a five-second (soon to be changed to three-second) average peak gust. ASOS provides a two-minute average, continuously updated each minute, for the hourly observation (Lockhart, 1995, 1996a, 1996b). Possible sources for differences in wind direction (Lockhart, 1996b) are that measurements may not be taken at exactly the same time, the instruments are not co-located which would affect the character of the wind flow, and the wind direction is determined differently. ASOS provides unweighted (objective) averages (scalar or unit vector) from one-second samples taken for two minutes, whereas the conventional observation is the (subjective) average direction and speed inferred by an observer watching a dial for one minute. Analysis of five-second wind averaging indicates ASOS peak winds are lower than the previous subjective measurements (Lockhart, 1996b; McKee et al., 1996a). Differences in the hourly wind speed observation show a nonlinear wind speed-dependent bias (Lockhart, 1996b). A comparison of the wind direction distributions at two sites indicated that there was no significant difference between the ASOS and conventional hourly observations (Lockhart, 1996b).
The ASOS cloud height indicator (CHI) is a laser ceilometer that differs from the standard NWS ceilometer in the way it processes returns for low cloud base and total obscuration. Both ceiling height and cloud coverage (up to 12,000 feet only) are determined by time averaging over a 30-minute period the conditions directly overhead. In manual observations, the observer subjectively evaluates the ceilometer trace at a single point in time to determine ceiling height, and the cloud coverage is determined by visual examination of the cloud conditions over the entire sky then subjectively forming a spatial average (Cornick and McKee, 1993). ASOS ceiling reports were highly correlated to conventional ceiling reports most of the time (92.7 percent), but the high level of equality drops during periods of active weather (Cornick and McKee, 1993).
ASOS is not equipped to measure sunshine duration (NWS, 1992a). Conventional pressure observations are based on an aneroid altimeter indicator or a precision aneroid barometer with observations made at hourly and special observation times (NWS, 1992a, 1994a). The ASOS barometers consist of redundant digital pressure transducers utilizing capacitive sensors, which compute and update the pressure report once every minute from readings obtained every 10 seconds (NWS, 1992a).
Manual observation of weather phenomena, including obstructions to vision, has been based on personal interpretation of the human senses (NWS, 1994a) for almost all of history (Cornick and McKee, 1993), with intensity being based on visibility criteria. These phenomena include (a) rain, snow, fog, haze, and freezing precipitation; and (b) tornadoes, funnel clouds, water spouts, thunderstorms, hail, ice crystals, snow pellets, snow grains, ice pellets, drizzle, blowing obstructions (snow, sand, dust, spray), and smoke. The automated observation of these elements required a fundamental change in observational technique and perspective. The ASOS Precipitation Identification (PI) sensor can discriminate between the occurrence of rain and snow (and identify intensity) from an algorithm based on sensor response (Cornick and McKee, 1993). Fog is reported if visibility drops below seven statute miles and dew point depression is 4°F or less. If the dew point depression is greater than four degrees and no present weather is indicated, then haze is reported. ASOS cannot report the weather phenomena from group (b) above (NWS, 1992a). In a study of 13 sites, ASOS and human observers reported approximately the same number of total minutes of freezing rain, however the coincidence rate (ASOS and human reporting freezing rain at the same time) was about 66 percent (Ramsay, 1997).