the water as a heat reservoir, but if the air is dry, they are also cooling the air with evaporative cooling. The heat in the air goes into evaporating some of the water, and will not be released until the water re-condenses. This is a very effective way of cooling dry air. Simply moving the air also has a cooling effect. Humans cool themselves using evaporative cooling when they sweat. A draft disrupts the boundary layer of body-warmed and water-saturated air clinging to the skin, so a human will feel cooler in moving air than in stagnant air of the same temperature. Airflow forces The windcatcher can function in two ways: directing airflow using the pressure of wind blowing into the windcatcher, or directing airflow using buoyancy forces from temperature gradients (stack effect). The relative importance of these two forces has been debated. The importance of windpressure obviously increases with increasing wind speed, and is generally more important than buoyancy under most conditions in which the windcatcher is working effectively. Airflow speed is also important, especially for evaporative cooling (since it only works on dry air, and humidifies the air). It is possible for a windtower-ventilated building to have very high flow rates; 30 air changes per hour were measured in one experiment. Uniform, stable flow with no stagnant corners is important. Turbulent flow should therefore be avoided; laminar flow is more effective at maintaining human comfort (for an extreme example, see Tesla valve). Other elements are often used in combination with the windcatchers to cool and ventilate: courtyards, domes, walls, and fountains, for instance, as integral parts of an overall ventilation and heat-management strategy. Wind pressure If a windcatcher's open side faces the prevailing wind, it can "catch" it, and bring it down into the heart of the building. Suction from the lee side of a windtower is also an important driving force, usually somewhat more constant and less gusty than the pressure on the upwind side (see Venturi effect and Bernoulli's principle). Routing the wind through the building cools the people in the building interior. The air flows through the house, and leaves from the other side, creating a through-draft; the rate of airflow itself can provide a cooling effect. Windcatchers have been employed in this manner for thousands of years. The windtower essentially creates a pressure gradient to draw air through the building. Windtowers topped with horizontal