Cross ventilation
Cross-ventilation pertains to wind, fresh air or a breeze entering through an opening (namely a window) that flows directly through the occupied space and out through an opening on the opposite side of the building, where the air pressure is lower, whereby creating a flow of cool air and as well as a current of air across the room from the exposed area to the sheltered area. Windows or vents positioned on opposite sides of the room allow passive breezes a pathway through the structure, which circulate the air and provide passive cooling. Cross-ventilation is a  wind-driven effect and requires no energy, in addition to being the most effective method of wind ventilation. A commonly used technique to remove pollutants and heat in an indoor environment, cross-ventilation can also decrease or even obviate the need for an air-conditioner and can improve indoor air quality. Others terms used for the effect include, cross-breeze, natural cross-ventilation, cross-draft, through-draft, wind-driven ventilation, wind effect ventilation and cross-flow ventilation. Process. The phenomena occurs when openings in an environment (including vehicles) or building (houses, factories, sheds, etc) are set on opposite or adjoining  walls, which allow air to enter and exit, thus creating a current of air across the interior environment. There is also a pressure difference between the opposite sides of the establishment. The effect is mostly driven by the wind, whereby the air is pulled into the building on the high pressure windward part and is pushed out on the low pressure downwind side of the establishment (because of the pressure difference between the openings). A wind's effect on a structure creates regions that have positive pressure on the building's upwind area and a negative pressure on the downwind  side. Thus, the building shape and local wind patterns are critical in making wind pressures that force airflow through its openings. If the windows on both sides of the buildings are opened, the overpressure on the side facing the wind, and/or low pressure on the adjacent protected side, will make a current of air through the room from the uncovered side towards the sheltered side. If there are windows on both sides in a building, cross ventilation is appropriate where the width of the room is up to five times the floor-to-ceiling height. If openings are only one  side then wind-driven ventilation is more suited for structures where the width is around 2.5 times the floor to ceiling height. Factors. Cross ventilation relies on many factors, such as the tightness of the establishment, wind direction and how much wind is available, its potential travel through chimneys, vents and other openings in the home. Casement windows can be installed to improve to improve cross-breezes. Air quality may also affect cross ventilation. Although cross ventilation is generally more direct at its job than stack ventilation, its cons include its effects being unproductive on hot, still days, when it  is most necessary. Moreover, cross ventilation is generally only suitable for narrow buildings. The contrasting height of the openings (walls, sill, panels or furniture) ordered by the space also immediately influence the level and velocity of ventilation. Effectivity. Cross ventilation works well in climates with hotter temperatures, where the system allows continual changes of the air within the building, refreshing it and reducing the temperature inside the structure and also when the window on the windward side of the building is not opened as much as the one on the leeward side. Cross ventilation will not be efficacious if the  windows are more than 12m apart and if a window is behind a door that is regularly shut. An opened window that faces a prevailing wind and is conjugated with another window on the opposite side of a building will supply natural ventilation for fresh air. A decent and effective cross ventilation will remove heat from the interior and keep indoor air temperatures approximately 1.5 C° (2.7 F°) below the outdoor air temperatures, ensuring that there is a steady inflow and outflow of fresh air inside the building. Besides windows, brise soleils, doors, louvers or ventilation grills  and ducts can also work as effective ventilation openings, though an awning window provides the least effectivity. The wind surrounding building structures is important when it comes to assessing the air quality and thermal comfort indoors since both air and heat exchange rely heavily on the wind pressure on the exterior of the building. For the best airflow, the windward windows of the occupied space should not be opened as much as those on the leeward side. Disadvantages of wind-driven ventilation include capricious wind speeds and directions (which may create a strong unpleasant draft), and the polluted air from  the outside that may tarnish the indoor air quality. Types. There are four different types of cross ventilation: Equation. For a simple volume with two openings, the cross wind flow rate can be calculated using the following equation: formula_1 where formula_2 is the far-field wind speed; formula_3 is a local pressure drag coefficient for the building, defined at the location of the upstream opening; formula_4 is a local pressure drag coefficient for the building, defined at the location of the downstream opening; formula_5 is the cross-sectional area of the upstream opening; formula_6 is the cross-sectional area of  the downstream opening; formula_7 is the discharge coefficient of the upstream opening; and formula_8 is the discharge coefficient of the downstream opening. For rooms with single opening, the calculation of ventilation rate is more complicated than cross-ventilation due to the bi-directional flow and strong turbulent effect. The ventilation rate for single-sided ventilation can be accurately predicted by combining different models for mean flow, pulsating flow and eddy penetration. The mean flow rate for single-sided ventilation is determined by: formula_9 where "l" = width of the window; "h" = elevation of the top edge of the window ; "z" = elevation of neural level (where inside and outside pressure balance); "z" = reference elevation where the wind velocity is measured (at 10 m) and formula_10 = mean wind velocity at the reference elevation. As observed in the equation (1), the air exchange depends linearly on the wind speed in the urban place where the architectural project will be built. CFD (Computational Fluid Dynamics) tools and zonal modelings are usually used to design naturally ventilated buildings. Windcatchers can assist wind-driven ventilation by guiding air in and out of structures.