Pressure Tendency

1. 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

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

3. “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

4. 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

5. 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