Pressure Tendency How pressure changes with time at a particular point Pressure in a column is due to the weight of the air above Lower the mass of the air above, lower the pressure

We can use the continuity equation (conservation of mass) to substitute for ∂ρ/∂t (a future lab will deal specifically with the continuity equation) Pressure tendency Horizontal mass convergence 0 w = 0 near ground

“The pressure tendency at an altitude of z is due to the net mass change of the air above that altitude” However, mass does not change significantly in a column, so other techniques must be employed to measure the pressure tendency (later) Pressure tendency at the surface is easily calculated from pressure data

Isallobaric Analysis Isallobar – Line of constant pressure tendency Pressure systems tend to move towards maximum and minimum pressure tendency regions –Low moves toward maximum pressure fall –High moves toward maximum pressure rise

Isallobaric Analysis Process of analyzing a map of raw pressure tendency data to illuminate patterns in the data Some rules: –Use pencil –Label contour lines at edges (middle of line for circles) –Contour lines can never cross –Contour lines should be as smooth as possible –Darken lines with marker/pen after you are certain it is a reasonable analysis