Final answer:
Wind pressure is the most challenging force for architects to address in curtain wall systems due to its variability and potential to create significant pressure differences that affect water movement.
Step-by-step explanation:
The most difficult force for architects to address when designing curtain walls is wind pressure. Wind pressure affects water movement through openings in curtain walls and can be highly variable and unpredictable.
For instance, as with the illustration of shower curtains bulging inward due to the high-velocity stream of water and air causing a region of lower pressure inside the shower stall, wind can create similar differential pressure on a curtain wall system. In contrast, gravity, surface tension, and capillary action are generally more consistent and predictable phenomena that can be calculated and accommodated in the design.
Examples of pressure differences created by wind can be seen when vehicles are close to one another, such as a car passing a truck on the highway; the air movement can cause the car to veer toward the truck. Similarly, architects must account for wind pressure when designing curtain walls to ensure stability and manage water infiltration effectively.