It was the aim of the Navy to keep their weather reconnaissance aircraft below the level of clouds, where the aerologist could watch the surface of the sea as much of the time as is possible within the limits of reasonably safe operation. When in a tropical storm, the aerologist guided the pilot around or into the center. Down near the water, say one hundred to three hundred feet altitude, turbulence is apt to be very bad, sometimes extremely violent. Above seven hundred feet, clouds are likely to interfere and this was extremely dangerous at that altitude in those early years because the altimeter which they used to indicate height of the aircraft by pressure of the atmosphere was sometimes badly in error in a tropical storm. If the pilot and the aerologist lost sight of the water’s surface for a few minutes, they suddenly found the aircraft about to strike the precipitous waves of a storm-lashed sea. Pressure of the atmosphere falls with increase of elevation, roughly one inch drop in pressure for each one thousand feet. If we put an ordinary barometer reading 29.90 inches in a plane on the ground and go up one thousand feet, it will read about 28.90 inches. The pressure altimeter is a special type of barometer that shows elevation instead of pressure. When the pressure is 29.90 inches and the altimeter is set at 0, we go up to where the pressure is 28.90 inches and it reads one thousand feet. But if the pressure over the region falls to 28.90 inches and the altimeter is not adjusted, it will read one thousand feet at the ground and be roughly one thousand feet in error when we go up to where the reading is 27.90 inches. In ordinary weather, big changes in the barometer take place slowly and there usually is plenty of time for correction. In a flight into a hurricane, big changes take place rapidly. The change caused by the plane going up may be confused with the drop in pressure in the hurricane. If the plane is in the clouds when these changes take place, the pilot may have a frightening surprise on coming into the clear again. More recently, the hunters have been equipped with radar altimeters which give the absolute altitude for check. They send a radar pulse downward and it is bounced back from the sea surface to the instrument. The time it takes to go down and back depends on the height—the higher, the longer it takes—and the instrument is designed to give the indication very accurately in feet. Thus, the radar altimeter removed some of the dangers of low level flight.