occurring during mid to late autumn after the first frost, a period of unusually warm weather with clear skies and cool nights infrared radiation: the portion of energy that has a wavelength between 0 8 micrometers (just above visible radiation) to 0 1 millimeters (microwave radiation) instability: a state of the atmosphere when a parcel of air will keep moving either up or down from its starting position when moved An unstable atmosphere is often associated with severe weather Intertropical Convergence Zone (ITCZ): belt of clouds and thunderstorms that runs parallel to the equator and marks where the trade winds of both hemispheres come together Shifts seasonally, and is associated with monsoons and dry seasons in some regions isobar: a line of equal pressure resembling a contour line on a map isotherm: a line passing through points of equal temperature on a map