Air being pulled into a storm makes clouds of its own, and two of them are long, low and sinister-looking from the wrong angle.

In this lesson you will

  • Distinguish a tail cloud, attached to the wall cloud, from a beaver tail, attached to the updraft base and higher.
  • Identify inflow bands, and state that cyclonically curved bands may indicate a mesocyclone.
  • Explain that inflow-band cloud motion is toward the updraft.

Inflow bands

The Field Guide describes inflow or feeder bands as "arranged parallel to the low-level winds and moving into or toward a thunderstorm". They are the visible signature of the storm drawing its fuel in.

And then it gives you something to act on: "Spotters should be especially wary of inflow bands that are curved in a manner suggesting cyclonic rotation; this pattern may indicate the presence of a mesocyclone."

Teaching point
Direction of motion is the tell that costs nothing to check. Inflow-band cloud moves toward the storm. If the low cloud you are looking at is traveling away from the storm, you are looking at outflow, and lesson 5 is about you.

Two long features, one difference

Tail cloud
A horizontal cloud extending from the precipitation cascade region toward the wall cloud, with its base at about the same height as the wall cloud. Motion is toward the wall cloud, with rapid upward motion where the two meet.
Beaver tail
A broad, flat inflow band attached to the updraft base, so its base is noticeably higher than a wall cloud, oriented roughly along the pseudo-warm front. Slang rather than a formally defined term, though the NWS glossary carries it.

Attachment point and base height separate them. A tail cloud joins the wall cloud and sits at wall-cloud height. A beaver tail joins the updraft base and sits higher. Once you have lesson 6, that difference is easy; before it, the two look like the same long ragged thing.

Neither is reportable on its own. Both are context: they tell you where the inflow is and therefore where the updraft is, which is where you should be looking.

Why this lesson exists

NWS Northern Indiana published a page specifically about low hanging clouds being mistaken for funnel clouds and tornadoes, and beaver tails and scud are what it is about. A long horizontal cloud viewed end-on, from several miles away, in poor light, looks very much like something descending from the base. Lesson 11 is the full treatment of that problem. The short version: a feature that is horizontal in reality can look vertical from the wrong angle.

Check yourself

You see a long, low, ragged cloud extending away from a storm base. What single observation most quickly tells you whether it is an inflow feature?

  1. Which way its cloud material is moving. Inflow moves toward the storm
  2. Its color. Inflow features are darker than outflow features
  3. Whether it is attached to the base. Only inflow features attach
  4. How ragged the edges are. Inflow bands have smooth edges
Hint

One of these you can check in about fifteen seconds without knowing anything else about the storm.

Answer

Which way its cloud material is moving. Inflow moves toward the storm

Motion. Inflow-band cloud travels toward the updraft, because that is the air being pulled in. Outflow features travel away from the storm, ahead of it. Color, raggedness and attachment all vary too much between storms and lighting conditions to decide it on their own.

Sources

  1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), glossary I (inflow bands) and T (tail cloud)
  2. NWS online glossary (beaver tail, listed as slang)
  3. NWS Northern Indiana, "Low hanging clouds mistaken for Funnel Clouds and Tornadoes" (August 2013)