Download air pressure

Weather Wisdom - with Martin Babakhan
AIR PRESSURE
Knowledge of atmospheric pressure is of great assistance for weather observers. Because
air acts like a fluid; high pressure in a locality will result in a air moving outwards towards
comparatively low pressure areas.
The actual atmospheric pressure at a station is called ‘Station level pressure”. This pressure
at a given place and time is dependent upon: The altitude of a station. The effect of
gravity, and the amount of air above the station.
The movement of pressure systems: The passage of a well-developed pressure system
often is accompanied by a change of 5 hPa or more in atmospheric pressure. Change in
intensity of pressure system can occur: This is because of the deepening or filling of a low
pressure system.
A daily variation: This variation, which may be thought of as an atmospheric tide, is fairly
strong in equatorial latitudes but vanishes about latitude 60º. The pressure is greatest due to
this effect at 10 a.m and 10 p.m local standard time and lowest at 4 a.m and 4 p.m. This
variation amounts to about one (hPa) in middle latitudes and more than 5 hPa in tropical
regions. This is significant in tropical region because falling pressure may be due only to
the daily variations and may not indicate the approaching of a storm.
BAROMETER
1. A barometer will not give a true reading of static pressure if it is subject to the influences
of gusty winds. It may be necessary to relocate the barometer to a more sheltered location.
2. Barometers must not be installed in "sealed" Room , where the air pressure inside the
building is liable to be different to that outside
Mercury barometers:
These should be placed in a room in which the temperature is constant or changes only
slowly, and in which temperature gradients do not occur. They should be shielded from
direct sunshine at all times and should not be placed near any heating or cooling device. It
is preferable to hang a mercury barometer on a solid inside wall which is free from
vibration. Accurate work, the best position is in an insulated, unheated windowless room,
such as a basement, with a small fan to prevent any stratification of temperature.
2
Aneroid barometers:
Most aneroid barometers used for meteorological purposes are temperature
compensated for fluctuations in temperature in the ranges likely to be
experienced in sheltered conditions in Australia. However, they should be
protected from differential heating from direct sunlight, heaters or any form
of air conditioning.
You also find great information from the Bureau of Meteorology Web site “How to
measure air pressure using a Barometer”
http://www.bom.gov.au/info/weatherkit/section2/barom.shtml
HOW TO CALLIBRATE YOUR BAROMETER
Open the Australian Bureau or Meteorology homepage http://www.bom.gov.au/
Clickable Map
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Click on NSW on the weather map
Click on All Latest NSW weather observations
Click on Clickable Map
Click on Armidale Airport or Tamworth Airport
Refer to the second last column headed “Press MSL hPa” which will give sea level
pressure which is updated every 3 hours.
Wait until just after an update before calibrating your barometer.