Surface Pressure

Unit: Hectopascals (hPa)

Surface pressure is the weight of the atmosphere above a given point, measured at ground level. Standard sea-level pressure is approximately 1013 hPa.

Why it matters for fire weather

Pressure itself doesn’t directly affect fuels or fire behaviour. But pressure patterns are the large-scale drivers of fire weather — they determine which air masses arrive, how long they stay, and what weather they bring:

• High pressure systems (anticyclones) bring sinking air, clear skies, low humidity, and often light surface winds — conditions under which fire danger builds progressively over days.
• Low pressure systems bring rising air, clouds, and precipitation — generally reducing fire danger.
• Pressure gradients (the difference between nearby high and low systems) drive wind speed — tight gradients mean strong winds.

How it works

Persistent high pressure

The most significant fire weather pattern in southern Europe is a blocking high — a high pressure system that stalls for days or weeks. This produces:

• Sustained hot, dry conditions through subsidence (sinking air that warms and dries as it descends)
• Light surface winds (though mountain/valley winds can still be locally strong)
• Day after day of fuel drying with no rain relief
• The progressive drought accumulation that drives the DC (Drought Code) and BUI upward

The extreme Mediterranean fire seasons of 2021 and 2023 were both characterised by persistent high-pressure blocking patterns that maintained temperatures above 40°C and humidity below 20% for weeks (European Commission JRC, 2023).

Pressure change

The rate of pressure change (tendency) can be more informative than the absolute value:

• Rapidly falling pressure often precedes wind events and weather changes — including the Mediterranean wind events (mistral, tramontana, foehn) that trigger wind-driven fires.
• Steadily rising pressure after a low suggests improving conditions with progressive drying to follow.

How to read it in Wildflyer

Surface pressure appears on the weather timeline. You can also toggle the pressure isobar layer on the map to visualise pressure patterns and anticipate wind direction and strength. Isobars (lines of equal pressure) packed closely together indicate strong pressure gradients and therefore strong winds.

Sources

• European Commission Joint Research Centre (2023). Current wildfire situation in Europe. EFFIS Annual Reports.
• World Meteorological Organization (2018). Guide to Meteorological Instruments and Methods of Observation. WMO-No. 8.