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heat dome

A heat dome is a persistent high-pressure ridge that traps warm air beneath sinking, compressing air. Two consequences matter for fire. First, the ridge suppresses cloud and rainfall for days or weeks. Second, it keeps overnight minimum temperatures high, which prevents the humidity recovery that normally lets fires lie down after dark. Crews rely on those hours to build containment lines. When the night stops cooling, containment stops being achievable and suppression becomes damage limitation.

• Dead fuel — leaf litter, fallen branches, cured grass — responds to humidity within hours. It governs how easily a fire starts and how fast it runs.
• Live fuel — shrubs and tree canopy — responds over weeks to months. It governs whether a surface fire climbs into the crown.
• When live fuel moisture drops below roughly 80 per cent of dry weight in Mediterranean shrubland, crown fire becomes plausible on almost any windy afternoon.
• Spring rainfall deficits are therefore a better predictor of a severe August than August temperatures alone.