March 9, 2000
Below is a table which should help simplify the explanation of the ASOS present weather algorithm processing. The algorithm uses input from the Light Emitting Diode Weather Identifier (LEDWI), the frequency from the freezing rain sensor, the 10-minute harmonic average visibility, the dew point depression computed from the 5-minute average ambient and dew point temperatures, and the 5-minute average ambient temperature. Based on these parameters, ASOS will evaluate the data and provide an automated present weather entry (i.e., both precipitation and/or obscurations) to be included in the METAR/SPECI reports.
| LEDWI Output | Freezing Rain Indication | Visibility (VIS) | Dew Point Depression (DD) | Temperature (T) | ASOS PRESENT WX Algorithm Output |
| P | Yes or No | NA | NA | T > 38F | -RA |
| P | Yes | NA | NA | T = 37F | UP |
| P | Yes | NA | NA | 0F < T < 36F | -FZRA |
| P | No | NA | NA | 0F < T< 38F | UP |
| P | Yes or No | NA | NA | T < 0F | UP |
| R | Yes | NA | NA | T > 36F | RA |
| R | Yes | NA | NA | 0F < T < 36F | FZRA |
| R | No | NA | NA | T > 33F | RA |
| R | No | NA | NA | 0F < T <32F | BLSN, UP, or No
Entry (See BLSN Algorithm Paper - 10/22/98 ) |
| R | Yes or No | NA | NA | T < 0F | No Entry |
| S | Yes or No | NA | NA | T > 38F | No Entry |
| S | Yes or No | NA | NA | T < 38F | SN |
| R, S, None | Yes or No | 5/8 SM < VIS < 7 SM | DD < 4F | Any T Value | BR |
| R, S, None | Yes or No | VIS < 5/8 SM | DD < 4F | T > 32F | FG |
| R, S, None | Yes or No | VIS < 5/8 SM | DD < 4F | T < 32F | FZFG |
| None | Yes or No | VIS < 7 SM | DD > 4F | NA | HZ |
Legend:
P or UP - Light Unkown Precipitation None - No Precipitation Detected from
LEDWI
R or RA - Rain, (-RA is Light Rain) NA - Not Used in Algorithm
S or SN - Snow FG - Fog
FZRA - Freezing Rain, (-FZRA is Light Freezing Rain) FZFG - Freezing Fog
BLSN - Blowing Snow HZ - Haze
BR - Mist