Station Model - Pressure Tendency

Pressure Tendency

See also: Atmospheric tides and Atmospheric pressure

Below the pressure will lie the pressure tendency figure, which indicates the pressure change over the past three hours. The number depicting the pressure change will usually have two digits and indicate the pressure change in 0.1 millibar increments. There are nine different figures which represent the pressure change. A slant angled up and right indicates a steady rise, while a downward slant to the lower right indicates a steady fall. Steady rises can indicate improving conditions and approaching high pressure, and normally occur in the wake of a cold front. Steady falls can indicate deteriorating conditions and an approaching low pressure area, with the largest falls occurring in advance of a surface cyclone and its attendant warm front.

The time of the day must be considered, as there are two natural rises per day (locally around 10 am and 10 pm) and two natural falls per day (locally around 4 am and 4 pm). These daily changes in pressure can mask the movement of pressure systems and fronts past a location. The lowest natural dip in the pressure, in a stagnant weather pattern, occurs around 4 pm while the highest natural peak in pressure occurs around 10 am. Once plotted on a map, the analysis of isallobars (lines of equal pressure change) can be plotted on a map, which can indicate the direction of motion of high and low pressures systems across the map domain.