How to read a skew-T diagram
Skew-T log-P diagrams are how meteorologists visualize the atmosphere in the vertical. They look intimidating. They aren't — once you understand the four lines that matter.
The four line types
A skew-T has four families of lines that never change:
- Isobars — horizontal lines. Pressure decreases upward. 1000 mb at the surface, 100 mb near the tropopause.
- Isotherms — diagonal lines skewed 45° right. Warmer to the right, colder to the left. "Skewed" is what puts the T in "skew-T."
- Dry adiabats — curved lines showing how an unsaturated parcel cools as it rises.
- Moist adiabats — curved lines showing how a saturated parcel cools as it rises. Cools slower than dry because condensation releases latent heat.
The two lines that get plotted
A weather balloon sends back two continuous curves — one for temperature, one for dew point.
- Temperature (T) — always the rightward curve.
- Dew point (Td) — always the leftward curve.
- The gap between them indicates humidity. Touching = saturated (cloud). Wide apart = dry.
The critical levels
- LCL — Lifted Condensation Level. Where the parcel becomes saturated if lifted. Marks cloud base.
- LFC — Level of Free Convection. Where a lifted parcel becomes warmer than its environment and starts accelerating upward on its own.
- EL — Equilibrium Level. Where the rising parcel is finally cooler than its surroundings. Approximate storm top.
Reading CAPE and CIN
CAPE (Convective Available Potential Energy) is the area between the parcel path and the environmental temperature above the LFC — literally an area on the diagram. Bigger area = more instability.
CIN (Convective Inhibition) is the small area below the LFC where the parcel is cooler than its environment — the "cap" that must be broken.
Rough visual test: if the parcel path stays far to the right of the environmental temperature for most of the mid-troposphere, you have big CAPE. If the parcel dips just left of the environment near 850-700 mb, you have a cap.
The hodograph
Attached to most skew-T displays is a hodograph — a plot of wind vector tips at different heights. Straight hodograph = splitting supercells. Curved (clockwise in the Northern Hemisphere) = right-moving supercells favored. Sickle-shaped low-level curve = high tornado potential.
Practice reading real soundings
- SPC soundings page — spc.noaa.gov/exper/soundings/
- University of Wyoming atmospheric soundings — weather.uwyo.edu/upperair/sounding.html
- Iowa State Environmental Mesonet