PRINCIPAL INVESTIGATOR
      Amey Bailey
      USDA Forest Service
      Hubbard Brook Experimental Forest
      West Thornton, NH
      Telephone: (603) 726-8902

BEGIN DATE
       January 1, 1966

END DATE
      On-going

LOCATION
      Headquarters station

DATA DESCRIPTION
       Hygrothermographs have operated at Weather Stations HQ, 1, 6, 14, 23 and INT for the period of their records. Data have only been read from the hygrothermograph charts for three periods: January 1966 - April 1966, June 1967 - Sept. 1967, and May 1971 - April 1973. Beginning in 1981, a humidity sensor has operated as part of the automatic weather station at Headquarters.

SAMPLING DESIGN
      The hygrothermographs at Stations HQ, 1, 6, 14, 23 and INT record relative humidity by sensing the expansion and contraction of a set of horsehairs. The accuracy of their calibration and frequency of maintenance has varied considerably over the years. For a time in the early 1960's, they were not maintained at all. Beginning around 1965 they were calibrated occasionally (less than once a year) by operating in a chamber with saturated salt solutions. Since the 1970's, maintenance and salt calibration has been conducted annually, normally in the spring. Minimum relative humidity at the time of maximum daily temperature was converted to vapor pressure. Because vapor pressure varies little throughout a day and among locations, data from all available hygrothermographs were averaged in order to reduce error. For 1971 - 1973, averaging gave triple weighting to HQ and double weighting to Station 1 to reflect the relative quality of their calibrations.
      Beginning in 1981, a humidity sensor has been operated as part of the automatic weather station at Headquarters. This was originally a Brady Array. Brady Array values were corrected by calibration with an Assmann psychrometer. When the weather station blew over on August 1, 1983, this sensor was destroyed and data collection stopped. On April 26, 1985 a Lambda temperature-humidity probe was installed. The humidity sensor has been replaced about annually since then; however, missing data still occur for a variety of reasons. The manufacturer's calibration is used and is checked occasionally with an Assmann psychrometer.
      For missing data since 1986, estimates have been made by taking the average vapor pressure calculated from the HQ and Station 1 hygrothermographs. Vp is calculated using the minimum humidity and maximum temperature for the date.

ACCESS
      Public

DATA LOCATION
      USDA Forest Service, Durham, NH

LAST UPDATED
      May 24, 2001

CONTACT PERSON
      John Campbell
      PO Box 640
      Durham, NH 03824
      Telephone: (603) 868-7643
      E-mail: jcampbell@lternet.edu

VARIABLES

Column	Variable	Description	Units	Coded	Missing Value
1	DATE	Sample Date	yymmdd	n	none
2	VP	Vapor pressure	pKa	n	none
3	FLAG	Flag	none	y	none

CODES

Variable	Code	Description
FLAG	e	Estimated

DATA FILE NAME(S)
      vp.txt