A Skew-T Log-P diagram is a thermodynamic chart that plots an atmospheric sounding with temperature lines tilted about 45 degrees to the right and pressure decreasing logarithmically up the y-axis, so that stability, moisture, and parcel behavior are all readable at a glance. The environmental temperature and dewpoint are drawn on top of a background of dry adiabats, moist adiabats, mixing ratio lines, and isotherms.
Why the axes are strange
The logarithmic pressure axis gives each layer of the atmosphere vertical space proportional to its geometric depth. On a linear pressure scale the surface layer would compress into a thin strip and the stratosphere would dominate the plot. On a log scale, the troposphere gets room to breathe.
Skewing the temperature lines to the right does two things. It spreads the temperature and dewpoint curves apart in a typical tropospheric profile so a forecaster can actually see them, and it aligns dry adiabats and moist adiabats so that a lifted parcel path can be drawn as a natural curve. The combination is what makes CAPE and CIN visible as areas rather than integrals a person has to compute mentally.
What a forecaster reads first
The two curves closest together tell a moisture story. Where the temperature and dewpoint lines are nearly on top of each other, the layer is saturated or nearly so, and clouds are likely. A large spread aloft, especially between about 700 and 500 hPa, points to a dry mid-level layer that can support strong evaporational cooling in downdrafts.
Lapse rate is the slope of the temperature curve relative to the dry adiabats. A steep slope through the mid-levels indicates a rapid drop of temperature with height and steeper conditional instability, which supports stronger updrafts if a parcel can reach that layer.
The parcel path is then drawn upward from a chosen starting point. It follows a dry adiabat until it saturates at the LCL, then follows a moist adiabat above that. Where the parcel path crosses back to the right of the environmental temperature curve, the parcel becomes positively buoyant at the LFC. Where they meet again near the tropopause is the equilibrium level. The enclosed positive area is CAPE. The negative area below the LFC is CIN.
How forecasters use it
Reading a Skew-T is a rapid ingredient audit. Moisture in the boundary layer, an inversion or cap, the depth and steepness of the elevated mixed layer, mid-level dryness, and the height of the LCL are all read in the first few seconds. Wind barbs alongside the chart give the shear profile and feed the hodograph.
The chart is also a scenario tool. Modifying the surface parcel for expected afternoon heating and moistening, then re-lofting it, gives an estimate of afternoon CAPE and cap strength. Comparing a morning observed sounding with an afternoon model sounding at the same site is a standard way to check whether models are handling the boundary layer sensibly.
Where the chart misleads
A Skew-T shows an environment, not a storm. It assumes an undiluted parcel with no entrainment and no water loading, and real updrafts fall short of pure parcel CAPE by an amount that varies with the environment and the storm itself.
The chart also inherits every limitation of its source sounding. A twelve-hourly RAOB, a model column, or a VAD wind profile each carries its own biases and resolutions. A crisp-looking cap on the plot may be broken by a boundary that never appeared in the sample. Treat the Skew-T as a strong hypothesis about the column, and pair it with surface data, satellite trends, and the nearest real observations before making a call.