Isallobar

Seasonal variations of atmospheric pressure are illustrated by the isallobaric chart […] which gives the differences between mean pressure in July–August, and in January–February. The position of the isallobars on this chart is largely indicative of the role of monsoonal factors […]

Successive values of pressure provide figures for the barometric tendency, permitting the plotting of isallobars […] This is called tendency analysis.

If your barometer has dropped 0.05 of an inch in the last three hours, you are on the -0.05 isallobar.