Atmospheric pressure is defined as the force or pressure exerted by a column of air in a particular area as a result of its weight and motion. Measuring it can help you analyze the surface weather and determine surface troughs, high pressure systems, frontal boundaries, etc., all of which have a crucial role to play in weather forecasting.
Atmospheric pressure is measured using a barometer―a scientific instrument which measures the pressure exerted by air using water, air, or mercury.
In terms of altitude, pressure increases as you start ascending and continues to increase until you reach the top of the mesosphere. The fact that air pressure increases with height was discovered by French scientist, Blaise Pascal, who also suggested that the pressure changes near the Earth's surface are associated with daily weather changes.
Basically, high and low pressure systems evolve as a result of various factors including the temperature differentials in the atmosphere, amount of solar heating, movement of wind, etc. While high pressure system is often associated with clear weather, low pressure system is associated with cloud formation and precipitation.
Barometric Pressure Readings
If you have ever referred to a weather chart, you must have come across contours with numbers and abbreviations like 'hPa' and 'mb' (1013 hPa for instance) on it. These are the barometer readings for the region which they represent in the form of isobars―lines connecting points with equal barometric pressure at any given point of time.
The abbreviations 'mb' and 'hPa' stand for Millibars and Hectopascal Scale respectively. 1013 hPa is the standard atmospheric pressure at the sea level at 45° latitude. As the same varies from one place to another, you need to adjust your barometer to the local settings to get the correct readings for your area.
Basically, there exist three types of barometers: water-based barometer, mercury barometer, and aneroid barometer. Irrespective of which of these you use, you will have to maintain a proper timetable and check the readings at set time of the day on a regular basis for accuracy.
Ideally, you should maintain a log of these readings, which will help you compare it in order to determine whether pressure is rising or falling. Furthermore, you also need to take into consideration the fact that barometric pressure readings are affected by region, season, and prevailing weather conditions.
For instance, the density of mercury changes with temperature and therefore, the reading has to be adjusted in accordance to this if you are using a mercury thermometer.
When high pressure system prevails, the air becomes dense, as a result of which it begins to sink gradually and becomes warm; the end result is clear weather. On the other hand, during a prevailing low pressure system, the air is not at all dense and thus, it begins to rise.
In course of time, it reaches a significant altitude, wherein it cools and (when there is moisture in the air) condenses to form clouds. Low pressure system is also associated with strong winds, as wind blows from a high pressure area to a low pressure area.
If you get barometric readings which show that the pressure is rising, you are likely to experience a clear weather over the period of next 12 - 24 hours. Similarly, if you see a fall in barometer reading, it implies that the weather will become rough over the period of next 12 - 24 hours.
If there is rise, and it is there for some time, it means that clear weather conditions will prevail over the next few days. The same rule applies for a fall in the readings.
You need to go by the simple rule of the thumb―rise in barometric pressure readings hints at a clear weather, while a fall in the same hints at rough weather. At times, you may also experience unsettled readings on your barometer, which will imply that the weather conditions over the next 12 - 24 hours are 'uncertain'.
Such uncertainties are quite common when it comes to weather forecasting. Some things are only perfected with time; weather forecasting is definitely one of them.