Definition
An air mass that forms over cold ocean waters at high latitudes, producing cool, moist, and generally unstable air. Maritime polar air masses affecting North America originate over the cold North Pacific and North Atlantic Oceans. As they move inland, they typically bring low clouds, fog, drizzle, showers, and reduced visibility, especially along windward coasts and mountain slopes.
Plain English
A body of air that picked up its character from a cold ocean far to the north. It arrives cool and damp, often bringing clouds, fog, and showers — particularly on coasts facing the wind.
Context Anchor
Seen in weather chapters, air mass source-region charts, and forecast discussions that describe where a large body of air came from.
Derivation
‘Maritime’ comes from the Latin maritimus, meaning ‘of the sea’ — so the air mass took on its moisture from spending time over ocean water. ‘Polar’ refers to the cold high-latitude region where it formed, not the actual poles. Together: cold ocean air.
Why Pilots Care
An mP air mass can bring fog, low ceilings, drizzle, and icing conditions that affect visibility and flight safety.
Grounding Statement
Picture cool, damp ocean air moving inland and spreading its moisture and lower temperatures across the route.
Intuition Check
Do not read “polar” as meaning the air must come from the exact North Pole. Here, “polar” means the air formed in a cold high-latitude region, and “maritime” means it picked up moisture over water.
Example Sentence 1
A maritime polar air mass moving in off the Pacific brought low ceilings and steady drizzle along the Oregon coast.
Example Sentence 2
When the mP air mass moved over land, the student pilot noted the drop in ceilings and prepared for possible instrument conditions.