What's the difference between clag and regular fog?

Clag is cloud that forms when moist air is forced upwards by hills and mountains (orographic cloud), whereas fog typically forms when the ground cools overnight. Clag is usually in motion, streaming over ridges, while valley fog tends to sit still—and clag often feels wetter and windier because of how it's created.

Why does walking in clag feel so much colder than the temperature suggests?

It's mostly the wind chill. The air creating clag is being forced over the hill with considerable force, so it's often moving fast and driving moisture at you. That combination of wind, dampness, and fine water droplets makes it feel much colder than the actual temperature.

Can I navigate safely in thick clag with just my phone GPS?

You can, but it's risky to rely on it alone. Batteries die, signals can bounce oddly off hillsides, and screens become hard to read when wet. A map and compass work regardless of conditions and don't need batteries—proper navigation skills become genuinely necessary when visibility drops to a few metres.

Weather The Elements

What Clag Actually Is (And Why It's Not Just Thick Fog)

Hill walkers use 'clag' to describe the specific low cloud that sits on summits and ridges when you're walking in it—thick, damp, navigationally useless, and distinctly different from valley fog in both how it forms and how long it stays. It's orographic cloud at ground level, and understanding that tells you whether it'll lift or settle in for the day.

21 August 20268 min read