The updraft base and the precipitation

Why the name changed

You were probably taught "rain-free baseThe smooth, flat, dark cloud base on the rear flank of a storm, where warm air is flowing in and rising. It is named for the air going up, not for being dry: hail and large drops may still fall from it.". 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 updraftThe column of rising air inside a storm. Everything a thunderstorm does, from hail to tornadoes, is downstream of how strong and how organised this is. 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 supercellA thunderstorm with a single rotating updraft that can persist for hours. The storm type that produces most strong tornadoes, very large hail and the most damaging winds.. Module 2 called this the forward-flank downdraftForward-Flank DowndraftThe main precipitation area of a supercell, ahead of and to the left of the updraft. Heavy rain and most of the hail fall here..

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 shaftThe visible column of rain falling between the cloud base and the ground. At a distance one can be mistaken for a wide tornado. carrying hail, often looking whiter, brighter or faintly turquoise-green against a grey rain shaft. Sizing hail is module 5; recognising 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 footA horizontal bulge near the ground at the bottom of a precipitation shaft, where the downdraft has hit the surface and spread outward. It is a visual sign of a wet microburst.

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 microburstA downburst arriving inside heavy rain, which is the dominant damaging-wind mode in the humid Southeast. The rain hides it, so the warning you get is short.."

That is module 2 lesson 4 seen from the ground. The shaft is splaying outward at the bottom because the downdraftAir descending through and out of a storm, driven by the weight of the precipitation and by rain evaporating into drier air below. It is what produces the gust front and the damaging winds. has hit the surface and has nowhere left to go. It is a wind report, and it is often a very short-notice one.