from
http://www.theweatherprediction.com/habyhints/18/
CHANGES IN ATMOSPHERIC PRESSURE
METEOROLOGIST JEFF HABY
One of the earliest forecasting tools was the use of atmospheric pressure. Soon, after the invention of the barometer, it was found that there were natural fluctuations in air pressure even if the barometer was kept at the same elevation. During times of stormy weather the barometric pressure would tend to be lower. During fair weather, the barometric pressure was higher. If the pressure began to lower, that was a sign of approaching inclement weather. If the pressure began to rise, that was a sign of tranquil weather. There is also a small diurnal variation in pressure caused by the atmospheric tides. The barometric pressure can lower by several processes, they are:
1. The approach of a low pressure trough
2. The deepening of a low pressure trough
3. A reduction of mass caused by upper level divergence (vorticity, jet streaks)
4. Moisture advection (moist air is less dense than dry air)
5. Warm air advection (warm air is less dense than cold air)
6. Rising air (such as near a frontal boundary or any process that causes rising air)
When the barometric pressure is lowering, it will be caused by 1, 2 or a combination of the 6 processes listed above. All the processes above deal either with decreasing the air density or causing the air to rise in order to lower the barometric pressure. When forecasting, try to figure out which physical processes in the atmosphere are causing the pressure to lower or rise over your forecast region. When looking at upper level charts, instead of looking for changes in barometric pressure you will be looking for height falls or height rises. Important: Barometric pressure is ONLY plotted on SURFACE CHARTS. Any upper level chart you examine will be taken on a constant pressure surface (e.g. 850, 700, 500, 300, 200). Because upper level charts use a constant pressure surface, height falls or height rises are used to determine if a trough/ridge is approaching and/or deepening. When heights fall it is due to a reduction in mass above the pressure level (i.e. if heights fall on an 850 mb chart, it is because the air is rising or low level cold air advection is occurring). On upper level charts you must consider what is happening above or below the pressure level of interest. If heights fall at 700 mb for example, it could be due to the fact that cold air advection is occurring in the PBL, therefore decreasing the overall height of the troposphere and decreasing the 700 mb height. Just to give you some complexity, barometric pressure can fall at the surface but heights can rise over the same region on upper level charts or vice versa. An example would be a large magnitude of warm air advection in the PBL. The warm air is less dense than the air it is replacing, therefore the surface pressure will fall. However, since warm air expands the height of the troposphere (because it is less dense and takes up more space) the heights aloft will rise. When I start throwing in vorticity, jet streaks, and topography this discussion will become even more complicated.
The more you learn about meteorology and forecasting the more you will realize the pure complexity of the atmosphere, the interaction of many physical processes at the same time and that learning about meteorology and forecasting lasts a lifetime. For the most part, you can interpret height falls and rises the same way as surface barometric rises or falls. Increment weather is associated with height falls and lowering barometric pressure and fair weather is associated with height rises and rising barometric pressure. Other tips:
1. Low pressure troughs tend to move toward the region of greatest height falls
2. Ridges build most strongly into regions with the greatest height rises
