What is a shelf cloud?

A shelf cloud is a low, horizontal, wedge-shaped arcus cloud attached to the base of a thunderstorm, formed where the storm’s gust front lifts the air ahead of it. It marks rain-cooled air moving out of the storm, and it usually arrives just ahead of that air’s wind and rain.

The overview of squall lines, bow echoes and derechos describes the gust front as the wind arriving before the rain. This page covers the cloud that often sits on that boundary, and what the radar shows of the boundary when it cannot show the cloud.

The definition

The National Weather Service glossary defines a shelf cloud as “a low, horizontal wedge-shaped arcus cloud, associated with a thunderstorm gust front (or occasionally with a cold front, even in the absence of thunderstorms).” It adds that the shelf cloud “is attached to the base of the parent cloud above it (usually a thunderstorm),” and that “rising cloud motion often can be seen in the leading (outer) part of the shelf cloud, while the underside often appears turbulent, boiling, and wind-torn.”

Two related terms from the same glossary set the context. A gust front is “the leading edge of gusty surface winds from thunderstorm downdrafts; sometimes associated with a shelf cloud or roll cloud.” An outflow boundary separates thunderstorm-cooled air from the surrounding air and is “similar in effect to a cold front.”

How it forms

Rain and evaporation chill the air in a thunderstorm’s downdraft. That dense air reaches the ground and spreads out as a cold pool. The NSSL Severe Weather 101 page describes the gust front as “the leading edge of rain-cooled air that clashes with warmer thunderstorm inflow,” marked by a wind shift, a temperature drop and gusty winds ahead of the storm, and adds: “Sometimes the winds push up air above them, forming a shelf cloud or detached roll cloud.”

The outflow behaves like a gravity current. Klingle and Smith (1985), reviewing gust-front structure for the Indiana Academy of Science, describe the cooler, denser outflow intruding into the warmer environment so that “the warm air is lifted up and over the outflow boundary.” Air inside the outflow moves faster than the boundary itself and piles up into a raised “head” at the leading edge. When the air being lifted over that head is moist enough, it condenses, and the cloud takes the shape of the wedge beneath it.

Shelf cloud or roll cloud

Both are arcus clouds. The NWS glossary defines arcus as a low, horizontal cloud formation along the leading edge of thunderstorm outflow, and names shelf and roll clouds as its two types. The difference is attachment.

• A shelf cloud is wedge-shaped and joined to the storm base above it.
• A roll cloud is “a low, horizontal tube-shaped arcus cloud” that is “completely detached from the thunderstorm base or other cloud features.” The glossary calls them relatively rare.

Shelf cloud or wall cloud

This is the confusion that matters for reports, because the two mark opposite halves of a storm. The wall cloud guide sets out the airflow, location, shape and motion side by side. The NWS’s Advanced Spotters’ Field Guide adds cues that work over several minutes of watching:

• What it signifies. “Shelf clouds signify an area of downdraft and outflow while wall clouds indicate an area of updraft and inflow.”
• How it moves relative to the rain. Watched for several minutes, a shelf cloud tends to move away from the precipitation area. A wall cloud tends to keep its position relative to it.
• Which way it slopes. Shelf clouds tend to slope downward away from the precipitation. Wall clouds tend to slope upward away from it.

The same guide warns that a shelf cloud can form under the rain-free base and look like a wall cloud, and that a small shelf cloud can form along the clear slot of a rear-flank downdraft near a real one.

What spotters watch for

A shelf cloud is a sign of outflow, and the wind is the report. NWS Louisville’s spotter material says the main threat with a squall line is “severe damaging winds associated with the shelf cloud,” while noting that brief spin-up tornadoes can occur. The glossary’s gustnado, a usually weak and short-lived vortex along the gust front, is “more likely to be associated visually with a shelf cloud than with a wall cloud.”

The SKYWARN Weather Spotter’s Field Guide (2011) asks spotters to report “estimated or measured wind speed and wind damage,” and notes that “a detailed description of moving objects or damage is often more useful” than a speed estimate.

What radar shows at the leading edge

Radar does not show the shelf cloud. It can show the gust front the cloud rides on, in two ways.

A fine line in reflectivity. Out ahead of the precipitation, the gust front often appears as a thin arc of weak echo. The American Meteorological Society’s Glossary of Meteorology attributes such fine lines to scattering from refractive-index gradients across the boundary and from insects, and the birds that eat them, concentrated along it. Klingle and Smith (1985) give the same two explanations. Wilson and colleagues (1994), comparing several radars in different regions, concluded that insects are the main source of clear-air echo in the mixed boundary layer. The NWS office in Austin/San Antonio describes the practical version in its review of the 24 October 2022 south-central Texas storms: gusty winds “pick up dust, leaves, bugs, and other particles, which register as weak power returns,” and in correlation coefficient those non-meteorological targets stand apart from rain and hail.

A shear line in velocity. Behind the fine line, the outflow wind shows as a change in radial velocity across the boundary. In a set of ten gust fronts Klingle and Smith summarize, a thin line echo appeared in seven and a line of radial shear in nine. The shear was greatest where the gust front lay perpendicular to the beam — that is, where the outflow blew along the beam. They also found the thin line appears only once the gust front separates from its parent storm, while the shear line can be seen at any stage. Their conclusion is worth keeping: relying on reflectivity alone is unwise, because low-reflectivity outflow can carry significant wind shear.

Where it sits in a line or a bow

The SKYWARN field guide describes a multicellular line as having “a continuous, well developed gust front along the leading edge,” and NWS Louisville ties shelf clouds to “a solid line of storms.” When a segment of the line surges forward into a bow echo, it is the outflow that is surging, and the gust front, with any shelf cloud riding on it, pushes out fastest where the wind behind it is strongest. The field guide’s own example is a KTLX image from 7 September 2001 showing a bow echo in an unbroken line “with a gust front leading the line south of the bow.”

The distance between the fine line and the storms is informative. In the Texas case, NWS Austin/San Antonio found most of the convection in three counties sat behind the gust front, and read that as storms “undercut by cold, outflowing air, which acts to cut their updrafts off from the surface.”

WeatherStory Radar decodes reflectivity, velocity and correlation coefficient from full-resolution Level II, so the fine line and the shear behind it can be read gate by gate.