seasonmap field guide

How to read a skew-T sounding

A skew-T/log-P diagram plots one vertical column of temperature, moisture, and wind from the surface to the stratosphere. Five elements carry most of the operational information: the temperature and dewpoint traces, CAPE and CIN, inversions, the dendritic growth zone, and the hodograph.

Axes and traces

Pressure decreases upward on a logarithmic axis (1000 mb near the surface, 200 mb near the tropopause), and the temperature axis is skewed 45° to the right — the tilt gives the diagram its name and keeps typical atmospheric profiles running vertically rather than leaning across the chart.

The right trace is temperature, the left trace is dewpoint. The gap between them indicates saturation: where the traces converge, the layer is saturated (cloud, fog, or precipitation); where they diverge, the air is subsaturated. A sounding supporting severe convection typically shows the traces nearly together in the lowest kilometer (moist boundary layer) and several degrees apart in mid-levels (dry air aloft, which strengthens evaporative downdrafts).

CAPE and CIN

Lift a parcel of surface air and compare its temperature to the environment at each level. Where the parcel is warmer, it is positively buoyant; the area between the parcel curve and the temperature trace over that layer is CAPE (convective available potential energy), in J/kg. A few hundred J/kg is sufficient for ordinary thunderstorms; above 3,000 J/kg indicates extreme instability (AMS Glossary; Markowski & Richardson 2010).

CIN (convective inhibition) is the corresponding area where the parcel is colder than the environment, usually a warm layer just above the boundary layer that functions as a cap. Weak CIN lets storms initiate readily; moderate CIN suppresses convection while surface heating builds CAPE through the afternoon; above roughly 100–150 J/kg it typically prevents surface-based initiation. Large CAPE under CIN that daytime heating can erode is the profile behind the most intense convective outbreaks.

Inversions

A layer where temperature increases with height is a temperature inversion, and the type sets the sensible-weather signature. A surface-based radiational inversion on a clear winter night traps cold air, fog, and pollutants near the ground. A frontal (warm-nose) inversion — above-freezing air overriding a subfreezing surface layer — is the vertical structure behind freezing rain: snow melts falling through the warm nose, then refreezes on contact with the subfreezing layer below. Check this profile whenever a precipitation-type forecast shows an ice swath.

Dendritic growth zone

Dendritic snow crystals — the large, low-density, stacking kind — grow fastest in cloud layers between −12°C and −18°C when saturated with respect to ice. When strong upward motion coincides with a saturated dendritic growth zone (DGZ), snow-to-liquid ratios increase and totals run higher than raw QPF implies. seasonmap shades the DGZ on every sounding; check whether the temperature and dewpoint traces sit together inside the shaded band.

Hodograph and shear

The companion plot traces the wind vector through the column. A short hodograph with little curvature indicates weak vertical wind shear, favoring pulse or disorganized convection. A long hodograph with smooth, consistent curvature — winds veering with height — indicates strong shear organized into a rotational sense, the profile associated with rotating, long-lived storms. Length corresponds to shear magnitude; curvature indicates the source of that rotation (streamwise vorticity available to the updraft).

On seasonmap, selecting a model layer and clicking anywhere on the map returns the full skew-T with hodograph for that point — for GFS, the AI models (see the model guide), and the in-house runs — with hover readouts at every level. Comparing soundings across quiet and active days is the fastest way to build familiarity with the shapes. Related: how forecast verification works.