U. S. Department of Commerce • National Oceanic and Atmospheric Administration

NOAA Ocean Service • Office of Response and Restoration

National Environmental Satellite, Data, and Information Service • National Ice Center

Observers Guide to
For additional information contact:

NOAA Emergency Response Division

7600 Sand Point Way NE

Seattle, WA 98115

response.restoration.noaa.gov

U. S. Department of Commerce

National Oceanic and Atmospheric Administration

NOAA Ocean Service • Office of Response and Restoration

National Environmental Satellite, Data, and Information Service • National Ice Center

This Observers’ Guide to Sea Ice was prepared by Dr. Orson P. Smith, University of 

Alaska Anchorage, School of Engineering for the National Oceanic and Atmospheric 

Administration with thanks to Kathleen Cole, Anchorage Forecast Office Ice Forecaster. 

It is a product of the Cook Inlet and Prince William Sound Navigation Safety and Efficiency project. To order additional copies of this document, fax your request to (206) 

526-6329 or e-mail orr.library@noaa.gov.
1

Table of Contents

 Introduction 2

 Completing an Observation Report 4

 Sea Ice Observation Report Form 5

 Glacier Ice Observation Report Form 6

 Concentration 7

 Stages of Development 11

 Sea Ice Forms 16

 Glacier Ice Forms 23

O B S E R V E R S ’ G U I D E T O S E A I C E
2

Introduction

 This booklet is intended for use by volunteers to report aerial, shipboard, or shoreline observations of ice conditions at sea to authorities such 

as the National Weather Service, National Ice Center, U.S. Coast Guard, pilots’ 

associations, port authorities, or other maritime interests. Emphasis here is 

on conditions of concern to mariners with regard to safe passage of ships. 

Scientific observers may wish to note additional details following guidance of 

the World Meteorological Organization.

 Sea ice consists of frozen sea water. River (freshwater) ice is often 

indistinguishable when mixed with sea ice, especially with snow cover. 

Glacier ice, broken away from a tidewater glacier, is usually more irregular 

than river ice or frozen sea water. Tidal estuaries may also have thick, irregular 

pieces of sediment-laden beach ice, which have been grounded on tidelands, 

repeatedly submerged, and floated free by spring tides. 

 Sea ice is observed in terms of three basic parameters: concentration, stage of development, and form. Concentration refers to the fraction of 

the sea surface covered by ice, reported in tenths by international convention. Stage of development refers to age and structural characteristics of 

the ice that may be inferred from specific visible features and knowledge 

of regional conditions prior to the observation. Stage may be more directly 

observed from shipboard when ice breaking reveals the prevailing thickness 

of the ice. Stage classification does not apply to glacier ice. Form refers to 

the horizontal shape and dimensions of pieces of ice. The form of glacier ice 

is also reported in terms of freeboard, or maximum height above the sea 

surface. 

 Observers should report sea ice conditions with reference to the 

date, time, and geographical position of the observation and the particular 

perspective, or field of view. The field of view from an airplane is much different from the field of view from the bridge of a ship or from the shoreline. 

A reporting form for sea ice and a separate form for glacier ice are provided 

to aid observers in noting this essential information.
3

U.S. Sea Ice Reporting Agencies

National Weather Service

Alaska Region, Forecast Office

6930 Sand Lake Road

Anchorage, Alaska 99502

Ice Desk: (907) 266-5138

Marine Desk (24 hours): (907) 266-5106

Fax: (907) 266-5188

e-mail: nws.ar.ice@noaa.gov

http://pafc.arh.noaa.gov/ice.php

National Ice Center

Federal Office Building #4, Room 2301

4231 Suitland Road 

Washington, D.C. 20395 

Telephone: (301) 394-3100

Fax: (301) 394-3200

e-mail: liaison@natice.noaa.gov 

http://www.natice.noaa.gov
4

Completing an Observation Report

1. Use a dry-erase marker to fill out either the sea ice or the glacier ice 

observation report form, or to make a permanent record on a copy of 

the form.

2. Note the date, time, and time zone of the observation, e.g., “Alaska 

Standard,” “Alaska Daylight Savings (summer) Time,” or “Greenwich Mean 

Time” (GMT).

3. Note the latitude and longitude of the observation by reference to a 

chart or Global Positioning System (GPS). GPS also give accurate time of 

day.

4. Describe your position in words, as you would over the radio or 

telephone, noting the water body and nearby charted geographical 

features, e.g., “central Cook Inlet, 3 nautical miles east of Middle Ground 

Shoal.”

5. Describe your perspective in terms of altitude above the water, the compass direction toward which you are looking, and the most recognizable 

geographical features directly in view, e.g., “10 m (30 ft) above the water 

(bridge-level), looking west toward the center of Trading Bay.”

6. Check the choices of concentration that apply to the conditions you 

observe, noting the average and the range of concentrations in view.

7. Sea ice: Circle the stages of development you judge to be in view. Direct 

knowledge of recent local weather and prior nearby ice observations, 

as well as the appearance of the ice, may be applied to judge stage of 

development.

8. Use the length, or beam, of the ship or recognizable objects in view (like 

ships, docks, or oil platforms) to estimate the size of ice pieces observed 

and note the prevailing form and the range of ice forms visible.

9. Glacier ice: Note the form in terms of estimated maximum freeboard 

and waterline length. Also note the above-water shape.

10. Note any other observed features that might help mariners following 

you across the water body or that would help ice reporters match your 

observation with others.

11. Transmit your report as soon as possible to the National Weather 

Service, National Ice Center, or other marine service organization.
5

Sea Ice Observation Report Form

Date: Time: Time zone:

Latitude: Longitude:

Description of position:

Altitude of observer:

Looking toward (compass bearing):

Charted landmarks in view:

Concentration:

(see pages 7-10)

0 1 2 3 4 5 6 7 8 9 10

Stage of Development:

(see pages 11-15)

New Ni YNGYNGWFL FM FT Old

Form:

(see pages 16-22)

New Brash Belts Strips Pancakes

Cakes Small floes Medium 

floes

Big floes Vast floes Giant floes

Other description:
6

Glacier Ice Observation Report Form

Date: Time: Time zone:

Latitude: Longitude:

Description of position:

Altitude of observer:

Looking toward (compass bearing):

Charted landmarks in view:

Concentration:

(see page 7)

0 1 2 3 4 5 6 7 8 9 10

Glacier ice forms:

(see pages 23-27)

Growlers Bergy bits Small berg

Medium berg Large berg Very large berg

Above-water shape:

(see page 23) Blocky Tabular Domed

Pinnacled Drydocked Wedged non-tabular

Other description:
7

Concentration

Concentration

6 tenths “open drift”

less than 1 tenth “open water” 2 - 3 tenths “very open drift” 4 tenths “open drift”

5 tenths “open drift” 7 - 8 tenths “close pack”

9 tenths “very close pack” 10 tenths “compact”
8

Concentration

3 - 4 tenths “open to very open drift”

Photo provided by USCG MSD Kenai, Alaska

1 - 2 tenths “very open drift”

Photo provided by ConocoPhillips

Concentration
9

Concentration

5 - 6 tenths “open drift”

7 - 8 tenths “close pack”

Concentration
10

Concentration

10 tenths “compact”

9 tenths “very close pack”

Concentration
11

Stages of Development

New (N): 

Ice of the following initial stages of ice formation

Frazil: Separate fine needles or plates suspended in the water

Grease: A thin skin of frazil crystals coagulated on the sea surface having a dark, greasy appearance (Also called ice fat, lard ice)

Slush: Snow mixed with water in a viscous surface layer

Shuga: An accumulation of spongy white lumps

Nilas (Ni): 

A thin, elastic crust of ice, less than 10 cm (4 in) thick, easily bending on 

waves, often with a striped or chevron appearance

Young (YN): 

Ice 10 - 30 cm (4 - 12 in) thick of the following sub-stages

Gray (G): Young ice 10 - 15 cm (4 - 6 in) thick, less elastic than nilas, that 

breaks on swell and rafts (one layer over another) under pressure

Gray-white (GW): Young ice 15 - 30 cm (6 - 12 in) thick, that buckles to 

form ridges on its edges from pressure or collisions

First-year thin (FL): 

Sea ice that, in uniform level areas without ridges or other deformations, is 30 

- 70 cm (12 - 28 in) thick

First-year medium (FM): 

Sea ice 70 - 120 cm (28 - 48 in) thick

First-year thick (FT): 

Sea ice over 1.2 m (4 ft) thick

Old or multi-year (MY): 

Sea ice of any thickness that has survived at least one melting season, characterized by undulating, weathered ridges and a well-defined melt water 

drainage pattern

Stages of Development
12

New Ice (N)

Slush: Snow mixed with water

Shuga: An accumulation of spongy white lumps

Stages of Development

New Ice (N)

Frazil: Fine needles or plates suspended in the water 

Grease: A thin skin of frazil crystals coagulated on the sea surface having a dark, greasy 

appearance (Also called ice fat, lard ice)

Stages of Development
13

Stages of Development

Young (YN) Level ice 10 - 30 cm (4 - 12 in) thick, of the following sub-stages: 

Gray (G): Young ice 10 - 15 cm (4 - 6 in) thick, less elastic than nilas, that breaks on swell 

and rafts under pressure 

Gray-white (GW): Young ice 15 - 30 cm (6 - 12 in) thick, which buckles to form ridges on 

its edges from pressure or collisions

Nilas (Ni) A thin, elastic crust of ice, less than 10 cm (4 in) thick, easily bending on waves, often with a 

striped or chevron appearance

Stages of Development
14

Stages of Development

First-year thin (FL) Sea ice 30 - 70 cm (12 - 28 in) thick

First-year medium (FM) Sea ice 70 - 120 cm (28 - 48 in) thick

Stages of Development
15

Stages of Development

First-year thick (FT) Sea ice over 1.2 m (4 ft) thick 

(Photo provided by Jerry Galt)

Old or Multi-year (MY) Sea ice of any thickness that has survived at least 

one melting season, characterized by undulating, weathered ridges and a 

well-defined melt water drainage pattern

Stages of Development
16

Sea Ice Forms

New: Small, thin, newly formed, dinner plate-sized pieces

Brash: Broken pieces less than 2 m (6 ft) across

Pancake: Rounded floes 30 cm - 3 m (1 - 10 ft) across with ridged rims

Ice Cake: Level piece 3 - 20 m (6 - 65 ft) across

Small Floe: Level piece 20 - 100 m (65 - 328 ft) across

Medium Floe: Level, continuous piece 100 -500 m (328 - 1640 ft) across

Big Floe: Level, continuous piece 500 m - 2 km (1/3 - 1 mi) across

Vast Floe: Level, continuous piece 2 - 10 km (1 - 6 mi) across

Giant Floe: Level, continuous piece greater than 10 km (6 mi) across

Belt: A linear accumulation of sea ice from 1 km to over 100 km (0.6 - 60 mi) 

wide

Strip:A linear accumulation of sea ice less than 1 km (0.6 mi) wide

Beach Ice: Irregular, sediment-laden blocks that are grounded on tidelands, repeatedly submerged, and floated free, generally, by spring tides

Fast Ice: Ice formed and remaining attached to shore

Sea Ice Forms
17

32 ft

16 ft

200 ft

6 ft

300 ft

710 ft

Sea Ice Forms

Sea Ice Forms

Useful Size-Reference Objects

Brash: less than 2 m (6 ft) across

Growler: less than 5 m (16 ft)

Pancake: 30 cm - 3 m (1 - 10 ft)

Bergy Bit: 5 - 15 m (16 - 50 ft) 

Ice Cake: 3 - 20 m (6 - 65 ft) across

Small Berg: 15 - 60 m (50 - 200 ft)

Small Floe: 20 - 100 m (65 - 328 ft)

Medium Berg: 61 - 122 m (201 - 400 ft)

Large Berg: 123 - 213 m (401 - 670 ft)

Medium Floe: 

100 - 500 m (328 - 1640 ft)

Very Large Berg: 

greater than 213 m (670 ft)

Big Floe: 

500 m - 2 km (1/3 - 1 mi) 
18

Sea Ice Forms

Pancake Ice Circular floes 30 cm - 3 m (1 - 10 ft) across and up to 10 cm (4 in.) thick with raised rims 

Brash Broken pieces less than 2 m (6 ft) across 

(Homer, Alaska)

Sea Ice Forms
19

Sea Ice Forms

Small Floe Continuous level piece 20 - 100 m (65 

- 328 ft) across (altitude 500 ft, Anchorage, Alaska)

Ice Cake Relatively level piece less than 20 m (65 ft) across (altitude 500 feet, Cook Inlet, Alaska)

Sea Ice Forms
20

Sea Ice Forms

Medium Floe Continuous flat piece 100 - 500 m (328 - 1640 ft) across (altitude 800 ft, Cook Inlet, 

Alaska)

Big Floe Continuous flat piece 500 m - 2 km (1/3 - 1 mi) across (altitude 500 - 600 ft, Arctic Ocean)

Sea Ice Forms
21

Sea Ice Forms

Belt A linear accumulation of sea ice from 1 km to over 100 km (0.6 - 60 mi) wide

Strip A linear accumulation of sea ice less than 1 km (0.6 mi) wide (altitude 800 ft, Cook Inlet, Alaska)

Sea Ice Forms
22

Sea Ice Forms

Fast Ice Ice formed and remaining attached to shore (upper Cook Inlet, Alaska) 

Beach Ice Thick, irregular, sediment-laden pieces, which have been grounded on tidelands, repeatedly 

submerged, and floated free, generally, by spring tides (grounded beach ice, Cook Inlet, Alaska)

Sea Ice Forms
23

Form Freeboard (height above water) Length

Growler less than 1 m (3 ft) less than 5 m (16 ft)

Bergy Bit 1 - 5 m (3 - 16 ft) 5 - 15 m (16 - 50 ft)

Small Berg 5 - 15 m (16 - 50 ft) 15 - 60 m (50 - 200 ft)

Medium Berg 16 - 45 m (51 - 150 ft) 61 - 122 m (201 - 400 ft)

Large Berg 46 - 75 m (151 - 240 ft) 123 - 213 m (401 - 670 ft)

Very Large Berg greater than 75 m (240 ft) greater than 213 m (670 ft)

Blocky: Generally straight-sided polygonal shape

Tabular: Flat top with near-vertical sides

Domed: Rounded appearance above the waterline

Non-tabular or irregular: No regular geometric shape apparent

Wedged: Triangular wedge shape

Pinnacled: One or more prominent central spires

Drydocked: Melted out in center to form a U-shaped slot

Black Ice: Dark, sediment-laden ice from glacier medial moraines or 

associated frozen glacier surface ponds of turbid runoff water

Glacier Ice Forms

Glacier Ice Forms
24

Growler Glacier fragment less than 1 m (3 ft) freeboard and less than 5 m (16 ft) across 

(Prince William Sound, Alaska)

Glacier Ice Forms

Glacier Ice Forms
25

Bergy Bit Glacier fragment 1 - 5 m (3 - 16 ft) freeboard or 5 - 15 m (16 - 50 ft) across 

(Prince William Sound, Alaska, photo provided by Stan Stephens)

Glacier Ice Forms

Bergy bit (irregular)

Black ice bergy bit (blocky)

Glacier Ice Forms
26

Small Berg Glacier fragment 5 - 15 m (16 - 50 ft) freeboard or 15 - 60 m (50 - 200 ft) across 

(Prince William Sound, Alaska)

Glacier Ice Forms

Medium Berg Glacier fragment 16 - 45 m (51 - 150 ft) freeboard or 61 - 122 m (201 - 400 ft) 

across (North Atlantic, photo provided by USCG International Ice Patrol)

Glacier Ice Forms
27

Glacier Ice Forms

Large Berg Glacier fragment 46 -75 m (151 - 240 ft) freeboard or 123 - 213 m (401 - 670 ft) across 

(Photo provided by Jerry Galt)

Very Large Berg Glacier fragment over 75 m (240 ft) 

freeboard or over 213 m (670 ft) across (Antarctica, photo 

provided by USCG International Ice Patrol)

Glacier Ice Forms
Carlos M. Gutierrez

Secretary, U.S. Department of Commerce

Vice Admiral Conrad C. Lautenbacher, Jr., USN (Ret.)

Under Secretary for Oceans and Atmosphere and 

NOAA Administrator

John H. Dunnigan

Assistant Administrator,

Ocean Services and Coastal Zone Management

NOAA Ocean Service

U.S. Department of Commerce

National Oceanic and Atmospheric Administration • NOAA Ocean Service

August 2007