This USMC manual, Marine Corps Reference Publication MCRP 12-10.1 (3-11.1B) Small-Unit Leader’s Guide to Weather and Terrain Change 1 April 2018, provides basic information on weather and terrain and their effects on military operations. This manual is applicable to small-unit leaders and commanders and to the staffs of small units. MCRP 12-10.1 is also a basis for instruction and for use by personnel developing equipment and doing studies for the Marine Corps.