How to Read a Skew-T

In short: a Skew-T draws one column of the atmosphere. The right-hand line is temperature, the left-hand line is dew point, and the dashed line is where a rising bubble of air would go. Read it bottom to top: how much moisture is near the ground, whether a warm lid sits over it, and how much warmer a rising parcel stays than its surroundings on the way up.

Every forecast number in CAPE & Instability comes off this chart. The SPC, every NWS office, and every serious chaser read it before a severe weather day.

The chart in one picture

The lines behind the traces

The background grid looks busy. Four main kinds of line do the work:

• Pressure levels, horizontal. Height is shown as air pressure, which falls as you climb. The 500 mb line sits near 18,000 feet. The scale is logarithmic, which is the “log-P” in the full name, Skew-T log-P.
• Isotherms, tilted 45 degrees to the right. To read the temperature at any level, follow the tilted line down, not straight down. Without the tilt, both traces would lean so far left the chart would be useless.
• Dry adiabats, curving up and to the left. The path unsaturated rising air follows: it cools about 9.8 °C per kilometer.
• Moist adiabats, curving more gently. The path saturated rising air follows. It cools more slowly, because water vapor condensing into cloud releases heat.

Many charts draw only some of these, faintly, so the two traces stay readable.

Temperature, dew point and the gap between them

The gap between the two traces is the dew point depression. It is the fastest read on the chart.

• Traces together means saturated air, usually a cloud layer. A deep layer of touching traces from the ground up is a soupy, rainy day.
• Traces together near the ground, then far apart above is the classic spring setup: humid air from the Gulf under dry air from the west.
• Traces far apart near the ground, together aloft is the inverted V: a moist layer high up over a deep dry layer near the surface. Rain falling into that dry layer evaporates, chills the air, and drives strong downdrafts. It is the high-plains signature for dry microbursts and big outflow winds.

The parcel line

The dashed line is the path a lifted parcel would follow: up a dry adiabat until it saturates at cloud base, then up a moist adiabat. Where it runs right of the temperature trace, the parcel is warmer than its surroundings and rises on its own. The area between the two is CAPE. Where it runs left, the parcel has to be forced: that area is CIN.

Which parcel is drawn changes the picture. A surface parcel, a mixed-layer parcel and a most-unstable parcel can give very different answers on the same chart. The CAPE guide explains which to read when.

Spotting the cap

A cap, or inversion, is a layer where temperature stops falling with height, or rises. On the chart, the temperature trace kinks to the right. Usually the dew point drops sharply away at the same level, because the warm air on top came from somewhere dry.

On a spring Plains day the cap is often a plume of hot, dry desert air a mile or so up. Underneath sits humid air from the Gulf. Forecasters call this a loaded gun sounding: big CAPE locked under a lid. If something breaks the lid in one spot, a storm there gets the whole air mass to itself. If nothing does, the day ends sunny.

Steep lapse rates above the cap, the temperature trace leaning hard to the left from about 700 to 500 mb, are part of the same plume. The SPC flags 700 to 500 mb lapse rates as one sign of where deep storms are more likely.

On the day, two checks tell you whether the cap is going. In the hourly model frames, watch CIN shrink where the front, dryline or outflow will be by mid-afternoon. In the sky and on visible satellite, look for towering cumulus that keep building in one place along that boundary instead of going up and shearing apart. A sounding can also give a convective temperature, the surface temperature at which a surface parcel breaks the cap without any lift; if thermometers are well short of it by mid-afternoon, something else has to do the lifting.

Winter: the warm nose

In winter the question is not storms but what falls. Follow the temperature trace against the 0 °C isotherm:

• Below freezing all the way down: snow.
• A layer above freezing aloft, deep enough to melt the snow, over a shallow freezing layer at the ground: freezing rain. The drops fall as rain and freeze on contact.
• The same warm layer over a deep freezing layer near the ground: sleet. The drops refreeze into ice pellets before landing.

That bulge above freezing is called the warm nose. A degree or two of difference in it decides between inches of snow and a glaze of ice. Rough guides: a warm nose whose wet-bulb peaks under about 1 °C (34 °F) rarely melts snow fully, so the flakes usually land as snow or refreeze into sleet; above about 3 °C (37 °F) it usually melts them all. Partly melted flakes refreeze into sleet easily; fully melted drops usually need a cold layer near the ground of about -10 °C (14 °F) or colder, or they land as freezing rain. The cold layer’s temperature matters more than its depth. Snow can still reach the ground with the surface at 1 to 2 °C (34 to 36 °F) if the air is dry enough for melting to cool it.

PWAT, freezing level and wet-bulb zero

Three numbers below the chart summarize it for rain and hail:

• PWAT, precipitable water: all the water vapor in the column, as the depth of water you would get if it all rained out at once. High PWAT for the place and season means storms can produce very heavy rain. See Flash Flooding.
• Freezing level: the height of 0 °C. A lower freezing level lets more hail reach the ground before it melts.
• Wet-bulb zero: the height where air cooled by evaporation would reach 0 °C. Forecasters use it alongside the freezing level for hail. See Hail. In winter it matters for a different reason: once precipitation starts, evaporation cools the air toward its wet-bulb temperature, so a low wet-bulb zero means rain can change to snow even when the thermometer says it should not.

Balloon soundings and model soundings

A radiosonde is a small instrument package carried up by a weather balloon. It measures temperature, humidity and pressure on the way up, and its drift gives the wind. The NWS network has 92 sites, 69 of them in the lower 48 states. Most launch at 00Z and 12Z, though some sites have cut back to one launch a day or paused since 2025, and extra launches are added on some severe days. A balloon measures the real air, but only at those places and times.

A model sounding is the same chart drawn from a forecast model at any point and any hour. It fills every gap between balloons. It also inherits the model’s errors: low-level moisture and the strength of the cap are where models most often go wrong, and those are exactly what decide whether storms form. When a balloon launch is near and recent, compare the two.

In MesoRadar: open Map tools, choose Skew-T, then tap anywhere on the map for a HRRR sounding, or on an amber dot for the observed balloon sounding (Pro). The table under the chart lists SBCAPE, MLCAPE, MUCAPE, ML CIN, LCL / LFC / EL, the shear and helicity rows, STP, SCP, PWAT and Freezing level. The More fold adds DCAPE, the effective inflow layer, the hail rows, lapse rates and Wet-bulb zero. The wind half of the sheet is in How to Read a Hodograph.

A six-step read

1. Bottom 100 mb: how humid? Dew points close to the temperature, and high in absolute terms, mean fuel.
2. Check the model against the morning balloon. Open the nearest amber dot for the 12Z launch, then tap the same spot for the HRRR’s Now hour. The tool starts at Now, so do this early in the morning, while the two are only an hour or two apart. If the model is too moist near the ground, or its cap is too weak or too strong, carry that error forward when you read its afternoon hours.
3. Above the bottom 100 mb: is there a kink to the right? That is the cap. How big is it, and is anything coming to break it?
4. The parcel: how far right of the temperature trace does it run, and through how deep a layer? That is CAPE.
5. Dry air: an inverted V near the ground or a dry slot aloft both point to downdraft wind.
6. The winds: the barbs on the right edge and the hodograph tell you what kind of storm the fuel will build.