Air masses, depressions and anticyclones

Understanding NI's changeable, Atlantic-dominated climate requires knowing about air masses — large bodies of air with consistent temperature and moisture characteristics — and the pressure systems they create.

Air masses affecting the British Isles

An air mass is a large body of air (hundreds of km across) that takes on the temperature and humidity of the surface over which it has travelled. Five main air masses affect NI:

Air mass	Source region	Temperature	Humidity	Associated weather in NI
Polar Maritime (Pm)	North Atlantic (near Iceland)	Cold	Moist	Showers, cool temperatures — the MOST COMMON air mass affecting NI
Tropical Maritime (Tm)	Sub-tropical Atlantic	Warm	Moist	Mild, cloudy, drizzly — brings NI's mildest winter weather
Polar Continental (Pc)	Northern Asia/Russia	Very cold	Dry	Snow and frost in winter; mainly affects eastern Britain
Tropical Continental (Tc)	North Africa/Spain	Hot	Dry	Heatwaves in summer — rare in NI but can bring very hot dry spells
Arctic Maritime (Am)	Arctic Ocean	Very cold	Moist	Heavy snow and blizzards — rare but severe when it occurs

Prevailing wind: south-westerly. NI is dominated by Polar Maritime and Tropical Maritime air masses (both from the Atlantic), which explains its mild, wet, maritime climate.

Depressions (low pressure systems)

Depressions form over the Atlantic where cold polar air meets warm tropical air along the Polar Front. The boundary between these air masses is unstable, and the warmer, lighter air begins to rise over the heavier cold air, creating low pressure at the surface.

Structure of a depression

• Warm sector: the area of warm air between the warm and cold fronts.
• Warm front: leading edge of warm air. Warm air rises gently over the cold air ahead → thick cloud layers (cirrus → altostratus → nimbostratus) → prolonged, steady rain (or drizzle).
• Cold front: trailing edge of warm air, where cold air aggressively undercuts the warm sector → steep, narrow band of tall cumulonimbus clouds → heavy rain, thunderstorms → then rapid clearing.
• Occluded front: as the faster-moving cold front catches the warm front, the warm sector is lifted off the ground → mixed, complex weather.

Weather sequence as a depression passes over NI (west to east)

1. Ahead of warm front: cirrus (high wispy clouds); pressure falling; winds increasing from the SW.
2. Warm front arrives: cloud thickens (altostratus then nimbostratus); steady, prolonged rain begins; temperature rises.
3. In the warm sector: mild, cloudy, light drizzle; pressure steady; SW winds.
4. Cold front arrives: heavy rain, possibly thunderstorms; rapid drop in temperature; wind may veer NW; pressure rises.
5. Behind cold front: bright, clear intervals; scattered showers; colder; NW winds; pressure rising.

Anticyclones (high pressure systems)

Anticyclones form where air is sinking and diverging at the surface, creating high pressure. They are associated with:

• Summer anticyclones: hot, sunny, dry. Long hours of sunshine. Risk of drought and heatwaves. Example: 2003 European heatwave; July 2022 UK heat event (40.3°C).
• Winter anticyclones: cold, clear and frosty nights (radiation fog in valleys); ice, frost. Fog may persist in valleys all day. Can bring stable, dry but cold conditions for weeks.

Anticyclones are often slow-moving or stationary (blocking high), meaning they can dominate UK weather for days or weeks at a time.

NI Climate summary

NI has a temperate maritime climate:

• Mild winters (rarely below -5°C), cool summers (rarely above 25°C).
• Rainfall year-round (no dry season), wetter in uplands (Mournes, Sperrins) than east coast.
• Persistent cloud cover (low sunshine hours).
• Changeable — depressions track across NI every few days, bringing rapid changes.