Station vs. sea-level pressure

• Station pressure is recorded by a barometer at a surface station (1.5 m above ground level)

• If the station is not at sea-level, its pressure must be adjusted to sea-level pressure (SLP)

– typical pressure difference between surface high and low pressure areas ≈ 100 mb

– Average sea-level pressure ≈ 1000 mb

– Pressure decreases very rapidly with height, ≈ 100 mb drop per kilometer in lower atmosphere

– Adjustment to SLP needed to reveal high and low pressure areas on surface map

Figure One represents a map of isobaric pressure readings in millibars.

    a) Draw isobars for every four millibars  (996, 1000, 1004, etc.). Label the isobars and the high and low pressure cells.

    b) Draw arrows to indicate wind circulation from the highs to the lows.

    c) Place a heavy arrow along the steepest pressure gradient (and therefore highest wind speeds).

Ex.: At about 100 mb decrease per kilometer of elevation, the barometer at the 1500 m high station at a higher elevation needs to be adjusted by about 150 mb.

1. The Tejon Pass on Interstate 5 is about 1200 m (4000 ft) above sea level.

a. If the station pressure there is 885 mb, estimate its SLP

b. If the SLP there is 998 mb, estimate its station pressure

2. Station "A" has a station pressure of 800 mb and an elevation of 2000 m. At station "B", which is at sea level, the pressure is 990 mb. At which station is the SLP lower?