page_title: 1st Weather Reconnaissance Squadron
section_title: Instruments
hierarchical_section_title: 1st Weather Reconnaissance Squadron / History / Equipment / Instruments
caption_attribution_description: English: 1st Weather Reconnaissance Squadron B-17 Weather Instruments.
context_page_description: The 1st Weather Reconnaissance Squadron is an inactive United States Army Air Force unit. It saw service between April 1943 and May 1945. On 21 December the squadron was inactivated at Grenier Field, New Hampshire.
  During World War II the United States had to move large numbers of aircraft to the European and Mediterranean Theaters via the North Atlantic ferry route, a series of short flights between Newfoundland, Labrador, Greenland, Iceland and the UK. Weather conditions in winter closed the route and made crossing perilous at any time. The amount of time the route was open could be increased and the losses en route decreased by accurate weather forecasting. Land based weather stations left large gaps; the solution was the development of airborne meteorology.
  The 1st Weather Reconnaissance Squadron was formed to gather weather data for the North Atlantic ferry routes, particularly the segments over the ocean. This data was used to provide forecasts for the safe passage of aircraft flying between North America and Britain. It became operational in April 1943 and operated throughout the North Atlantic region.
context_section_description: The TB-25s were outfitted with: ML-313/AM psychrometer, ML-175-T-1 psychrometer, Friez Aerograph, SCR-718A radio altimeter, Kollsman pressure altimeter, C-13A free air thermometer, cloud height meter.
  The RB-17s were outfitted with: ML-313/AM psychrometer, Friez or Pioneer Aerograph ML-715, SCR-518 or SCR-718 Radio Altimeter, K-20 camera for cloud photography, cloud height meter, weather observer's table, de-icing equipment.
  The psychrometers were a wet bulb-dry bulb pair of thermometers. The moisture evaporating from the wet fabric on one bulb cooled it, the lower the humidity the lower the temperature. By comparing the two temperatures, it was possible to calculate the relative humidity. The thermometers were inside the nose of the airplane. Air was trapped by probes attached to the outside and directed in over the thermometer bulbs. When the psychrometer was first tried in the airplanes, the readings were inaccurate. It was found that when exposed to an airstream moving over 200 mph, the friction of the air heated the thermometers. The amount of heating was unpredictable, making it impossible to calculate a correction. New psychrometers were developed that slowed the air before making the readings. With these devices, accuracy was 98–99%.
  The aerograph combined three instruments, an air pressure sensor, a humidity sensor and a temperature sensor. The sensors were connected to pens that drew graphs on a paper chart wrapped around a rotating drum. The paper chart was replaced with a new one when it was full. When all the charts from a mission were collected in the mission log, they provided a continuous record of conditions of the flight. The air pressure sensor was an aneroid barometer and the humidity sensor was strands of human hair. As the hair absorbed or evaporated moisture, it lengthened or shortened, moving the pen.
  The radio altimeter gave a true reading of the airplane's altitude when it was over the sea. Over land, the radio altimeter only gave the altitude over the land below. Without knowing the elevation of the land, it wasn't possible to know the true altitude. The pressure altimeter gave the same readings over land or sea, but it could not distinguish between a change in air pressure caused by a change in altitude and a change in air pressure caused by a change in barometric pressure. Therefore, the pressure altimeter was only accurate to a range of altitudes. By using both altimeters together over the ocean, it was possible to determine the true altitude, the true air pressure at that altitude and the pressure at other altitudes at that point.
  On some RB-17E missions, there was also a bucket in the event the weather officer got airsick.
caption: 1st WRS RB-17G Weather Instruments
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