Definition
In a METAR, BL is a qualifier used in the present weather group to indicate that a particle-based weather phenomenon (such as snow, dust, sand, or spray) is being lifted by wind to a height of 6 feet or more above the ground. It is paired with a phenomenon code, for example BLSN (blowing snow), BLDU (blowing dust), BLSA (blowing sand), or BLPY (blowing spray).
Plain English
BL means the wind is picking something up off the ground — snow, dust, sand, or spray — and lifting it high enough to reduce how far you can see. The 'high enough' part is the key: at least 6 feet above the surface.
Context Anchor
Seen in coded airport weather reports, usually combined with the thing being blown, such as blowing snow, blowing dust, or blowing sand.
Derivation
From the ordinary English word 'blowing.' In METAR coding, it is a fixed two-letter qualifier indicating wind-driven lifting of particles, distinct from 'DR' (drifting), which means the same particles are being moved but stay below 6 feet.
Why Pilots Care
Signals reduced visibility and strong surface winds that affect takeoff, landing, and ground operations.
Grounding Statement
Picture loose snow being picked up by strong wind and carried across the runway at windshield height.
Intuition Check
Blowing does not mean simply that the wind is strong. In METAR use, it means the wind is lifting particles like snow, dust, or sand to 6 feet or more above the ground.
Example Sentence 1
The METAR reported BLSN with visibility of 1/2 mile, so the pilot expected reduced surface visibility on the approach.
Example Sentence 2
Before takeoff the pilot noted BLSN in the report and prepared for possible runway obscuration from blowing snow.