Using Thermodynamic Diagrams in Meteorology

Abstract

Radiosonde measurements, or soundings, are plotted automatically on a thermodynamic diagram. These thermodynamic diagrams are useful tools for gaining a quick sense of the atmospheric properties. The Skew-T Log-P diagram is a type of thermodynamic diagram used by the U.S. National Weather Service. This diagram plots isotherms of temperature as lines slanted (hence, the “Skew-T”) upward from left to right. Isobars of constant pressure are shown as horizontal lines logarithmically along the y-axis (hence, the “Log-P”) and labeled in units of millibars (mb). A line of constant potential temperature (θ) is called an isentrope or an unsaturated adiabat, or a dry adiabat; these are shown as nearly straight but slightly curved, slanted lines upward, and allow meteorologists to trace the path of an unsaturated parcel of air up the troposphere. When the atmosphere is saturated, the path of an adiabatically moving air parcel can also be plotted on the Skew-T Log-P diagram; these more obviously curved lines correspond to lines of equivalent potential temperature, and are called saturated adiabats, or wet adiabats. This chapter describes the myriad of variables that can be deduced from the Skew-T Log-P diagram, including temperature, dew point temperature, relative humidity, cloud base (i.e., lifting condensation level), and available energy.