7.1 Atmospheric pressure: concepts and measurements
4 min read•july 31, 2024
Atmospheric pressure, the force exerted by air molecules, plays a crucial role in weather patterns. It's measured using various instruments, from traditional mercury barometers to modern digital sensors. Understanding pressure helps us predict weather and analyze atmospheric conditions.
High and systems drive wind patterns and influence weather. Pressure gradients, the rate of pressure change over distance, are key to understanding atmospheric dynamics. Meteorologists use pressure data to create weather maps and forecast future conditions.
Atmospheric pressure and its significance
Defining atmospheric pressure
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- Atmospheric pressure represents force exerted by weight of atmosphere per unit area at a given point on Earth's surface
- Measured in units of pascals (Pa), millibars (mb), or inches of mercury (inHg)
- 1 atmosphere equals 101,325 Pa, 1013.25 mb, or 29.92 inHg at sea level
- Drives wind patterns and weather systems
- Crucial parameter in meteorological forecasting and analysis
- Pressure gradients indicate rate of change of pressure over distance
- Fundamental in understanding atmospheric dynamics and weather pattern formation
High and low pressure systems
- High-pressure systems associated with fair weather
- Clockwise circulation in Northern Hemisphere
- Low-pressure systems often bring unsettled weather
- Counterclockwise circulation in Northern Hemisphere
- Isobars visualize pressure distributions on weather maps
- Essential tools for identifying weather systems
- Lines of constant pressure
- Examples of pressure systems:
- Hurricanes (low pressure)
- Anticyclones ()
Instruments for measuring atmospheric pressure
Mercury-based instruments
- Mercury barometers balance atmospheric pressure against weight of mercury column in glass tube
- Traditional and highly accurate method
- Barographs continuously record atmospheric pressure over time
- Use connected to pen
- Traces on rotating drum (paper chart)
- Fortin allows adjustment of mercury level for precise measurements
- Used as a reference standard in many weather stations
Modern electronic instruments
- Aneroid barometers use partially evacuated metal chamber
- Expands or contracts with pressure changes
- Connected to mechanical or electronic readout
- Digital barometers employ pressure sensors
- Convert pressure changes into electrical signals
- Often use piezoresistive or capacitive sensors
- Provide precise digital display
- Examples of digital barometers:
- Vaisala PTB330 (high-precision meteorological barometer)
- Kestrel 5500 Weather Meter (portable multi-function device)
Atmospheric profiling instruments
- Radiosondes measure vertical pressure profiles in atmosphere
- Weather balloons equipped with pressure sensors
- Provide data up to stratosphere (30 km altitude)
- Aircraft-based sensors in AMDAR system
- Measure pressure at various altitudes during flight
- Contribute to global weather observation network
- Examples of radiosonde systems:
- Vaisala RS41 (widely used modern radiosonde)
- Lockheed Martin LMG6 (GPS-enabled radiosonde)
Interpreting atmospheric pressure data
Surface weather observations
- Surface weather station reports include pressure readings
- Typically in millibars or inches of mercury
- Often adjusted to sea level for consistency across stations
- Pressure tendency indicates change in pressure over time
- Reported as rising, falling, or steady
- Crucial for short-term weather forecasting
- Examples of pressure tendencies:
- Rapidly falling pressure (approaching storm system)
- Steadily rising pressure (improving weather conditions)
Synoptic and upper-air analysis
- Synoptic weather maps display isobars to show pressure patterns
- Closely spaced isobars indicate strong pressure gradients and potential for high winds
- Upper-air charts present pressure data at various altitudes
- Often use constant pressure surfaces (500 mb chart)
- Analyze atmospheric flow patterns and jet streams
- Examples of upper-air chart types:
- 850 mb chart (low-level moisture and temperature analysis)
- 300 mb chart (jet stream analysis)
Advanced data sources
- Satellite-derived pressure data from microwave sounders
- Provide global coverage of atmospheric pressure at different levels
- Crucial for data-sparse regions (oceans, remote areas)
- Reanalysis datasets combine observations and model data
- Create comprehensive, gridded pressure fields
- Used for climate studies and historical weather analysis
- Examples of reanalysis datasets:
- NCEP/NCAR Reanalysis
- ERA5 (ECMWF Reanalysis v5)
Altitude vs atmospheric pressure
Pressure-altitude relationship
- Atmospheric pressure decreases with increasing altitude
- Due to reduced weight of air column above
- Rate of pressure decrease with height follows non-linear relationship
- Approximated by barometric formula or hydrostatic equation in meteorology
- Standard atmosphere model defines pressure values at different altitudes
- 1013.25 mb at sea level
- About 500 mb at 5,500 meters (18,000 feet) altitude
Pressure concepts in meteorology and aviation
- Pressure altitude used in aviation
- Altitude in standard atmosphere with given pressure
- May differ from true altitude due to non-standard conditions
- Geopotential height crucial in upper-air meteorology
- Height of pressure surface above mean sea level
- Used for analyzing atmospheric circulation patterns
- Scale height of atmosphere approximately 8.5 km
- Altitude where pressure decreases to 1/e (about 37%) of sea-level value
- Illustrates rapid decrease of pressure with height in lower atmosphere
Pressure adjustments and applications
- Pressure reduction to sea level standard practice in surface weather observations
- Allows consistent comparison of pressure readings across different elevations
- Altimeter setting in aviation adjusts aircraft altimeter to local pressure conditions
- Ensures accurate altitude readings for safe flight operations
- Examples of pressure-altitude relationships:
- Denver, Colorado (5,280 feet elevation) typical surface pressure around 850 mb
- Mount Everest summit (29,029 feet) pressure about 330 mb
Key Terms to Review (18)
Adiabatic processes: Adiabatic processes are thermodynamic changes that occur without any heat exchange between a system and its surroundings. This concept is crucial for understanding how air parcels behave as they move vertically through the atmosphere, impacting temperature and pressure changes in atmospheric layers and influencing various weather phenomena.
Aneroid Barometer: An aneroid barometer is a device used to measure atmospheric pressure without the use of liquid, utilizing a small, flexible metal capsule that expands or contracts based on the surrounding air pressure. This type of barometer is highly portable and often used in meteorological instruments for both surface and upper-air observations. Its readings can help predict weather changes, making it an essential tool in atmospheric science.
Anticyclone: An anticyclone is a weather system characterized by high atmospheric pressure at its center, where air descends and spreads outwards, leading to clear skies and stable weather conditions. This phenomenon plays a significant role in shaping the atmospheric composition and behavior, influencing pressure variations, global wind patterns, and how weather maps are analyzed for forecasting purposes.
Barometer: A barometer is an instrument used to measure atmospheric pressure, which plays a crucial role in weather forecasting and understanding meteorological processes. By tracking changes in pressure, barometers help indicate weather patterns, such as high and low-pressure systems, which are essential for predicting storms and other weather events.
Bernoulli's Principle: Bernoulli's Principle states that as the speed of a fluid increases, its pressure decreases, illustrating the relationship between fluid velocity and pressure. This principle plays a vital role in understanding atmospheric pressure and how it affects air movement, especially in high-altitude jet streams where variations in speed can create significant changes in pressure and influence weather patterns.
Cyclone: A cyclone is a large-scale air mass that rotates around a center of low atmospheric pressure, characterized by strong winds and heavy precipitation. Cyclones are significant weather phenomena that can influence global wind patterns, atmospheric pressure systems, and weather map interpretations, affecting local and regional climates dramatically.
Daniel Gabriel Fahrenheit: Daniel Gabriel Fahrenheit was a Polish-German physicist and engineer, best known for developing the mercury-in-glass thermometer and the Fahrenheit temperature scale. His innovations significantly improved temperature measurement, providing a more precise and reliable method that has influenced scientific practices and the understanding of atmospheric conditions.
Evangelista Torricelli: Evangelista Torricelli was an Italian physicist and mathematician best known for his invention of the barometer in the 17th century, which was crucial for measuring atmospheric pressure. His work laid the groundwork for the understanding of how pressure changes with altitude and its relationship with weather patterns, making it a fundamental concept in meteorology.
Front: A front is a boundary that separates two different air masses, often with distinct temperature and humidity characteristics. These boundaries are crucial in weather patterns because they create conditions for precipitation, storms, and various weather phenomena. The interaction at fronts is responsible for significant changes in atmospheric pressure, temperature, and wind direction, making them key players in the dynamics of Earth's atmosphere.
High pressure: High pressure refers to regions in the atmosphere where the pressure is higher than that of the surrounding areas. These areas are typically associated with descending air, leading to clear skies and stable weather conditions. High pressure systems play a significant role in influencing wind patterns and overall atmospheric dynamics.
Ideal gas law: The ideal gas law is a fundamental equation in physics and chemistry that describes the relationship between pressure, volume, temperature, and the number of moles of an ideal gas. It is expressed as PV = nRT, where P is the pressure, V is the volume, n is the number of moles, R is the ideal gas constant, and T is the absolute temperature. This law connects various important properties of gases and helps in understanding atmospheric pressure and behavior under different conditions.
Isobar: An isobar is a line on a weather map that connects points of equal atmospheric pressure. These lines help meteorologists visualize pressure patterns and analyze weather systems, including cyclones and anticyclones, providing critical insight into wind patterns and storm movements.
Low pressure: Low pressure refers to a region in the atmosphere where the atmospheric pressure is lower than that of surrounding areas. This phenomenon typically leads to cloud formation and precipitation, making it essential for understanding weather patterns. In areas of low pressure, air rises and cools, which can create unstable weather conditions, influencing both local climates and broader meteorological systems.
Millibar: A millibar is a unit of pressure commonly used in meteorology to measure atmospheric pressure. It is defined as one-thousandth of a bar, where one bar is approximately equal to the atmospheric pressure at sea level. Millibars are crucial for understanding weather patterns, as they provide a standardized way to quantify air pressure changes in the atmosphere, which can indicate different weather systems and conditions.
Pascal: A pascal (Pa) is the SI unit of pressure, defined as one newton per square meter. This measurement is essential for understanding atmospheric pressure, as it quantifies how force is distributed over a given area. The pascal connects to other significant concepts such as weather forecasting, atmospheric science, and meteorological instruments, providing a standardized way to express pressure readings across various environments.
Pressure changes: Pressure changes refer to the variations in atmospheric pressure due to factors such as temperature, altitude, and weather systems. These changes play a critical role in meteorology, influencing wind patterns, cloud formation, and precipitation. Understanding how pressure changes impact weather dynamics is essential for forecasting and analyzing atmospheric behavior.
Pressure gradient: A pressure gradient is the rate at which atmospheric pressure changes over a certain distance. This concept is crucial in understanding how winds develop, as the greater the difference in pressure over a distance, the stronger the wind that results. Additionally, the pressure gradient plays a significant role in weather systems and helps to explain phenomena such as fronts and cyclonic behavior.
Squall Line: A squall line is a narrow band of thunderstorms that can form along a cold front or within a moist and unstable air mass, characterized by strong winds, heavy rain, and often severe weather. These lines can be several hundred miles long and are typically associated with rapid changes in atmospheric pressure and instability, making them critical in understanding storm development and severe weather patterns.