An inflow band is a low cloud band arranged parallel to the low-level wind and oriented so that it appears to feed into a thunderstorm's updraft. Multiple inflow bands are often visible at once on a well-organized storm, running from the ambient sky into the base of the storm.
How they form
Strong updrafts pull moist boundary-layer air toward the storm from tens of miles away. Where that inflow converges and rises slightly along the way, the air cools to its lifting condensation level and forms low cloud. Because the flow is organized, the resulting cloud tends to line up as a band along the inflow direction rather than as scattered puffs.
The bands get their appearance from the difference between the moist inflow layer and the drier air above it. Where inflow is strong and the low-level air is very humid, the bands are long, well-defined, and continuous. Where inflow is weak or the air is drier, they are broken and diffuse.
How spotters use them
Sharp, well-defined inflow bands that visibly stream into the updraft are a sign that the storm has vigorous low-level inflow. On a supercell, inflow bands that begin to curve as they approach the base can suggest that cyclonic low-level rotation is organizing, and this deserves closer attention to the wall cloud region.
The direction of the bands also confirms the storm's actual inflow direction, which is not always what a nearby observation station reports. This can help chasers position themselves where they can safely observe the updraft.
Important limits
Inflow bands are a strength indicator, not a hazard forecast. A storm can have strong inflow bands and never produce a tornado, and a storm can produce a tornado with only modest visible banding when the moisture profile does not favor prominent low-level cloud.
The pseudo-warm-front-parallel version of an inflow band, broad and flat and running roughly along the boundary between inflow and forward-flank downdraft, is called a beaver tail. It is a specific type of inflow band, not a separate storm feature.