?
  3. What are the three rules starting with the letter “L” that you need to do at Child’s Glacier?  
  4. Do you move to higher or lower ground when you see a glacier calving?
  5. One of the rules is to “Look” at Child’s Glacier. What can you probably see in the woods?  
  6. There are photographs taken of Child’s Glacier calving. How high did the wave reach?
  7. What town in Alaska is Child’s Glacier close to?
  8. If an iceberg had lots of bubbles inside, what color might it be?
  9. What does a darkly-striped iceberg consist of?

Brain Challenge!
What would it be like if you were in a boat and a glacier started to calve? Do you think it would rock your boat if you were right next to the glacier?

Exercise: Circle the Calving Glacier
Circle the things a calving glacier could do.

A. Cause chunks of ice to fall into the water
B. Knock down the Empire State Building
C. Knock down a person
D. Cause a glacial wave to rise over 20 feet when it breaks over the beach
E. “Moo” like a cow

Project: Think About It

Look for ragged ice debris on top of a normally smooth glacier surface. This is evidence of recent avalanching. Now think, would there be any connection between avalanche activity and the time of day? How about with time of year?

(Courtesy of Glaciers of North America, By S. Ferguson)

A typical side-looking radar pointing perpendicular to the flight direction.       Credit: NASA.

What is SAR?

SAR: The Power Tool of Remote Sensing

Synthetic aperture radar (SAR), used to create the majority of the imagery available in the ASF archive, is one of the power tools of remote sensing. Synthetic aperture radar (SAR) bounces a microwave radar signal off the Earth’s surface to detect physical properties.

Through Snow and Rain and Dark of Night
Unlike optical technology, synthetic aperture radar (SAR) can “see” through darkness, clouds, and rain, detecting changes in habitat, levels of water and moisture, effects of natural or human disturbance, and changes in the Earth’s surface after events such as earthquakes or sinkhole openings.

Synthetic aperture radar (SAR) has been used in a wide range of applications, from studying Antarctic icebergs, to tracking the paths of oil spills into sensitive marshes, to mapping the wetlands of Alaska.

SAR Applications

Discover examples of how researchers around the world are using SAR

ASF welcomes suggestions for examples to consider adding to this graphic. Email with “Submission-SAR Uses” on the subject line.

Basic SAR Concepts and Terminology

Also see: Synthetic Aperture Radar (SAR) and InSAR Guides.

How Does it Work?
A synthetic aperture radar (SAR) is an active sensor that first transmits microwave signals and then receives back the signals that are returned, or backscattered, from the Earth’s surface.

Flight and Directional Terminology
The instrument measures distances between the sensor and the point on the Earth’s surface where the signal is backscattered. This distance is slant range (see illustration), which can be projected on the ground representing the ground range. The flight direction is also referred to as along-track or azimuth direction, and the direction perpendicular to the flight path is the across-track or range direction. The angle between the direction the antenna is pointing and the nadir is the look angle. The angle between the radar beam center and the normal to the local topography is the incidence angle. Both angles are sometimes used synonymously, which is only valid if the InSAR geometry is simplified neglecting the Earth’s curvature and the local topography. Because the look angle of the sensor significantly affects the behavior of backscatter, it is one of the main parameters determining the viewing geometry and the incidence angle of the backscattered signal. Depending on the characteristics of the illuminated terrain, areas of layover and shadow may occur in the imagery.

Wavelength and Effects
The wavelength of the sensor determines the penetration depth of the transmitted signal into the vegetation layer of the terrain surface. The longer the wavelength, the deeper the penetration can be, particularly in forests.

  • The energy of an X-band sensor is mainly returned at the top layer of the canopies
  • Most of the L-band signal penetrates through the upper vegetation layer and is returned at the ground surface.
  • The backscatter behavior of C-band is less predictable. Due to volume scattering effects, the layer of backscattering is less determined and does not correspond directly to a terrain surface — neither the vegetation surface nor the ground surface.

A typical side-looking radar pointing perpendicular to the flight direction. Credit: NASA.

Resolution and Speckle
The spatial resolution of the radar sensor defines the minimum separation between the measurements the sensor is able to discriminate and determines the amount of speckle introduced into the system. Speckle is a scattering phenomenon that arises because the spatial resolution of the sensor is not sufficient to resolve individual scatterers. Speckle can be reproduced if the acquisition conditions are identical, while noise is random in nature. Speckle is removed by multi-looking. The higher the spatial resolution of the sensor, the more objects on the ground can be discriminated. The term spatial resolution is often confused with the pixel size, which is the spacing of the pixels in the azimuth and ground range direction after processing the data.

SAR Frequently Asked Questions

Unlike the aperture in a camera, which opens to let in light, radar aperture is another term for the antenna on the spacecraft or aircraft. The radar antenna first transmits electromagnetic energy toward Earth and then receives the returning energy after it reflects off of objects on the planet. In the NASA image below, the radar antenna is the rectangle at the Earth end of the 1978 Seasat satellite. The data collected by the radar antenna are then transmitted to another kind of antenna on Earth — such as the antennas of the ASF Satellite Tracking Ground Station — so they can be stored and processed.

In general, the larger the antenna, the more unique information scientists can obtain about an object — and the more information, the better the image resolution. However, antennas in space are not large. So scientists use the spacecraft’s motion, along with advanced signal-processing techniques, to simulate a larger antenna.

Synthetic aperture radar (SAR) interferometry (InSAR) detects motion or elevation by comparing radar signals from two or more images of the same scene. The images are taken at different times from the same vantage point in space. SAR interferometry is often used to detect surface changes (for use in seismology, for example) or to generate digital elevation maps. The InSAR image below shows deformation on Okmok, a volcano in the Aleutian Islands. Image courtesy of Zhong Lu, © ESA 2008.

Image courtesy of Zhong Lu, © ESA 2008.

Because the radar wavelength is longer than particles in a cloud, such as droplets, the signal traveling through a cloud is mostly unaffected by any refraction at the boundaries of the different media. 

In microwave remote sensing, scientists measure the time and magnitude of the signal backscattered from the ground to the radar antenna. The magnitude of the signal defines the brightness of a given pixel in the image. The resulting image has a grayscale. Scientists sometimes colorize SAR images to highlight certain data or features.

The interpretation of synthetic aperture radar (SAR) images is not straightforward. The reasons include the non-intuitive, side-looking geometry. Here are some general rules of thumb:

  • Regions of calm water and other smooth surfaces appear black (the incident radar reflects away from the spacecraft).
  • Rough surfaces appear brighter, as they reflect the radar in all directions, and more of the energy is scattered back to the antenna. Rough surface backscatter even more brightly when it is wet.
  • Any slopes lead to geometric distortions. Steeper angles lead to more extreme layover, in which the signals from the tops of mountains or other tall objects “lay over” on top of other signals, effectively creating foreshortening. Mountaintops always appear to tip towards the sensor. 
  • Layover is highlighted by bright pixel values. The various combinations of the polarization for the transmitted and received signals have a large impact on the backscattering of the signal. The right choice of polarization can help emphasize particular topographic features.
  • In urban areas, it is at times challenging to determine the orbit direction. All buildings that are perfectly perpendicularly aligned to the flight direction show very bright returns.
  • Surface variations near the size of the radar’s wavelength cause strong backscattering. If the wavelength is a few centimeters long, dirt clods and leaves might backscatter brightly.
  • A longer wavelength would be more likely to scatter off boulders than dirt clods, or tree trunks rather than leaves.
  • Wind-roughened water can backscatter brightly when the resulting waves are close in size to the incident radar’s wavelength.
  • Hills and other large-scale surface variations tend to appear bright on one side and dim on the other. (The side that appears bright was facing the SAR.)
  • Due to the reflectivity and angular structure of buildings, bridges, and other human-made objects, these targets tend to behave as corner reflectors and show up as bright spots in a SAR image. A particularly strong response — for example, from a corner reflector or ASF’s receiving antenna — can look like a bright cross in a processed SAR image.

In ASF’s full-resolution synthetic aperture radar (SAR) images, objects can be distinguished as small as about 30 meters wide. Some of the smaller items scientists have spotted have been ships and their wakes. When the synthetic aperture radar (SAR) happens to be aligned at a certain angle, long thin objects such as roads or even the Alaskan oil pipeline can also be seen. Objects can be much smaller than the resolution and still be observable such as bright point objects. They only need to be perfectly aligned with the look direction of the synthetic aperture radar (SAR) sensor.

As the spacecraft moves along in its orbit, the radar antenna transmits pulses very rapidly in order to obtain many backscattered radar responses from a particular object. The synthetic aperture radar (SAR) processor could use all of these responses to obtain the object’s radar cross-section (how brightly the object backscattered the incoming radar), but the result often contains quite a bit of speckle. Generally considered to be noise, speckle can be caused by an object that is a very strong reflector at a particular alignment between itself and the spacecraft, or by the combined effect of various responses all within one grid cell. To reduce speckle, the data are sometimes processed in sections that are later combined — called looks. The more looks used to process an image, the less speckle. However, resolution is reduced, and information is lost in this process. Several research groups are developing/improving algorithms to reduce speckle while saving as much accurate information as possible.

Noise is defined as random or regular interfering effects that degrade the data’s information-bearing quality. Speckle is a scattering phenomenon that arises because the resolution of the sensor is not sufficient to resolve individual scatterers. Physically speaking, speckle is not noise, as the same imaging configuration leads to the identical speckle pattern. Speckle is removed by multi-looking. See “What is a ‘look'” above.

After the radar sends its microwave signal toward a target, the target reflects part of the signal back to the radar antenna. That reflection is called backscatter. Various properties of the target affect how much it backscatters the signal.

  • Sentinel
  • PALSAR (Faraday rotation can be a factor.)
  • RADARSAT-1 (The most suitable RADARSAT-1 data for InSAR was acquired during and after the Modified Antarctic Mapping Mission in the fall of 2000.)
  • ERS-1 
  • ERS-2
  • JERS-1

IfSAR is another term for InSAR. InSAR is the more common term, particularly for satellite-borne sensors. IfSAR has been used more by the military and/or for airborne sensors.

Layover is a type of distortion in a synthetic aperture radar (SAR) image. The radar is pointed to the side (side-looking) for imaging. Radar signals that return to the spacecraft from a mountaintop arrive earlier or at the same time as the signal from the foot of the mountain, seeming to indicate that the mountaintop and the foot of the mountain are in nearly the same place, or the mountaintop may also appear “before” the foot. In a synthetic aperture radar (SAR) image with layover, the mountains look as if they have “fallen over” towards the sensor.

Where features are shifted from their actual location, the resulting geolocations are incorrect. This effect can be removed by the technique of terrain correction (also see “What is terrain correction?” below).

As with shadows from sunlight, shadows in synthetic aperture radar (SAR) images appear behind vertical objects. Mountains may appear to have black shadows behind them, depending on the steepness of the slope. The shadows appear black because no radar signals return from there.

Radiometric correction involves removing the misleading influence of topography on backscatter values. For example, the correction eliminates bright backscatter from a steep slope, leaving only the backscatter that reveals surface characteristics such as vegetation and soil moisture.

ASF DAAC 2014; Includes Material
© JAXA/METI 2008.

Terrain correction is the process of correcting geometric distortions that lead to geolocation errors. The distortions are induced by side-looking (rather than straight-down looking or nadir) imaging, and compounded by rugged terrain. Terrain correction moves image pixels into the proper spatial relationship with each other. Mountains that look as if they have “fallen over” towards the sensor are corrected in their shape and geolocation.

ASF DAAC 2014; Includes
Material © JAXA/METI 2008.

The RTC products ASF generated for the ALOS PALSAR mission include a copy of the DEM used to process the RTC. Note that this DEM is not generated from current SAR data, but is a geoid-corrected version of the best publicly-available DEM for the area covered by the RTC product. Sources vary depending on the area, but generally use either the SRTM (Shuttle Radar Topography Mission) or 3DEP (3D Elevation Program, including seamless products formerly called NED) DEMs.

For more information on the DEMs included in the RTC product, visit the ALOS PALSAR RTC page.

Most digital elevation models (DEM) are geoid-based and require a correction before they can be used for terrain correction. The DEM included in an ASF radiometrically terrain corrected (RTC) product file was converted from source DEM orthometric height to ellipsoid height using the ASF MapReady geoid_adjust tool. This tool applies a geoid correction so that the resulting DEM relates to the ellipsoid. 

An online tool is available that computes the height of the geoid above the WGS84 ellipsoid, and will show the amount of correction that was applied to the source DEM used in creating an RTC product.

Orthorectification corrects geometric distortions in imagery, just as terrain correction does (see “What is terrain correction?” above). The term ‘orthorectification’ is used more often in association with aerial and optical imagery. Terrain correction generally refers to synthetic aperture radar (SAR) imagery.

A georeferenced image has the location of the four corners of the image and the information needed to put the data into a projection. Geocoded data is already projected. Each point in the image is associated with a geographic coordinate.

C-band (~5.3 GHz)
Applies to ERS-1, ERS-2, RADARSAT-1, Sentinel-1
– Variety of applications, but particularly sea ice, ocean winds, glaciers.

L-band (~1.2 GHz)
Applies to PALSAR, UAVSAR, AIRSAR, JERS-1, Seasat
– Provides vegetation penetration
– Applications included sea ice, tropical forest mapping, soil moisture
– Subject to ionospheric effects

P-band (~0.4 GHz)
Applies to some products of UAVSAR
– Greatest penetration depth through vegetation and into soil
– Ideal for soil moisture and biomass
– Difficult to operate from space due to ionospheric effects

SAR Reference Guides

Fundamentals of Remote Sensing
Natural Resources Canada – Online comprehensive guide: concepts, image interpretation, applications. Clear writing, helpful illustrations.

Earth Observation College
Highly Recommended. Online In-depth comprehensive lessons and tutorials from concepts to applications.

ASF SAR User’s Guide
In 1993, when ASF was still named the Alaska SAR Facility, ASF produced a definitive synthetic aperture radar (SAR) User’s Guide that scientists continue to use today. Dense text, equations.

NOAA SAR Marine User’s Manual
The 2004 NOAA synthetic aperture radar (SAR) Marine User’s Manual, which starts with principles of synthetic aperture radar, is 464 pages long, with over 240 images and figures.

Radar and SAR Glossary
Comprehensive European Space Agency (ESA) list of synthetic aperture radar (SAR) terms and concepts, with figures.

The ASAR User Guide
ESA’s guide to the Advanced Synthetic Aperture Radar (ASAR) instrument on the ENVISAT mission includes a great deal of widely relevant content, including a Scientific Background section.

InSAR Reference Guides

GMTSAR: An InSAR Processing System Based on Generic Mapping Tools
2011 guide for an open-source InSAR processing system designed for users familiar with Generic Mapping Tools; the GMTSAR system is available here.

Delft Object-oriented Radar Interferometric Software User’s manual and technical documentation
2008 technical documentation and information for Delft open-source software, one of many options for processing InSAR.