Using this airmet over the NW of the country:
SLCZ WA 011445
AIRMET ZULU UPDT 2 FOR ICE AND FRZLVL VALID UNTIL 012100
.
AIRMET ICE...ID MT WY NV UT CO WA OR CA AND CSTL WTRS
FROM 50NNW ISN TO 50SSW BFF TO ALS TO DVC TO ELY TO 40SSW FMG TO
120WSW PYE TO 150WSW FOT TO 20NNE DSD TO 60SW YXC TO 50NNW ISN
MOD ICE BTN FRZLVL AND FL180. FRZLVL SFC-100. CONDS CONTG BYD 21Z
THRU 03Z.
And the corresponding graphic (Valid 1500Z, 12/1/10):
http://aviationweather.gov/data/products/gairmet/combined/201012011500_us_ICE.gif
I interpret that upper number is the max altitude for icing, and the lower set is the minimum boundary. In this case, the freezing level varies between the surface and 10,000 ft. The freezing level can be ascertained via graphic or via this:
FRZLVL...RANGING FROM SFC-130 ACRS AREA
MULT FRZLVL BLW 070 BOUNDED BY 40SE YQL-50NNW ISN-40WSW SNY-
40SSW CYS-30NNE LAR-40SW BIL-30NNE BIL-40SE HVR-40NW GTF-
40SE YQL
MULT FRZLVL BLW 110 BOUNDED BY 30WNW HVE-30SSE HVE-DVC-70ESE
RSK-20W FTI-60SE ABQ-20SSE INW-20NW TBC-30WNW HVE
MULT FRZLVL BLW 080 BOUNDED BY REO-BAM-30NNE FMG-20ESE LKV-
REO
SFC ALG 20SW FMG-40SSE OAL-50NNE PGS-20NE SSO-50SSE SSO
--------------------------------------
The small Airmet next to it in ND reads:
CHIZ WA 011445
AIRMET ZULU UPDT 2 FOR ICE AND FRZLVL VALID UNTIL 012100
.
AIRMET ICE...ND SD NE
FROM 100WSW YWG TO 40SSW GFK TO 60WSW FSD TO SNY TO 40ESE CYS TO
40NNW ISN TO 100WSW YWG
MOD ICE BLW 150. CONDS CONTG BYD 21Z THRU 03Z.
is just two numbers representing MOD ICE BLW 150.
So, in a nutshell, the top number is the max altitude expected, and the bottom is the range of minimum altitudes expected, depending on the freezing level.
