Slope and aspect

Aspect is the clearest case in viticulture where a rule of thumb has to be stated with its hemisphere attached, and a great deal of wine writing forgets to.

It matters most in cool, marginal regions, where the extra energy is the difference between ripening and not, and progressively less as you move towards the warm end.

Solar energy per unit of ground is greatest when the surface is perpendicular to the incoming radiation. Tilting a vineyard towards the sun increases the energy each vine receives, and steep slopes at high latitude can gain the equivalent of a substantially warmer site.

The Mosel is the standard illustration: extremely steep slopes angled towards the sun, at a latitude that would otherwise struggle to ripen Riesling at all.

Cold air is denser than warm air and flows downhill. Mid-slope sites therefore drain cold air away overnight and are markedly less frost-prone than valley floors — the reason the best sites in many regions sit partway up.

Slopes drain water, which limits vigour and imposes the mild water stress associated with better fruit. They also erode, and terracing or cover cropping is often required.

The costs are real. Steep slopes cannot be mechanised, so labour costs per bottle are several times those of flat vineyards, which is a large part of why Mosel and Douro wines cost what they do.

In hot regions the calculation inverts. East-facing slopes catching morning sun and avoiding afternoon heat are often preferred, and north-facing sites in the Northern Hemisphere become an asset rather than a liability.