Latitude and day length

The old shorthand places viticulture between roughly 30 and 50 degrees in each hemisphere. It was always approximate and it is becoming less useful.

Altitude, ocean currents, continentality and local topography routinely override latitude, which is why the belt has never been more than a first approximation.

Day length varies more with season at higher latitudes. A vineyard at 50°N has very long midsummer days, which partly compensates for lower sun angle and cooler temperatures.

The frost-free season shortens with latitude, which constrains which varieties can ripen before autumn cold arrives — the practical limit in England, Germany and Canada.

Low-latitude viticulture exists and works, but generally requires altitude to supply the cooling that latitude does not. Parts of Brazil, Bolivia, Kenya and India are cases in point.

Very low latitudes can lose winter dormancy entirely, and vines there may be manipulated into producing two crops a year — viable commercially, and not how fine wine is generally made.

Ocean currents shift the picture substantially. The cold Humboldt and Benguela currents allow viticulture at latitudes that would otherwise be too warm on the western edges of South America and Africa.