The provisions of this section and its subsections are not applicable at LAWRS. Observing practices and procedures for LAWRS personnel are given later in Section 4 of this chapter. The method of obtaining temperature and dew point varies according to the system in use at the station. The data may be read directly from digital or dial readouts, or calculated from other measured values. A partial Fahrenheit to Celsius Conversion Table is provided in appendix E. The formulas utilized for conversion are:
C= 5/9 (F-32) or F= 9C/5 + 32.

13-18.   DRY- and WET-BULB TEMPERATURES (NA LAWRS)

The observer shall obtain data from instruments in the following order of priority:

a.  Dry-bulb

(1)  Hygrothermometer, if the temperature is above -50ºF (-45°C), or

(2)  Psychrometer, equipped with a mercury thermometer, if the temperature is above -35ºF (-37°C), or

(3)  Psychrometer, equipped with a spirit thermometer, if the temperature is -35ºF (-37°C) or less, or

(4)  A thermograph

b.  Wet-bulb values (when needed to compute dew point)

(1)  Psychrometer, if the dry-bulb temperature is above -30ºF (-34°C)

(2)  Dry-bulb reading of the psychrometer, if the dry-bulb temperature is -30ºF (-34°C) or below

The observer shall obtain dew point temperatures from:

a.  A hygrothermometer, if the dry-bulb temperature is above -30ºF (-34°C), or

b.  Dry-bulb and wet-bulb temperatures, if a hygrothermometer is not available and the dry-bulb temperature is above -30ºF (-34°C), or

c.  The dry-bulb temperature, if the dry-bulb temperature is -30ºF (-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. When the dew point temperature from the system in use equals or exceeds the dry-bulb temperature and the system is within operational limits:

d.  The observer shall assume the wet-bulb and dew point temperatures with respect to water to be the same as the dry-bulb temperature if the wick of the wet-bulb is not frozen or liquid fog is present, or

e.  Assume the wet-bulb and dew point temperatures with respect to ice to be the same as the dry-bulb and convert them to their water equivalent if the wet-bulb wick is frozen or ice fog is present.

13-20.   OBTAINING PSYCHROMETRIC DATA

The method of obtaining temperature, dew point, and relative humidity values varies with the system in use at the station. The following three paragraphs describe the various methods.

13-21.   HYGROTHERMOMETER (NA LAWRS)

The observer shall obtain psychrometric data from the station's standby system whenever any of the following occur in relation to the station's hygrothermometer.

a.  Errors that exceed 2°F (1.1° C) in ambient air temperature or, for dew point temperatures that exceed the limits in Figure 13-1, Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° F, or Figure 13-2, Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° C.

b.  If the dew point is higher than the dry-bulb temperature or if the comparison checks indicate that the sensor is out of calibration, the observer shall discontinue use of the sensor until it has been serviced and calibrated.

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.  Unobtainable Depression. When the wet-bulb is covered with water and a depression cannot be obtained, the relative humidity shall be regarded as 100% and the temperature of the dew point the same as that of the wet-bulb. If the wet-bulb is covered with ice and a depression cannot be obtained, the observer shall use the relative humidity and the dew point converted to their equivalent value with respect to water, unless liquid fog is present at the station. In this latter instance, the relative humidity shall be regarded as 100% with respect to water and the dew point the same as the wet-bulb temperature.

Figure 13-1. Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° F (e.g., HO-83, 1088, etc.)

Figure 13-2. Chilled Mirror Hygrothermometers - Dew Point Comparison Difference in ° C
(e.g., HO-83, 1088, etc.)

The observer shall use the dry-bulb and wet-bulb temperatures to calculate the dew point and relative humidity with psychrometric calculators or psychrometric tables based on atmospheric pressures of 23, 25, 27, 28, 29, or 30 inches of mercury. If available, the observer shall use a psychrometric calculator with the appropriate range rather than tables.

a.  Psychrometric Calculator. The observer shall use the appropriate psychrometric calculator to convert dew point values with respect to ice to a corresponding value over 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 equivalent to 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.