How to Read a Hodograph

In short: a hodograph is the wind profile drawn as one curve. Its length is how much shear there is. Its shape is how the wind turns. The storm’s motion is a point on the same chart, and the relationship between that point and the curve near the ground is most of what decides whether a supercell has the right low-level spin for a tornado.

The left half of a Skew-T tells you about fuel. The hodograph beside it tells you about wind, which decides what the fuel turns into. Read Wind Shear & Helicity first if shear and SRH are new.

Building the curve

Draw the wind at each height as an arrow starting from the center of the chart. The arrow points where the air is going, and its length is the speed, read off the rings. Then drop the arrows and keep only their tips, joined from the ground up. That curve is the hodograph.

Two habits make the chart readable fast:

• The center is calm. A wind of 30 kt from the southwest plots as a dot 30 kt out toward the northeast, because the arrow points where the air is going.
• Any line between two dots is shear. The longer the line, the stronger the shear in that layer. The whole chart is a stack of these.

What the shape says

Where the storm sits on the chart

A storm’s motion can be plotted on the same chart as a single point. The wind a storm feels is the difference between the real wind and its own motion: an arrow from the storm point to the wind at each height.

Supercells do not move with the average wind. The usual estimate, the Bunkers method, takes the mean wind through 0 to 6 km and steps about 7.5 m/s (15 kt) to the right of the shear for the right mover, and to the left for the left mover. On a clockwise-curved hodograph the right-mover point lands below and to the right of the curve, and the area between them, the shaded fan in the diagram, is large. That area is storm-relative helicity.

Watch for storms that turn harder right than the estimate. Moving the storm point further right and down on the chart sweeps a bigger area, so a deviant right mover can be ingesting much more helicity than the map’s number says.

In MesoRadar: the sheet's Wind rows include RM moving to; the More fold adds LM moving to, SRH 0-3 (LM) and Mean wind from (Pro).

Storm-relative wind

Re-plot every wind as the storm sees it, with the storm’s motion subtracted, and you get a storm-relative hodograph. It answers practical questions directly:

• Inflow at the ground: the arrow from the storm point to the ground wind. Strong inflow, 20 kt and more, feeds the updraft well.
• Upper-level wind relative to the storm: decides where rain and hail fall out. Very strong storm-relative wind aloft blows precipitation far from the updraft, the low-precipitation look. Moderate wind gives the classic look. Weak wind drops it back around the updraft, which wraps rain around the rotation, the high-precipitation look in Hook Echoes & Tornado Signatures. Rasmussen and Straka (1998) put rough numbers on it: anvil-level storm-relative wind above about 54 kt (28 m/s) with LP storms, 35 to 54 kt with classic storms, and below about 35 kt (18 m/s) with HP storms. Treat them as bands, not cutoffs.

In MesoRadar: the SR wind view, relative to the right mover, marks the 0-2, 4-6 and 9-11 km AGL means; the 9-11 km mean is the anvil-level number. The Wind by layer table lists SR wind for each layer.

The critical angle

Esterheld and Giuliano (2008) measured one angle on the chart: between the storm-relative inflow at 10 m and the 0 to 500 m shear. Near 90 degrees, the near-ground spin points straight into the storm, all of it streamwise. In their Oklahoma sample, tornadic supercells, especially the significant ones, clustered near 90 degrees and non-tornadic ones did not.

The sample was small (67 events). A much larger later study (Coffer and colleagues 2019) found the angle peaked near 60 degrees for both tornadic and non-tornadic supercells; only very large angles (over 120) or very small ones (under 30) pointed away from tornadoes. Treat it as a tiebreaker at most, on a day that already has shear, low cloud bases and discrete storms.

In MesoRadar: the Critical angle row, under the sheet's More fold (Pro).

Streamwise spin: SW% and SWV

Shear stores spin in the air. Storm-relative helicity counts how much of that spin a moving storm can draw into its updraft. But SRH mixes three things together: how strong the inflow is, how strong the spin is, and how well the two line up. Two numbers pull the alignment out.

• Streamwise vorticity (SWV) is the part of the horizontal spin that points along the storm-relative inflow, measured as a rate, per second. Picture the spiral on a thrown football. Spin along the path is streamwise. Spin across the path, like a log rolling sideways, is crosswise. Only the streamwise part enters the updraft already rotating about the way it is moving.
• SW% is the share of the spin that is streamwise. 100% means all of it points into the storm. Near zero means it rolls across the inflow, which helps the storm’s updraft much less.

Why it is useful: a layer can have large SRH only because the inflow is very fast, while most of the spin rolls across it. Another layer can have modest SRH but nearly pure streamwise spin. Coffer and colleagues (2019), using more than 20,000 right-moving supercells, found that SRH in the lowest 500 m separated significantly tornadic supercells from others better than deeper layers. In that study, the alignment numbers added little beyond SRH itself, so read SW% and SWV as a way to see why SRH is large, not as a separate signal. Like every number here, they describe the environment a storm would meet, not what a given storm will do.

The Wind by layer table’s 0-500 m row is that layer. On a model sounding it can rest on only two or three wind levels; the table marks it with an asterisk when it does.

In MesoRadar: the Wind by layer table, under More (and under the chart when the 3D view is expanded), has an SW% column beside SRH, and an SWV column on wider screens such as an iPad. A shared sounding card adds SW% 0-1/0-3 to its Wind group (Pro).

The 3D view

The flat chart hides one thing: height. A dot at 1 km and a dot at 3 km sit on the same sheet of paper, and a curve that turns sharply in the lowest 500 m looks no different from one that turns over 3 km unless you read the labels.

The 3D view lifts the same wind column off the floor so height is a real axis. Each kilometer sits at its true height, with a dashed drop to its spot on the rings, and the right mover’s motion stands as a column carrying the height marks. Turning it shows at a glance where in the column the wind does its turning, which is exactly what the near-ground research says to look at.

In MesoRadar: the hodograph's three views are Hodograph, SR wind and 3D. Expand the 3D view and drag to turn it (Pro).

A quick read of the hodograph

1. How long is it? Long means strong shear: organized storms.
2. Which way does it bend in the lowest 1 to 3 km? Clockwise favors the right mover.
3. Where is the right-mover point, and how big is the area under the low end of the curve? That is SRH.
4. How strong is the inflow at the ground, and how well does the near-ground spin line up with it? Critical angle, SW%, SWV.
5. What does the storm-relative wind do aloft? Decides classic or high-precipitation.