As the winter sets in and my hands go numb, I’m thinking about those damn hulks and how they move every bit as fast. I’ve heard the odd suggestion about zombies freezing in winter, but I think that would be a problem from a balance perspective. Here’s what I’d like to suggest.
Temperature above freezing: zombies behave normally
Temperature freezing to about -5°C /20F: zombie speed 80%, but bashing and cutting armour increases by 50%
Temperature about -5°C to -10°C (20F to 15F): zombie speed 50%, but armour is doubled and hit points increase by 70%
Temperature about -10°C to -20 (15F to -5F): zombie speed to 50%, but armour and HP doubled.
Below that, most zombies freeze solid with speed of zero (but are frozen solid and so have the armour and HP buffs). Tough, fat, hulk, brute, shocker, acid, firefighter, police, and soldier zombies cap at the final effect and can still move. This is likely only for cold labs anyway.