The observer shall use the first available operative system of the following for obtaining temperature and/or psychrometric data.

a.  Hygrothermometer, stand-alone temperature measuring device, or equivalent system.

b.  Psychrometer.

13-32.   HYGROTHERMOMETER OPERATING RANGE (ONLY LAWRS)

a.  Temperature: -50° F to 120° F (-45°C to 48°C).

b.  Dew Point: -20° F to 80° F (-28°C to 26°C).

13-33.   EVALUATING HYGROTHERMOMETER DATA (ONLY LAWRS)

If data is within the operating range of the hygrothermometer, the observer shall obtain data in accordance with the following:

a.  Face each indicator on as direct a line of sight as possible to minimize parallax errors.

b.  Observe temperatures to the nearest tenth of a degree Celsius or Fahrenheit from the left edge of the indicator.

c.  Do not report data if standby equipment is not available and the reading exceeds the operating range of the hygrothermometer.

d.  If the temperature is below -30° F (-34°C), disregard the dew point indicated on the instrument and assume the temperature of the dew point to be the same as the dry-bulb temperature with respect to ice. Convert the dry-bulb temperature to the corresponding dew point temperature with respect to water using a psychrometric calculator or nomograph.

13-34.   PSYCHROMETER (ONLY LAWRS)

The observer shall use a psychrometer equipped with mercury thermometers to determine dry-bulb and wet-bulb temperatures if the dry-bulb temperature is above -30° F (-34°C). If the dry-bulb temperature is -30° F (-34°C) or less:

a.  The observer shall use a psychrometer equipped with a spirit dry-bulb thermometer, in the range of -50° F to 110° F (-45°C to 43°C) or -80° F to 110° F (-62°C to 43°C), to obtain dry-bulb temperature, and

b.  Disregarding the wet-bulb reading, use the dry-bulb temperature to compute the dew point as described in paragraph 13-35, Dew Point Temperatures (Only LAWRS).

13-35.   DEW POINT TEMPERATURES (ONLY LAWRS)

The observer shall obtain dew point temperatures from:

a.  Dry-bulb and wet-bulb temperatures if the temperature is above -30° F (-34°C), or

b.  The dry-bulb temperature if the dry-bulb temperature is -30° F (-34°C) or below. To do this, the observer shall assume that the dew point with respect to ice is the same as the dry-bulb temperature and convert it to the corresponding dew point with respect to water using a psychrometric calculator or nomograph.

When readings are being made, the observer shall:

a.  Stand as far away from the thermometer as possible to prevent body heat from affecting the indicated readings.

b.  To minimize errors of parallax, make sure that the line of sight from the observer's eye to the top of the liquid column is level.

13-37.   PSYCHROMETRIC EVALUATIONS (ONLY LAWRS)

a.  Near Freezing Temperature. At wet-bulb temperatures near freezing, the observer shall determine visually that the wet-bulb is unfrozen before using wet-bulb depression data.

b.  Temperature Below Freezing. At wet-bulb temperatures below 0°C, if the wick is not frozen, the observer shall touch the wick with clean ice, snow or another cold object to induce freezing. The resulting dew point values with respect to ice shall be converted to corresponding values over water.

c.  Unobtainable Depression. When the wet bulb is covered with water and a depression cannot be obtained, the relative humidity shall be regarded as 100 percent. Consider the temperature of the dew point to be the same as the dry-bulb. If the wet-bulb is covered with ice and a depression cannot be obtained, consider the dew point to be the same as the dry-bulb temperature with respect to ice and convert it to its water equivalent. If liquid fog (not freezing fog) is present, the dew point is the same as the dry-bulb temperature.

Dry-bulb and wet-bulb temperatures shall be used to calculate relative humidity and dew point with psychrometric tables, nomograms or calculators based on atmospheric pressures of 23, 25, 27, 28, 29 and 30 inches of mercury. If a psychrometric calculator is available, it shall be used over its range rather than tables.

13-39.   DEW POINT CONVERSION (ONLY LAWRS)

a.  Psychrometric Calculator. The observer shall use the appropriate psychrometric calculator to convert dew point values with respect to ice to corresponding dew point values with respect to water. On the low temperature face of the calculator, equivalent values of dew point appear opposite each other on the DP (or T_W, DP) and T_i scale; e.g., a dew point of 20° F (-6.7°C) with respect to ice is the equivalent of 18.5° F (-7.5°C) with respect to water.

b.  Psychrometric Tables. For psychrometric data outside the range of calculators, the observer shall use the Smithsonian Meteorological Tables. The observer shall use Table 99 for reduction of the data and Tables 100 and 101 for converting between relative humidities with respect to ice and with respect to water. The observer shall use Table 102 for similarly converting dew points.