Understanding Atmospheric Pressure and Barometric Systems

Atmospheric Pressure

Atmospheric pressure is the force exerted by the air per unit area on all bodies immersed within it. It is determined by the amount of dry air and vapor tension present, varying across different locations and over time.

Variation of Pressure

Atmospheric pressure fluctuates based on temperature, altitude, and humidity:

• Temperature: If temperature increases, air expands, decreasing pressure between molecules. If temperature decreases, air contracts, leading to an increase in pressure.
• Altitude: The lower layers of the atmosphere are denser and support the weight of the upper layers. Consequently, pressure decreases as altitude increases.

Geographic Distribution of Pressure

• Isobars: Lines connecting points of equal barometric pressure.
• Isallobars: Lines connecting points of equal pressure variation.
• Barometric Isanomals: Lines connecting points of equal barometric anomaly, representing the difference between current and normal pressure for a given time.

Pressure Gradient

A vector perpendicular to the isobars, directed toward low-pressure areas.

Pressure Systems

When mapping isobars on a weather map, the following systems can form:

• Anticyclones (High Pressure): Areas where isobars are closed around a high-pressure center.
• Cyclones (Low Pressure/Storms/Depressions): Areas where isobars are closed around a low-pressure center.
• High-Pressure Wedges: Formed when isobars are not closed but are open in a U or V shape, with pressure increasing toward the interior.
• Troughs (Vaguadas): Areas where isobars are curved at one end, with decreasing pressure toward the interior.

Measuring Instruments

A barometer is used to measure atmospheric pressure.

Mercury Barometers

• Fortin (Mobile) Barometer: Features a screw to adjust the mercury level and includes a thermometer to monitor the barometer's temperature.
• Kew (Fixed) Barometer: Does not require a zero-adjustment screw. It utilizes a specific ratio between the internal areas of the tank and the barometric tube to allow for direct pressure readings.

Aneroid Barometers

• Altimeter: Features an altitude scale instead of, or in addition to, a pressure scale. Since pressure decreases with height (typically 10 mb per 100 m in the lower atmosphere), it helps determine relative height.
• Barograph: Contains a vacuum-sealed metal capsule (made of a beryllium and copper alloy) sensitive to pressure changes. Deformations are transmitted via a system of levers to a pen, which graphs pressure variations onto a rotating drum driven by a clockwork mechanism.