Surprise (DMG)

Before an encounter begins, a check for surprise may be necessary. Given the right conditions, it is possible for either side in an encounter to surprise the other. In essence, the encounter is just as random for the monsters as it is for the player characters.
As noted in the Player's Handbook, surprise is not always assured nor is the check always necessary. Light, excessive noise, and other types of prior warning can cancel the need for the check. Surprise isn't usually possible when no form of concealment is possible (as in the case of two ships at sea), though darkness, storms, fog, and the like do act as concealment.
In some cases, one side may be able to surprise the other without the other group having the same opportunity. This is particularly true when the player characters are using lanterns or torches and the monsters are not. Seeing the light, the monsters can try to sneak closer and get the jump on the player characters.
When making a surprise roll, there are many factors that can increase or decrease the chance of surprise. Some of these are very exotic or very particular to a situation, but others can be anticipated. The more common modifiers are listed on Table 57. By comparing other situations to these modifiers, the DM has a guideline for making appropriate adjustments.
 Table 57: Surprise Modifiers
Other Party is: 	Group'sModifier
Silenced 	-2
Invisible 	-2
Distinctive odor (smoke, powerful stench, etc.)	+2
Every 10 members 	+1
Camouflaged 	-1 to -3
PC Party is: 	
Fleeing 	-2
In poor light 	-1
In darkness 	-4
Panicked 	-2
Anticipating attack* 	+2
Suspicious* 	+2
Conditions are: 	
Rainy 	-1
Heavy fog 	-2
Extremely still 	+2
:* A party anticipates attack when they have good cause to suspect immediate danger and know the likely general direction of an attack. A suspicious party is one that has grounds to believe another group might try to make a hostile move against them.