The thickness (in decameters) of the layer of air between 1000 mb and 500 mb. The leading 5 (or, more rarely, 4) is not transmitted. This parameter is used most frequently in snow forecasting, since the thickness of a layer of air between two pressure levels is proportional to its mean temperature. The critical thickness varies from place to place, but typically a thickness less than 534 implies snow while a thickness greater than 540 implies rain. That's assuming anything falls in the first place.
TTPTTR1R2R3 VVVLI PSDDFF HHT1T3T5
BOS//605860 00719 131410 56050504
06000818448 04209 091622 56080605
12008935114 00802 002126 59121210
18000773014 -1811 022828 55120804
24000743520 -0913 092924 49090400
30000643339 -2216 102818 48080300
36000624749 03318 112519 47080300
42000615956 -0318 112621 42050299