The model: Fog-light rule: rain versus fog

In rain: dipped beam always, front fog light may be used with seriously impaired visibility (<200 m), rear fog light NEVER (Book p.187-188) — not even in heavy showers. In very dense fog or heavy snowfall: front fog light may be used (<200 m) AND rear fog light may be used (<50 m), but only for as long as it serves your safety (Book p.191).