Evaporative cooling is a critical greenhouse design component as more greenhouses are used year-round. To better understand the relationship of ventilation, evapotranspiration, temperature and relative humidity (VETH) in greenhouse evaporative cooling, and to help design an evaporative cooling system, an interactive Visual VETH software was developed for greenhouse engineers, teachers and students in controlled environment agriculture worldwide. This software can create VETH charts automatically, by inputting environmental conditions inside and outside the greenhouse, and physical properties of greenhouse, based on a steady state energy balance of greenhouse. In order to demonstrate applications of Visual VETH, a cooling strategy was proposed for semiarid greenhouses where water use for evaporative cooling is considered significant. Under typical semiarid mid-day weather (40°C air temperature, 10% relative humidity, and 1000 W m-2 solar radiation), having typical physical properties of a single layer plastic greenhouse, Visual VETH showed that a ventilation rate greater than 2 m3 m-2 min-1 was necessary to achieve a selected air temperature set point (25°C), and that the amount of evapotranspiration necessary to achieve the set point air temperature increased linearly with increasing ventilation rate of the greenhouse, while relative humidity inside the greenhouse decreased and stabilized at around 67%. This suggests that optimizing greenhouse ventilation is critical to reduce water use and also to effectively cool a semiarid greenhouse. Other example applications are outlined in the paper.