35 °C outside, freezing sea: upwelling and cold water on hot days, explained

Quick answer: why is the sea cold when it's hot outside?

Most often because of upwelling — a wind-driven process where warm surface water is pushed away from the coast and cold, deep water rises to take its place, cooling the sea near shore even on a blazing day. Winds blowing along or off the coast (common in summer on certain coasts) trigger it, and the water can drop several degrees quickly. The broader reason is that air and sea temperature are decoupled: the sea has enormous thermal inertia and its own dynamics (currents, depth, upwelling), so it doesn't simply match the air. A hot day doesn't heat the whole sea; it can even coincide with cold water if the wind is upwelling.

So the cold sea on a hot day is usually upwelling bringing deep cold water to the surface, plus the general fact that the sea is not governed by the day's air temperature. It's physics, not chance — and on upwelling-prone coasts, it's a regular summer occurrence.

How upwelling works

Upwelling is a wind-and-rotation-driven process. When wind blows along a coast in the right direction, the combination of wind stress and the Earth's rotation (the Coriolis effect) pushes the surface water away from the shore (offshore). That surface water has to be replaced, so cold, nutrient-rich water from the deep rises up to fill the gap near the coast — chilling the surface and the beach water. The stronger and more persistent the wind, the more upwelling, and the colder the near-shore water gets, sometimes dropping several degrees within a day. When the wind stops or reverses, the upwelling eases and the surface can warm again.

So upwelling is essentially the wind pumping cold deep water up to the surface at the coast. It's why a steady summer wind can leave you with icy water under a hot sun — the wind is actively drawing up the cold from below, overriding the sunshine.

• Alongshore/offshore wind + Earth's rotation pushes surface water offshore.
• Cold, deep water rises to replace it, chilling the near-shore sea.
• Stronger, steadier wind = more upwelling = colder water, fast.

Where and when it happens

Upwelling is strongest on certain coasts, typically west-facing ones with the right summer wind patterns: notably Portugal and northwest Spain, California and the US West Coast, parts of northwest Africa, and in a more localised way, spots around the Mediterranean and elsewhere when winds align. It's often a summer phenomenon, because that's when the driving winds blow, which is exactly why these coasts can have cold water in the hottest months. So the cold-sea-on-a-hot-day experience is characteristic of these upwelling coasts, where locals know a windy day means a cold swim, while non-upwelling coasts (like much of the enclosed Mediterranean) don't get this effect as strongly.

So expect upwelling and cold-water surprises on the classic upwelling coasts — Atlantic Iberia, the US West Coast, northwest Africa — especially on windy summer days. On these coasts, cold water under hot sun is a known, recurring pattern, not an anomaly.

Beyond upwelling: why sea and air differ

Upwelling is the dramatic cause, but the deeper reason cold water can coincide with hot air is that the sea and the air are largely decoupled. Water has huge thermal inertia — it heats and cools far more slowly than air — so the sea reflects the accumulated season, not the day: it stays cold in spring even during a heatwave (it hasn't caught up), and warm in autumn after the air cools (it hasn't cooled down yet). Add the sea's own dynamics — depth, currents, mixing — and its temperature simply isn't set by today's air. A hot afternoon warms the air quickly but barely touches the vast, slow sea.

So even without upwelling, don't expect the sea to match the air: it lags by weeks or months and follows its own physics. The hot day heats the air and sand, but the sea's temperature is a slow, separate story — which is why spring heatwaves still come with cold water.

How to predict and avoid a cold swim

You can anticipate cold-water days. Check live sea-temperature data (buoys, marine services, apps) before you go — it tells you the actual water temperature regardless of the air. On upwelling coasts, watch the wind: steady alongshore or offshore winds signal likely upwelling and colder water, while calm or onshore conditions let the surface warm. Be aware of the season's lag (spring water is cold despite warm air). And locally, sheltered bays or spots out of the upwelling-favourable wind may be warmer than exposed, wind-hit beaches nearby. If cold water is a dealbreaker, these checks help you pick a warmer day or spot.

So predicting a cold swim comes down to checking live sea temperature and reading the wind on upwelling coasts. The air temperature won't tell you — but the water data and the wind will, letting you avoid the shock of an icy sea under a hot sky.

The upside of upwelling

Cold as it is for swimmers, upwelling has a big upside: the deep water it brings up is rich in nutrients, which fuels plankton growth and, in turn, abundant marine life. That's why the world's great upwelling coasts (like off Portugal, northwest Africa, California and Peru) are among the most productive fishing grounds and richest marine ecosystems on the planet. So the same process that chills your swim feeds the food web that makes those coasts teem with life. It's a reminder that the cold water isn't a flaw but part of a productive natural system.

So while upwelling means a bracing swim, it also means a living, productive sea. On upwelling coasts, the cold water and the rich marine life go together — the chill you feel is the deep ocean's nutrients rising to sustain the ecosystem, which is no small compensation for a cold dip.