Sunlight loses in intensity if the angle with which they hit the ground is less than 90 degrees. This is partly due to it having to travel through more air, and thus more energy being absorbed, but mostly due to the fact that it spreads out over more land area.

E.g.: A column of sunlight illuminating one hectare of land when falling perpendicularly will cover 1.5 ha if it falls at 45º and 2.5 ha if it falls at 22.5º.

A field that slopes in the direction of the sun will compensate for this to some extent, thus increasing the intensity of sunlight that it receives.
To compensate for the effect cited in the example above, the slopes should have a gradient of 45º(1:1) and 67.5º(1:2.4) respectively.

Some Rhine vineyards have a gradient of 1:2, but this is too steep for tractors to be used, and so vine growing becomes very expensive. The steeper the slope, the greater the effects of soil erosion, though other factors should be taken into account such as rain intensity and the texture of the soil.

The best slopes in England are south facing, as although SSW slopes are slightly superior from a 'sunlight-catching' point of view, the foliage takes longer to dry out in the mornings, which is an important factor in the development of fungal diseases. South-facing slopes are far better than level fields in spring and autumn, but about the same in summer. The reason for this is that in summer the sun rises slightly north of East and sets slightly north of West. This creates a 'slope shadow' effect, reducing the number of sunlight hours.