COLD WEATHER

A. Chemical reactions (explosion of gun powder) take twice the amount of time to occur for every for every 10º C (18°F) drop on temperature. The converse is also true.

B. Cold air is dense offering a greater resistance to a shotstring which causes your pattern to open up faster than in warm air. Hence you may wish to change to a tighter choke in very cold weather.

Example: 12GA 1 1/4oz #4 lead
@70ºF the muzzle velocity=1330 ft/sec
@-40F muzzle velocity=1240 ft/sec