A wet barometer puts a vacuum above a liquid and applies air pressure to the surface of the liquid - the height of the liquid above to reservoir surface is the measure of air pressure. The liquid can be in a J tube with an open end or a straight tube in a pool. Air pressure will support about 30" of mercury or about 32 feet of water. <span>A dry barometer uses a sealed chamber with a diaphragm with a lever touching it or a spiral tube with a link connected to a display needle. In the former, the slight movement of the diaphragm with changed pressure is translated to a large movement of the end of the lever movement across the scale. In the spiral, the expanding/contracting air inside the tube changes its shape and pulls the needle over. </span> <span>In both case, the scale of pressure for the needle has to be calibrated to the design. </span> <span>- </span> <span>How Tropical Cyclones are Observed </span> <span>* Direct measurements of tropical storm and hurricane dimensions and wind speeds are taken primarily by reconnaissance </span> <span>aircraft, although ships and buoys also take important measurements. Once a hurricane is near and/or on land, Automated </span> <span>Surface Observation Systems (ASOS) provide surface conditions, and radio sondes take upper air measurements. </span> <span>* Indirect observational methods include satellite imagery and Doppler radar. In particular, satellites have greatly improved </span> <span>our ability to monitor and understand hurricanes. Radar data are important once the storm comes close to shore </span> <span>and after landfall for forecasting hurricane-related weather. </span>
