High-pressure area Hadley cells help form the subtropical ridge, steer tropical waves and tropical cyclones across the ocean and is strongest during the summer. The subtropical ridge also helps form most of the world's deserts. On English-language weather maps, high-pressure centers are identified by the letter H. Weather maps in other languages may use different letters or symbols. The direction of wind flow around an atmospheric high-pressure area and a low-pressure area, as seen from above, depends on the hemisphere. High-pressure systems rotate clockwise in the northern Hemisphere; low-pressure systems rotate clockwise in the southern hemisphere. The scientific terms in English used
Pressure experiment Pressure experiment Pressure experiments are experiments performed at pressures lower or higher than atmospheric pressure, called low-pressure experiments and high-pressure experiments, respectively. Pressure experiment are necessary because substances behave differently at different pressures. For example, water boils at a lower temperature at lower pressures. The equipment used for pressure experiments depends on whether the pressure is to be increased or decreased and by how much. A vacuum pump is used to remove the air out of a vacuum vessel for low-pressure experiments. High-pressures can be created with a piston-cylinder apparatus, up to () and . The piston is shifted with