Understanding Pressure and Atmospheric Phenomena
Concepts
Pressure: Force acting on a unit area. The SI unit of pressure is the Newton per square meter (N/m²), which is called the Pascal (Pa).
Pascal: Pressure exerted by a force of one Newton on a surface area of one square meter.
Hydrostatic Pressure: Pressure originating inside a liquid under its own weight.
Hydraulic Press: A mechanism consisting of two cylinders of different sections, connected by a tube containing a liquid that reaches the same height in both. These cylinders are closed by different-sized plungers that are in contact with the liquid.
Atmosphere: A mixture of gases surrounding the Earth that exerts pressure on its surface.
Barometer: Measures the pressure exerted by the atmosphere on the surface of bodies in contact with it.
Pascal’s Principle: The principle articulated by Blaise Pascal, which demonstrates that the pressure exerted on a point of a liquid, considered incompressible and enclosed in a container, is transmitted equally to every point of the liquid and the container walls. The pressure is transmitted equally in all directions. It is applied in the construction of presses, hydraulic scales, etc.
Theory
Hydrostatic Pressure Variation:
- The pressure inside a liquid acts in all directions.
- Higher pressure is found at greater depths.
- A higher density of liquid results in greater pressure.
- The pressure does not depend on the shape or the width of the container.
Torricelli’s Experiment: Torricelli filled a thin glass tube, 1 meter in length and 1 cm² in section, closed at one end, with mercury. He plugged the free end with a finger and inserted the tube upside down in a bucket also full of mercury. When he removed his finger, the mercury level dropped until the column in the tube was 760 mm above the mercury surface of the tray. A vacuum was created at the upper end of the tube.
- Atmospheric pressure is shown on the free surface of the mercury, as the level always reaches the same height.
- The atmospheric pressure acts on the mercury from the tank and holds the column contained within the tube, preventing it from descending further.
The pressure exerted by a column of mercury 760 mm high is called normal atmospheric pressure and is used to measure the atmosphere; it is equivalent to 101,300 Pa.
Relationship Between Atmospheric Pressure and Altitude: As we ascend, there is less air above us; namely, the atmospheric pressure is lower, and thus the length of the mercury column decreases.
Squalls and Anticyclones
At sea level, the pressure has a value of 1013 mb, so higher and lower pressures are called high and low pressure, respectively.
Anticyclone: Area of high pressure.
Borrasca (Squall): Area of low pressure.
The formation of these zones is due to the uneven temperature of the atmosphere: if the air is heated, it expands and rises, leaving behind an area of low pressure; if the air is cold, it is denser, so it descends, causing high-pressure zones.
Isobars: Lines that connect points of equal pressure. The maps are drawn with an interval of 4 mb. Cyclones and storms are not motionless but move and change shape. In an area where the pressure is low, the air will attract a storm in the surrounding regions where the pressure is higher. In these cyclonic areas where air masses of different origins converge and contact, the weather is unstable. Around a high-pressure area, air travels to the areas which have a lower pressure. As the air currents are divergent, heterogeneous contact between air masses does not occur, and the weather is good. It follows that the air moves from high-pressure areas to low-pressure areas, causing wind currents.
Front: Intersection of the land surface with the front.
Warm Front: Hot air dominates the cold, and vice versa.