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Which instrument measures precipitable water continuously?

  • A.A radiosonde
  • B.A ceilometer, which determines the height of the cloud base from the time taken for a light pulse to return
  • C.A ground based GPS receiver
  • D.A rain gauge, which collects the precipitation reaching the ground and therefore measures the same quantity

Answer: C

The wet delay in navigation satellite signals passing through the atmosphere is proportional to the integrated water vapour, so a network of geodetic receivers doubles as a humidity observing system. It provides continuous measurement at a few minute intervals, unlike a radiosonde which gives two profiles a day. The technique was a by-product of geodesy and is now assimilated operationally.

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