at a sufficiently oblique angle, it will tend to slip around the tower, instead. Windcatchers in areas with stronger winds will have smaller total cross-sections, and areas with very hot wind may have many smaller shafts in order to cool the incoming air. Windtowers with square horizontal cross-sections are more efficient than round ones, as the sharp angles make the flow less laminar, encouraging flow separation; suitable shaping increases suction. Taller windcatchers catch higher winds. Higher winds blow stronger and cooler (and in a different direction). Higher air is also usually less dusty. If the wind is dusty or polluted, or there are insect-borne illnesses such as malaria and dengue fever, then air filtering may be necessary. Some dust can be dumped at the bottom of the windcatcher as the air slows (see diagram below), and more can be filtered out by suitable plantings or insect mesh. Physical filters generally reduce throughflow, unless the flow is very gusty. It may also be possible to fully or partially close the windcatcher off. The short, wide right-triangle-prism are usually bidirectional, set in symmetrical pairs, and are often used with a (evaporative cooling unit) and a (roof lantern vent). Wide s are more often used in damper climates, where high-volume air flow is more important compared to evaporative cooling. In hotter climates, they are narrower, and air is cooled on its way in. They are more commonly used in Africa. , on the other hand, are multisided (usually 4-sided), and they are typically tall towers (up to 34 meters tall) which can be closed in winter. They are more common in the Persian Gulf region and in areas with dust storms. Taller windcatchers also have a stronger stack effect. Cooling methods Night-flushing cools the house by increasing ventilation at night, when the outdoor air is cooler; windtowers can assist night flushing. A windcatcher can also cool air by drawing it over cool objects. In arid climates, the daily temperature swings are often extreme, with desert temperatures often dipping below freezing at night. The thermal inertia of the soil evens out the daily and even annual temperature swings. Even the thermal inertia of thick masonry walls will keep a building warmer at night and cooler during the day. Windcatchers can thus cool by drawing air over night- or winter-cooled materials, which act as heat reservoirs. Windcatchers that cool by drawing air over water use