seasonmap field guide

How to read a skew-T sounding

A skew-T is the atmosphere in cross-section: one vertical column of temperature, moisture, and wind, from the ground to the stratosphere. It looks intimidating and it is not. Five things carry almost all of the information.

1. The two traces: temperature and dewpoint

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 — that tilt is the "skew-T" and it exists so that typical profiles stand up tall instead of leaning into the corner of the chart.

The right trace is temperature, the left trace is dewpoint. Everything starts with the horizontal gap between them: where they hug, the layer is saturated — cloud, fog, or precipitation. Where they spring apart, the air is dry. A classic storm-chaser sounding has them touching in the lowest kilometer (rich boundary-layer moisture) and gaping in mid-levels (dry air aloft that fuels strong downdrafts).

2. CAPE and CIN: the fuel and the lid

Lift a parcel of surface air upward and compare it to the environment. Wherever the parcel is warmer than its surroundings it accelerates upward — the area between the parcel curve and the temperature trace is CAPE (convective available potential energy), the fuel gauge for thunderstorms. A few hundred J/kg supports garden-variety storms; 3,000+ is a loaded spring.

CIN is the same area where the parcel is colder than the environment — usually a warm layer just above the boundary layer, the "cap." A modest cap keeps storms from firing too early, letting instability build all afternoon; a strong cap ends the show entirely. Big CAPE beneath a breakable cap is the setup severe forecasters watch closest.

3. Inversions: where temperature goes the wrong way

Any segment where temperature rises with height is an inversion, and each kind tells its own story. A surface-based inversion on a winter morning traps cold air (and fog, and pollution) in the valley. A frontal inversion — warm air riding over cold — is the signature of freezing rain: snow melts in the warm nose, then refreezes on contact below. When you see a p-type forecast paint an ice swath, this is the vertical structure behind it.

4. The dendritic growth zone

Snowflakes — the big, fluffy, stacking kind — grow fastest where the cloud is between −12°C and −18°C and saturated. Winter forecasters live inside that band: if strong upward motion runs through a saturated dendritic growth zone, snow-to-liquid ratios climb and totals overperform. seasonmap shades the DGZ automatically on every sounding, so the check takes one glance: are the traces together inside the shaded band?

5. The hodograph: wind with height

The small companion plot traces the wind vector as you rise. A short, crumpled hodograph means weak shear — storms that rain themselves out. A long, smoothly curving hodograph means the updraft will ingest winds that veer with height and begin to rotate: supercell territory. Length is shear magnitude, curvature is where the rotation comes from.

The fastest way to learn is volume: look at soundings on quiet days and stormy days until the shapes become familiar. On seasonmap, pick a model layer and click anywhere on the map — you get the full skew-T with hodograph for that point, for GFS, the AI models, and our in-house runs, with hover readouts at every level.