Table 3-3.—Thickness of Icing and Turbulence Layers (t_L) Code  Figure Thickness 1 1,000  feet 2 2,000  feet 3 3,000  feet 4 4,000  feet 5 5,000   feet 6 6,000  feet 7 7,000   feet 8 8,000   feet 9 9,000   feet Table 3-4.—Turbulence Type and Intensity (B) B = Turbulence Code Figure 0 None 1 Light Turbulence 2 Moderate  Turbulence  in  clear  air,  occasional 3 Moderate Turbulence in clear air, frequent 4 Moderate Turbulence in cloud, occasional 5 Moderate Turbulence in cloud, frequent 6 Severe Turbulence in clear air, occasional 7 Severe Turbulence in clear air, frequent 8 Severe Turbulence in cloud, occasional 9 Severe Turbulence in cloud, frequent X Extreme Turbulence 8.  Element  QNHP_IP_IP_IP_IINS  is  the  lowest altimeter setting in inches expected during the forecast period. 9 .    T h e    e l e m e n t s   TTTTT    GGG_e G_e    o r TTGGGG are change groups used to indicate changes in some or all of the elements forecast to occur at some intermediate  time  during  the  24-hour  forecast  period. Several change groups may be used to properly identify forecasted conditions as explained below. The   contraction   “FMGGGG”   indicates   the beginning  of  a  change  period  in  the  forecast.  All forecast  conditions  preceding  this  group  are  superseded by the conditions forecast in this group. For example, if the TAF period is 1515 and a change is forecast at 1930 UTC,  the  entry  "FM1930"  is  encoded.  The  elements entered  following  this  contraction  are  in  effect  from 1930  UTC  until  the  end  of  the  forecast  period,  1500 UTC. The  contraction  "BECMG"  is  used  to  indicate  a change  to  forecast  conditions  ‘expected  to  occur  at regular  or  irregular  periods  at  an  unspecified  time within the time period identified. The duration of the change should not normally exceed 4 hours. The   contraction   “TEMPO”   is   used   to   indicate frequent  or  infrequent  temporary  fluctuations  to  the forecasted meteorological conditions that are expected to last less than 1 hour in each instance, and an aggregate total  of  less  than  half  the  time  of  the  forecast  period indicated. 10.   Element   TT_FT_F/G_FG_FZ  is   the   temperature group. This is an optional group. The letter "T" is the temperature   indicator   and   T_FT_F   is   the   forecast maximum  or  minimum  temperature  in  whole  degrees Celsius, depending on the time of day. The G_FG_F is the time  at  which  the  maximum  or  minimum  temperature  is expected to occur. TAF  AMENDMENTS Occasionally  a  TAF  will  need  to  be  amended. Established  amendment  criteria  are  based  on  ceiling and  visibility  requirements  outlined  in  OPNAVINST 3710.7,   NATOPS   General   Flight   and   Operating Instructions, and other safety of flight considerations. Part of your job as an observer is to keep the forecaster aware of any changes to the meteorological situation 3-3