Terrain Obstacles and Hindrances (DMG)

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The movement point costs given above assume the best of conditions even in the worst of terrain. The mountains are assumed to be free of cliffs; the woods have no high-banked streams; rains haven't turned the plains to mud; the tundra hasn't been blanketed in snow. However, poor traveling conditions do occur, and when they do travel is slowed. Table 75 lists common obstacles and situations that slow movement. The modifiers for these are listed as either additional movement point costs or multipliers.
When additional movement costs are listed, these are added to the cost of the surrounding terrain. Thus, crossing a ridge in the high mountains costs nine movement points for that mile instead of the normal eight.
Multipliers increase the movement cost by the amount listed. Snow, for example, doubles the cost of crossing the plains. Indeed, severe weather or torrential rains—can actually bring all travel to a halt.
==Table 75: Terrain Modifiers==
Situation 	Modifier
Chasm* 	+3
Cliff* 	+3
Duststorm, sandstorm	x3
Freezing cold** 	+1
Gale-force winds 	+2
Heavy fog 	+1
Ice storm 	+2
Mud 	x2
Rain, heavy 	x2
Rain, light 	+1
Rain, torrential 	x3
Ravine 	+
Ridge 	+1
River*** 	+1
Scorching heat** 	+1
Snow, blizzard 	x4
Snow, normal 	x2
Stream*** 	+
: *These assume the player characters find a route around the obstacle. Alternatively, the DM can require the characters to scale or span the obstacle, playing out this encounter.
: **These extremes must be in excess of the norm expected of the character or creature. Thus, a camel is relatively unaffected by the scorching heat of a desert and a yak barely notices the cold of high mountains.
: ***This cost is negated by the presence of a bridge or ford.