CHAPTER 13.   TEMPERATURE AND DEW POINT

SECTION 3.   OBSERVING, DETERMINING, AND REPORTING PROCEDURES

The provisions of this section and its subsections are not applicable at LAWRS sites.

13-24.   STATION PRIMARY SYSTEM (NA LAWRS)

The observer shall use the first operable system from the following list to obtain temperature and additional psychrometric data.

a.  Hygrothermometer

b.  Psychrometer

c.  Mercury- or alcohol-in-glass extreme thermometers

d.  Corrected values from thermograph or hygrothermograph charts

13-25.   HYGROTHERMOMETER (NA LAWRS)

Record the temperature and dew point using the values displayed from the hygrothermometer.

13-26. LIQUID-IN-GLASS THERMOMETER (NA LAWRS)

The temperature from mercury- or alcohol-in-glass thermometers in psychrometers shall be observed as follows:

The observer shall stand as far from the thermometer as possible to prevent body heat from affecting the readings. To minimize errors of parallax, the observer shall make sure that the line of sight from the observer's eye to the top of the liquid column is level. The observer shall read the dry- and wet-bulb temperatures to the nearest tenth (0.1) of a degree.

13-27. THERMOGRAPH OR HYGROTHERMOGRAPH (NA LAWRS)

a.  The temperature shall be read at the point on the chart where the appropriate printed time curve intersects the temperature trace. The observer shall interpolate for values of temperature and time between printed lines.

b.  The humidity scale is based on values from 0 to 100 percent and humidity shall be determined similar to deriving temperature values from hygrographs or hygrothermographs. When the hygrograph position is adjusted to correspond with the psychrometric readings made at dry-bulb temperatures below freezing, the psychrometric value of relative humidity with respect to water shall be used. Hygrograms shall be evaluated according to local needs and disposed of when appropriate. Hygrograph data shall not be entered on observational records unless specifically authorized.

c.  Where less than 24 METAR observations are taken daily, the observer shall read (to the nearest degree) the corrected temperature from the thermograph for the hours when personnel are not on duty. If the thermograph is inoperative, the observer shall not interpolate between available recorded and observed data. This data shall be reported as missing.

13-28.   DRY-BULB AND DEW POINT TEMPERATURE (NA LAWRS)

Temperature (dry-bulb) and dew point shall be determined at all stations and reported in the body of the METAR and SPECI. The reporting resolution for the temperatures and the dew point in the body of the report shall be whole degrees Celsius. The reporting resolution for the temperatures and dew points in the remarks section of the report at designated stations shall be to the nearest tenth of a degree Celsius. The observer shall report the dry-bulb and dew point temperature in each METAR and SPECI. Dew point shall be calculated with respect to water at all temperatures.

13-29.   MAXIMUM AND MINIMUM TEMPERATURE (NA LAWRS)

Designated facilities shall determine and report in remarks the maximum and minimum temperatures that occurred in the previous 6 hours to the nearest tenth of a degree Celsius for the 0000, 0600, 1200, and 1800 UTC observations. Stations shall also determine and report the calendar day (LST) maximum and minimum temperatures to the nearest tenth of a degree Celsius. If the midnight LST observation is also a 6-hour synoptic observation, the observer shall determine and report both the 6-hour temperatures and the past 24-hour maximum and minimum temperatures. The format for reporting these temperatures is given in paragraphs 15-56, 6-Hourly Maximum Temperature, through 15-58, 24-Hour Maximum and Minimum Temperature. The observer shall obtain maximum and minimum temperature values from available equipment and determine them to the nearest tenth of a degree Celsius according to the following:

a.  Maximum/Minimum Extremes. Obtain values from maximum/minimum indicators in use (hygrothermometer or mercury- or spirit-in-glass thermometers) when properly reset for exposure throughout the 6-hour period. If for any reason a maximum or minimum indicator has not been properly exposed or reset for the observation period, disregard the indicated readings, and if a thermograph is not available for determination of these data, obtain the maximum and minimum values from the dry-bulb entries in Column 19, MF1M-10C. If none of the preceding procedures are possible, consider the value(s) as missing.

b.  Thermograph or Hygrothermograph. If values are not available from maximum/minimum indicators, determine the values from a corrected recorder chart of a thermograph or hygrothermograph.

13-30.   RELATIVE HUMIDITY COMPUTATIONS

When there is a local need for these data, determine the values to the nearest percent using a psychrometric calculator or table appropriate for the elevation of the station.