 The second type of case that produces screening off occurs when C is a common cause of A and E, such as in the barometer example depicted in  Figure 1.  A drop in atmospheric pressure (C) causes both a drop in the level of mercury in a barometer (A) and a storm (E). (This notation is slightly different from one used earlier.) The atmospheric pressure will screen off the barometer reading from the weather: given that the atmospheric pressure has dropped, the reading of the barometer makes no difference for the probability of whether a storm will occur.