The slides, written out
AI-drawn illustration. Simplified to explain the idea, not to scale.
Slide 1 · UNDERSTAND THE AIR
UNDERSTAND THE AIR. PREDICT THE WEATHER.
AIR MASSES & MARINE WEATHER
The Key to Safe Navigation & Smart Decision Making
THE RULES BEHIND EVERY FORECAST
Slide 2 · CLASSIFICATION
CLASSIFICATION
OF AIR MASSES
Where an air mass forms determines
its starting character.
An air mass is a large body of air with relatively
uniform temperatuer and humidity characteristics.
1 SOURCE REGION
ARCTIC /
ANTARCTIC AIR (A)
Very cold air
originating in
Arctic or Antarctic
regions.
POLAR AIR (P)
Air originating
over higher
latitudes.
Generally colder
than tropical air
but not as cold as
Arctic air.
TROPICAL AIR (T)
Air originating
over lower
latitudes.
Generally warm
or hot.
EQUATORIAL AIR (E)
A term used in
some classification
systems for air
originating in
equatorial regions.
2 SURFACE CHARACTER
MARITIME (m)
Forms over water
→ generally moist
CONTINENTAL (c)
Forms over land
→ generally dry
EXAMPLES
mP
Maritime Polar
cP
Continental Polar
mT
Maritime Tropical
cT
Continental Tropical
Source:
NOAA / NWS | WMO Terminology
Slide 3 · 02 THE CHARACTER OF AN AIR MASS
02 THE CHARACTER OF AN AIR MASS
WHAT DETERMINES
ITS PROPERTIES?
Source gives it a starting character.
The journey modifies it.
1 TEMPERATURE
Initially reflects the source region and is
subsequently modified as the air mass moves
over surfaces with different temperatures.
2 HUMIDITY
• Maritime → generally moist
• Continental → generally dry
But the air mass can be modified as it travels.
Example:
Dry continental air crossing a warm sea
can gain moisture and become progressively
more humid.
3 STABILITY
The temperature difference between the moving
air and the underlying surface strongly affects stability.
COLD AIR
Moving over
WARM surface
UNSTABLE AIR
Convection
Cumuliform cloud
Showers / possible
thunderstorms
WARM AIR
Moving over
COLD surface
STABLE AIR
Stratiform cloud
Fog / low cloud
Limited vertical
development
KEY TAKEAWAY
The air mass you receive at the ship may be
considerably different from the air mass at its source.
Slide 4 · 03 THREE KEY FACTORS
03 THREE KEY FACTORS
HOW AN AIR
MASS CHANGES
Origin determines the starting character;
path and time determine how the air
mass is modified.
01 ORIGIN
Determines the initial
temperature and moisture
characteristics.
Source region =
Starting character
KEY POINT
The source region
sets the baseline
temperature and
humidity.
02 PATH
As the air mass moves away from
its source, it interacts with the
surfaces it crosses.
Warm surface → lower layers
warm + moisture can increase
Cold surface → lower layers
cool + stability can increase
IN PRACTICE
By the time it
reaches your ship,
its character may
have changed
considerably.
03 AGE
The longer an air mass remains away
from its source region, the greater the
opportunity for its temperature, moisture
and stability characteristics to be modified.
Modification is not necessarily
uniform through the entire
vertical extent of the air mass.
EXAM LINE
Young air mass
carries source
signature; old
air mass is more
modified.
REMEMBER
Origin gives it a starting character.
Path and time determine how it changes.
Slide 5 · 04 THE MARINER'S WEATHER RULE
04 THE MARINER'S WEATHER RULE
READ THE SURFACE → PREDICT THE WEATHER
Use the temperature relationship between the air mass and the underlying surface.
COLD AIR
Moving over a WARM surface
Warmed from below
UNSTABLE AIR
Convection Cumuliform cloud
Showers / possible thunderstorms
WARM AIR
Moving over a COLD surface
Cooled from below
STABLE AIR
Stratiform cloud Fog / low cloud
Limited vertical development
FINAL TAKEAWAY
AIR MASS + SURFACE = WEATHER BEHAVIOUR
For a mariner, knowing the air mass is not enough. Ask what surface it is crossing.
Source: NOAA / NWS | WMO Terminology
Drafted by SailR from our daily lessons. AI-drawn illustration. Simplified to explain the idea, not to scale. Last updated 19 Oct 2026.
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