The thermal breeze

In fair weather and a weak gradient, this is what makes the day for the leisure sailor: the thermal breeze is born from the temperature difference between land and sea.

The engine: warm land, cool sea

Land warms up and cools down much faster than the sea. By day, the ground heats up, the air above it rises, and cooler air from the sea flows in to fill the gap: this is the sea breeze, blowing from the sea toward the land. At night the process reverses: the land cools below the sea temperature, and the air flows from the land toward the sea — this is the land breeze.

The rhythm of a typical day

• Morning: often calm, glassy sea. Sometimes a leftover land breeze.
• Early afternoon: the sea breeze sets in and builds (often 10 to 20 knots).
• Late in the day: it eases, then dies at sunset.
• Night: land breeze, lighter (a few knots).

When the breeze doesn't pick up

The thermal breeze needs sun and a weak synoptic wind. If the sky is overcast, or if a general wind (mistral, tramontane) dominates, the thermal is masked or absorbed into the main flow. A fine-mesh model (AROME) reproduces these coastal breezes far better than a global model.