The local rate of change of a vector or scalar quantity with time at a given point in space. Thus, in symbols, ∂p/∂t is the pressure tendency, ∂ζ/∂t is the vorticity tendency, etc. Because of the difficulty of measuring instantaneous variations in the atmosphere, variations are usually obtained from the differences in magnitudes over a finite period of time and the definition of tendency is frequently broadened to include the local time variations so obtained. An example is the familiar three-hourly pressure tendency given in surface weather observations; in fact, the term “tendency” alone often means the pressure tendency.