Meteorology (Weather)/barometric pressure and rain
I have always been curious about the usefulness of the barometric pressure reading. What does a high barometric pressure reading indicate versus a low barometric pressure reading? What does a rising or lowering barometric pressure mean? What kind of weeather is predicted? Does the average barometric pressure for an area vary from season to season? Any comments on this topic would be appreciated.
Air pressure is the force of the air pressing down on the Earth's surface. As air is all around us, there is air pressure all around us too. Near sea level there is more air above you than there would be on the top of Mount Everest, therefore the air pressure is higher near the sea than it is on the top of a mountain.
Air pressure does not only vary with height, there are many other factors, one main influence is temperature. The sun does not heat the earth uniformly, so different parts of the air are at different temperatures. If air is given more heat, this is extra energy for its molecules. If they have more energy, they can move further apart, so there will be fewer molecules occupying the same space and the air pressure will decrease.
Similarly, if the temperature is lower, the air pressure will increase. Warm air near the surface will tend to rise. A mass of low pressure is an area of air that is rising. As is rises it expands and cools. Cooler air cannot hold as much water as warmer air, so as the air rises the water will condense and form clouds. This is why an area of low pressure will often be accompanied by clouds and rain.
Conversely, an area of high pressure is a section of air which is sinking. As the air sinks it warms, so is able to hold more water, and therefore areas of high pressure are often accompanied by fair weather. Points of the same pressure can be joined up to form lines called 'isobars'. These form rings around high and low pressure centres.
Winds blow in an attempt to combat the differences in air pressure. They try to flow directly from a high to a low pressure, but due to the spinning of the earth and friction of the surface, actually flow around the pressure centres, following the isobars. The larger the difference in pressure the stronger the winds will blow.
When isobars on a weather chart are close together, it will be a blustery day, and when this is accompanied by an area of low pressure, it will also be wet.
An area of high pressure is usually associated with hot clear summer days, but it can bring fog, frost and even cloud.
High pressure areas are generally larger and move slower than low pressure. The winds circulate around the center in a clockwise, 'anticyclonic' movement. However, the winds are generally weaker than those around a low pressure, especially in the centre.
In a high pressure, the air is generally slowly sinking, or 'subsiding'. As air falls it warms, preventing clouds from forming. This is why highs are generally clear. However, sometimes the ground may be warm enough to cause some air to rise, and this can form a layer of cloud.
High pressure does not necessarily mean warm weather, a 'cold anticyclone' has cold air near the ground. These cold anticyclones are common in Siberia and Canada, where the cold air is cooling further and subsiding. Another cold anticyclone is an area of high pressure sandwiched between two areas of low pressure.
Sometimes a high pressure will 'get stuck', and can stay in the same position for over a week. This is called a 'blocking high' and forces other weather systems to go round it. These blocks often reoccur in the same place. Weather is usually stagnant and hazy. A common phrase used by weather forecasters is a 'ridge of high pressure'. This often indicates settled weather. A ridge is an area of high pressure that does not have a closed circulation, it either extends from a high pressure or is sandwiched between a couple of lows.
In an area of low pressure the air has a tendency to rise. This general upward motion means that there is less pressure from the air pushing down on the earth, in other words there is low pressure. As air rises, it cools and if there is enough water vapour it may condense to form clouds and rain. This is why a low pressure is generally associated with wet weather.
Low pressures often form on the boundary of warm and cold air. A small disturbance may cause the pressure to fall along this boundary, causing a circulation of air to develop. This is the beginning of a low pressure, and the boundaries of warm and cold air become weather fronts.
Other common types of lows are 'thermal lows'. One thermal low, a heat low, recurs over Spain and Portugal. During a summer day, the Iberian Peninsula gets much hotter than the surrounding sea. This in turn heats the air nearer the ground, which rises. This rising air lowers the pressure and a low forms. As the air rises, if there is enough moisture present, intense thunderstorms can develop.
Polar lows are another type of thermal low, but as the name suggests, this low forms in the polar regions. When air from the arctic moves south across the sea, it is heated from below and the rising air gives a region of low pressure. Polar lows are heated throughout the day and night, so can become very intense.
I know that this is a lot of information but I am sure it will help answering your questions.
Have a great day and if you have any further questions, ask.