Two regions under a storm, doing opposite things. Getting the boundary between them right is what makes every later lesson possible.

In this lesson you will

  • Identify the updraft base and the precipitation core in a photograph.
  • Locate the precipitation shaft and a hail shaft, which is often white or green-tinged.
  • Define the rain foot and state that it indicates a wet microburst or strong outflow.
  • Explain why "updraft base" is preferred over "rain-free base".

Why the name changed

You were probably taught "rain-free base". The Field Guide glossary now carries the correction itself: the base "may not actually be rain free; hail or large rain drops may be falling. For this reason, updraft base is more accurate."

This is not pedantry. A spotter who believes the updraft region is literally rain-free will discount the right part of the storm the moment a few large drops start falling on them, and will look somewhere else for the feature they should have been watching. The base is defined by what the air is doing, which is going up, not by whether anything is coming out of it.

Use "updraft base" in reports. If somebody on the net says "rain-free base", they mean the same thing.

The two regions

Updraft base
Smooth, flat, dark, and comparatively free of precipitation, on the rear flank of the storm. Warm air is flowing in underneath it and rising. Everything in lessons 4, 6 and 7 that you might have to report happens here.
Precipitation core
Where the rain and hail are actually falling out, ahead of and to the side of the updraft in a classic supercell. Module 2 called this the forward-flank downdraft.
Precipitation shaft
The visible column of falling rain between cloud base and the ground. From a distance a shaft can be mistaken for a wide tornado, which lesson 7 and lesson 11 both come back to.
Hail shaft
A precipitation shaft carrying hail, often looking whiter, brighter or faintly turquoise-green against a gray rain shaft. Sizing hail is module 5; recognizing the shaft is here.

The separation between these two is what makes a classic supercell readable. The precipitation falls out over there, the updraft is over here, and you can stand somewhere with a clear view of the second. Lesson 9 is largely about what happens to your job when that separation disappears.

The rain foot

The NWS glossary defines a rain foot as "a horizontal bulging near the surface in a precipitation shaft, forming a foot-shaped prominence. It is a visual indication of a wet microburst."

That is module 2 lesson 4 seen from the ground. The shaft is splaying outward at the bottom because the downdraft has hit the surface and has nowhere left to go. It is a wind report, and it is often a very short-notice one.

Watch the definition on this one
At least one NWS office handout has garbled "rain foot" into the rain-free base definition, and the error propagates through training material that copies from it. The correct definition is the one above: a bulge in a precipitation shaft, near the ground, meaning outflow. If you meet a version that describes a cloud base, it is wrong.

Check yourself

A storm is dropping large hail out of the flat, dark base you have been watching for rotation. Does that mean you were watching the wrong part of the storm?

  1. No. The updraft base is named for the air going up, and hail or large drops can still fall from it
  2. Yes. By definition a rain-free base has no precipitation falling from it
  3. Yes. Hail falls from the forward flank, so this must be the precipitation core
  4. It cannot be determined without knowing the storm motion
Hint

The NWS changed this term for exactly this reason.

Answer

No. The updraft base is named for the air going up, and hail or large drops can still fall from it

This is why the NWS prefers "updraft base". The Field Guide says the base "may not actually be rain free; hail or large rain drops may be falling". The region is defined by the air rising into it, not by what falls out of it, and a spotter who abandons it at the first hailstone has just stopped watching the part of the storm that produces tornadoes.

Sources

  1. NWS Weather Spotter's Field Guide (YPA-201154, June 2011), glossary R (rain-free base)
  2. NWS online glossary (rain foot)
  3. NWS Fort Worth, Spotter Handout 2022, and the Storm Spotter's Dictionary