Stupid ASOS Tricks!

Common Problems Due to Instrument Limitation of Automated Observations

This happens for an automated observation (AUTO near front of observation). Note whether the instrument is A01 (AWOS) or A02 (ASOS) as shown right after RMK.

Cloud Limitations

Example C1: Thunder-clear in Minnesota 7 September 2005

Compare the following METAR observations from Saint Cloud with the satellite pictures from the same time

Sky "clear below 12,000 feet" with thunder

KSTC 070953Z AUTO 00000KT 10SM CLR 12/11 A3026 RMK AO2 SLP255 T01220111=

KSTC 071053Z AUTO 00000KT 10SM CLR 13/12 A3027 RMK AO2 SLP255 T01280117=

KSTC 071153Z AUTO 03003KT 10SM CLR 13/11 A3027 RMK AO2 SLP258 T01330111 10150 20117 51009=

KSTC 071253Z 02005KT 10SM CLR 13/11 A3027 RMK AO2 SLP259 T01330111=

KSTC 071318Z 02004KT 10SM TS CLR 13/12 A3029 RMK AO2 TSB12=

KSTC 071330Z 03004KT 10SM TS CLR 14/12 A3029 RMK AO2 TSB12E27B30=

Scattered clouds with a base of 11,000 feet. Unknown above 12,000 feet

KSTC 071353Z 00000KT 10SM SCT110 14/12 A3030 RMK AO2 TSB12E27B30E44 SLP269 T01390117=

KSTC 071453Z 00000KT 10SM CLR 16/12 A3032 RMK AO2 SLP276 T01560117 53016=

The laser in the observing system can only detect clouds to a height of 12,000 feet. Note the 1318Z observation that reads thunder and clear. Note also the 1353Z observation where the laser does detect a scattered layer at 11,000 feet above MSL, but we still don't know what's above it.