Field 20 AIR TEMPERATURE. Wet Bulb reading. For units see Field 32 and Table IX. 21 BAROMETRIC PRESSURE. Tens, units, and tenths of millibars. The tenths digit is generally not reported prior to 1963.) 22 WEATHER. WMO Code L50L. 23 CLOUD TYPE AND AMOUNT. For cloud type see Table VI (WMO Code 0500) followed by cloud amount, see Table VII (WMO Code 2700.) 24 WAVE HEIGHT AND PERIOD. The first two columns are the height of the wind waves in whole feet or meters. (When height is given in meters the letter m will prefix the wave height entry.) The last two numerals give the wave period in seconds. Sea state (generally if Douglas Code) may be given in place of wave height and period. In this case the wave period is left blank and the Sea State Code is given with the letter X as prefix. 25 TCS. The TCS (Temperature Correction Slide) applied to the slide is given in degrees and tenths. For units see Field 32, Table IX. If TCS is not known (or known and not applied) field is blank. Some of the historical data has had a camera correction (after 0.0°F) entered in the TOS field rather than the computed TCS. If special slide corrections have been applied they are included in the TCS. NOTE: When the TCS is pre-fixed with the letter S, the value is to be applied to the read-out temperatures. (Add or subtract this number from the temperature values printed below.) 26 DSP. Depth correction applied to slide. Units are the same as grid depth units. This field is rarely reported for historical data. 27 REFERENCE TEMPERATURE CODE. See Table VIII. 28 THE REFERENCE TEMPERATURE. The reference temperature to tenths of a degree. For units, see Field 32 and Table IX. 3.