Sounding describes a plot of the vertical profile of temperature and dew point (and often winds) above a fixed location). Soundings are used extensively in severe weather forecasting, e.g., to determine instability, locate temperature inversions, measure the strength of the cap (capping inversion), obtain the convective temperature,
CAPE (Convective Available Potential Energy) and CIN (Convective Inhibition).
Soundings a usually carried out with an instrument called radiosonde an the results are usually plotted in a so-called skew-t / log (p) diagram.
Horizontal lines represent height in pressure coordinates (millibars or hPa); diagonal lines represent temperature. Heavy solid lines show the vertical profile of observed temperature (red) and dew point (blue). The grey line shows the temperature of a parcel of surface air and how its temperature would change if it would be lifted.