Atmospheric pressure defined as the force per unit area exerted on a surface. Pressure decreases with altitude as there is less overlying weight of air. Observations of pressure at a station, made by measuring the height of a mercury column in a mercury barometer, are reduced to pressure at mean sea level (MSL) by corrections for the latitudinal variation of gravity and for air temperature. There are two important pressure systems; high pressure system and low pressure system, which help to determine the weather of a region. More details about these pressure systems are described in the following sections.
We may not know everything about the weather, one thing is for certain; a high pressure system always means nice weather. The presence of a high pressure system does indicate nice, or at least tranquil (since high pressure systems can bring cold weather), weather often enough that the belief is understandable, but it's not always the case.
When a high pressure system results in air flow from a water source, such as a lake or an ocean, it can result in damp, dreary weather with drizzle. When a high pressure system results in overly tranquil weather, means that it stops all wind, the result is sometimes dense fog and polluted air. The results of a high pressure system are sometimes pollution, fog or rain- not the pleasant blue sky we always expect.
In a meteorological low pressure system, air flows towards the center in an attempt to bring about equilibrium. This convergence causes updrafts in the center of the low. Although the winds are generally light very near the low pressure system's center, winds farther away from the center but still associated with the system tend to be moderate, resulting in overall increased rates of ventilation. Low pressure systems typically cover fairly small areas and tend to be transient, seldom remaining in one place for very long. They are also often accompanied by clouds, which can cause precipitation that tends to remove some pollutants from the atmosphere through sedimentation. Cloudy skies also minimize the variation in mixing between levels of the atmosphere from day to night. Moderate horizontal wind speeds and upward vertical movement associated with low pressure systems generally result in good ventilation- that is, a significant volume of air moving past a given location. Typically, air pollution is low in an area where a low pressure system is present.
Most of us mistakenly assume that a low pressure system causes wind, but technically it doesn't. Wind is not caused by low pressure systems or high pressure systems; it's caused by the difference in pressure (pressure gradient) at any given location. The larger this gradient, the stronger the wind. The strongest gradient will often occur in a region that is between a high pressure system and a low pressure system, so the high pressure system is at least partially responsible for the wind. However, the low pressure system always gets the blame.
Water pressure refers to en energy level and is measured in pounds per square inch (psi). Volume and pressure are two different but mathematically related measurements. Water the is not moving has potential energy. When the water is moving, it has a combination of potential energy and kinetic energy. Both the quantity of water flowing and the pressure under a specific set of conditions must be measured as part of testing any water system, including hydrants.
Static pressure is the pressure in a system when the water is not moving. Static pressure is potential energy, because it would cause the water to move if there were some place the water could go. Static pressure causes the water to flow out of an opened fire hydrant.
Residual pressure is the amount of pressure that remains in the system when water is flowing. When fire fighters open a hydrant and start to draw large quantities of water out of the system, some of the potential energy of still water is converted in to the kinetic energy of moving water. The pressure remaining while the water is flowing is residual pressure. Residual pressure is important because it provides the best indication of how much more water is available in the system.
Advanced pressure systems offers a wide range of water blasting products. These are several categories from which to choose depending on the application and the intended working pressure.