Chapter 8 Notes:

Measuring Pressure

• Units for Air Pressure

  • Millibar (mb)

  • Hectopascal (hPa)

  • Inches of Mercury (in. Hg)

• Standard Sea Level Pressure: 1013.25 mb

• Tools for Measurement: Use barometers to measure atmospheric pressure.

Pressure in the Vertical

• General Principle: Pressure decreases with height.

• Pressure Formation: Pressure is determined by the amount of air above a given point.

Station Pressure

• Definition: Station pressure is the observed pressure at a location, representing the true barometric pressure.

• Influencing Factors: Station pressure varies based on temperature, density, and elevation.

• Comparative Analysis: Challenges arise in comparing pressure readings from locations at different elevations (e.g., Raleigh at 96 meters vs. Asheville at 650 meters above sea level).

Sea-level Pressure

• Adjustment Method: Pressure is adjusted to mean sea level (the average surface level of oceans).

  • This adjustment, known as mean sea-level pressure (MSLP), accounts for elevation differences.

• Importance: MSLP allows for comparison of pressure across different locations using the same reference point.

• Isobars: Lines connecting points of constant pressure on maps.

Surface Highs and Lows

• Surface Maps: Show sea-level pressure at surface level.

  • Areas of low pressure are labeled "L".

  • Areas of high pressure are labeled "H".

• Plotting Consistency: Meteorologists consistently plot sea-level pressure on surface maps.

• Isobaric Surfaces:

  • Definition: An isobaric surface is one where pressure is constant.

  • Weather Phenomena: Atmospheric phenomena such as fronts and pressure systems travel along these surfaces.

Analyzing Isobaric Surfaces

• Data Analysis: We analyze several isobaric surfaces for weather predictions (e.g., air columns).

• Hypothetical Scenario: Comparing effects of cooling and warming on two air columns:

  • Cooled Column: Denser air, decreased column height, pressure equal at different altitudes.

  • Heated Column: Less dense air, increased column height, pressure is consistent but varies more slowly with altitude.

• Pressure Behavior: Pressure decreases more rapidly with height in colder columns than in warmer columns.

Isobaric Surface Movement

• Behavior in Different Temperatures:

  • In warm air aloft, isobaric surfaces rise to higher altitudes.

  • In cold air aloft, isobaric surfaces sink to lower altitudes.

• Contour Lines: Used to show the height of isobaric surfaces relative to sea level.

  • Spacing: Closer contour lines indicate rapid height changes; further apart lines indicate gradual changes.

Troughs and Ridges

• Isobaric Surface Shape: Tilt in a wave-like manner.

• Indicators:

  • Lower heights indicate lower pressure and colder temperatures aloft.

  • Higher heights indicate higher pressure and warmer temperatures aloft.

• Definitions:

  • Ridges: Elongated areas of higher pressure aloft.

  • Troughs: Elongated areas of lower pressure aloft.

Forces of Wind

• Pressure Differential:

  • Concept illustrated with two individuals pushing against a wall, representing pressure exerted by air molecules.

  • Higher pressure leads to greater force against a surface, causing movement towards lower pressure.

• Net Force and Wind: Wind results from the net force directed from high to low pressure, leading to a pressure gradient force (PGF).

• Pressure Gradient Force (PGF): Defined as the net force directed from high to low pressure, driving wind strength.

Impact of Pressure Gradients

• Larger PGFs: Occur with larger differences in pressure.

• Isobar Spacing: Closer isobars correspond to a stronger PGF, driving wind to blow from high to low pressure.

Atmospheric Properties of Air Columns

• Behavior in Warm Air: Isobaric surfaces rise with increased temperature and decrease in cold air.

• Analysis Method: Changes in isobaric surface height are measured and depicted using contour lines.

Troughs and Ridges Revisited

• Wave-like Isobaric Surface Behavior: These features affect pressure dynamics.

• Temperature Relationships:

  • Lower heights correspond with colder temperatures.

  • Higher heights associate with warmer temperatures.

Wind Dynamics

• Pressure Gradient Relation: Strong (weak) pressure gradients lead to strong (weak) PGF and thereby stronger (weaker) wind.

• Coriolis Force:

  • Describes the apparent force due to Earth’s rotation that curtails airflow direction.

  • In the Northern Hemisphere, wind is deflected to the right.

Geostrophic Wind Concept

• Description: Wind at upper levels stabilizes in balance between PGF and Coriolis force, flows parallel to isobars.

• Surface Winds: Frictional drag at the surface influences wind speed, which increases with height.

• Surface Influence: Friction reduces wind speed and modifies the direction influencing pressure interaction.

Understanding Wind Flow

• Flow Patterns: Around low pressure systems, wind rotates counterclockwise, while high pressure rotates clockwise.

  • Cyclonic Flow: Wind patterns associated with low pressure systems (counterclockwise) in the Northern Hemisphere.

  • Anticyclonic Flow: Wind patterns associated with high pressure systems (clockwise).

Southern Hemisphere Wind Flow**

• High and Low Mechanics: Flow is reversed compared to the Northern Hemisphere.

  • Clockwise around low pressure/cyclones and counterclockwise around high pressure systems.

Surface and Upper-level Chart Analysis

• Representation: Solid black lines denote height in meters, dashed red lines represent temperature isotherms (in °C).

  • Upper-level Winds: Flow counterclockwise around upper-level lows and clockwise around highs, with wind parallel to height lines.

Vertical Air Motion**

• Low Pressure Systems: Air converging at surface lows rises and diverges upon reaching the higher altitude.

• High Pressure Systems: Air diverges at highs and sinks toward the surface as it spreads out.

• Units for Air Pressure

  • Millibar (mb): A unit commonly used in meteorology, where 1 mb is equivalent to 100 pascals.

  • Hectopascal (hPa): Equivalent to millibars, making it a standard unit in many meteorological contexts.

  • Inches of Mercury (in. Hg): Often used in aviation and weather forecasting, with 29.92 in. Hg being standard atmospheric pressure at sea level.

• Standard Sea Level Pressure: 1013.25 mb is considered the average atmospheric pressure at sea level, serving as a reference point for meteorological assessments.

• Tools for Measurement: Barometers, which can be aneroid or mercury-based, are primarily used to measure atmospheric pressure. Aneroid barometers are portable and widely used in weather stations, while mercury barometers provide accurate readings but are less common due to the toxicity of mercury.

Pressure in the Vertical

• General Principle: As altitude increases, air pressure decreases due to the diminishing amount of air above a given point. This decrease is not uniform, illustrating how pressure varies with altitude.

• Pressure Formation: Atmospheric pressure is contingent on both the weight of the air column above and external factors such as temperature and humidity, influencing air density.

Station Pressure

• Definition: Station pressure is defined as the actual measured air pressure at a specific location, adjusting for temperature and humidity while representing the true barometric pressure.

• Influencing Factors: Variations in temperature, density, and elevation significantly affect station pressure readings. For example, warm temperatures can lead to lower pressure at a given height, while colder temperatures can increase it.

• Comparative Analysis: Comparing pressure readings between locations at varying elevations presents challenges. For example, Raleigh at 96 meters elevation will have a different pressure reading compared to Asheville at 650 meters, complicating direct comparisons unless adjustments to sea level pressure are made.

Sea-level Pressure

• Adjustment Method: To standardize measurements, atmospheric pressure readings are adjusted to mean sea level (MSLP). This adjustment accounts for the differences in air pressure caused by elevation and provides a more accurate basis for comparison across different geographic locations.

  • Adjustments: The MSLP is derived by adding or subtracting a calculated value based on the elevation of the observation point above or below sea level.

• Importance: MSLP is essential for meteorologists and weather forecasters as it allows for the comparison of pressure readings across varying altitudes, facilitating more accurate weather predictions and analysis.

• Isobars: On weather maps, isobars are lines that connect points of equal pressure, helping to visualize pressure systems and predict wind patterns, with closer isobars indicating stronger winds.

Surface Highs and Lows

• Surface Maps: These maps depict the sea-level pressure at various surface locations, with areas of low pressure marked as 'L' and high pressure as 'H'. Understanding these pressure systems aids in foreseeing weather changes.

• Plotting Consistency: Consistent plotting of sea-level pressure by meteorologists ensures continuity in data interpretation across various weather observations.

• Isobaric Surfaces:

  • Definition: An isobaric surface is depicted where pressure remains constant. These surfaces are crucial for understanding weather systems and predicting movements in atmospheric pressure phenomena.

  • Weather Phenomena: Atmospheric phenomena including fronts and pressure systems tend to travel along these isobaric surfaces, influencing local weather patterns.

Analyzing Isobaric Surfaces

• Data Analysis: Meteorologists analyze multiple isobaric surfaces when predicting weather, examining the variations in air pressure and temperature at different altitudes (e.g., air columns)

• Hypothetical Scenario: When analyzing cooling and warming effects on two contrasting air columns:

  • Cooled Column: Characterized by denser air, resulting in decreased column height while maintaining pressure that is consistent across varying altitudes.

  • Heated Column: Features less dense air and an increased column height. Despite consistent pressure, variations with altitude occur more gradually in warmer air.

• Pressure Behavior: It is observed that pressure decreases more rapidly with height in cooler air columns due to denser air molecules compared to the gradual change seen in warmer columns.

Isobaric Surface Movement

• Behavior in Different Temperatures:

  • In warm air aloft, isobaric surfaces are elevated to higher altitudes due to the thermal expansion of air.

  • In contrast, in cold air aloft, isobaric surfaces descend to lower altitudes, compressing the air layers.

• Contour Lines: Meteorologists use contour lines to illustrate the height of isobaric surfaces relative to mean sea level, facilitating the understanding of atmospheric dynamics.

  • Spacing: The distance between contour lines indicates the rate of height change; closer lines reflect rapid variations, while wider spacing suggests more gradual alterations.

Troughs and Ridges

• Isobaric Surface Shape: Troughs and ridges exhibit a wave-like pattern, influencing both temperature and pressure dynamics in the atmosphere.

• Indicators of Pressure:

  • Lower heights often associate with lower pressure and cooler temperatures aloft, indicative of various weather conditions like storms.

  • Higher heights distinguish regions of higher pressure, generally correlating with warmer temperatures and clearer weather.

• Definitions:

  • Ridges: These are elongated areas that represent higher pressure at upper levels in the atmosphere.

  • Troughs: Extended zones of lower pressure aloft.

Forces of Wind

• Pressure Differential: The concept of pressure differential is explained through analogies, such as individuals pushing against a wall, highlighting the force exerted by air molecules. Increased pressure results in greater force against surfaces, thus propelling movement towards areas of lower pressure.

• Net Force and Wind: Wind is produced as a result of the net force directed from high-pressure areas towards low-pressure zones, leading to the generation of a pressure gradient force (PGF). This force is pivotal in driving wind systems.

• Pressure Gradient Force (PGF): The PGF is defined as the net force directed along the pressure gradient from high to low pressure, dictating wind strength and direction.

Impact of Pressure Gradients

• Larger PGFs: These gradients arise when there are substantial differences in air pressure over geographical areas, affecting wind speed and direction.

• Isobar Spacing: Analysis of isobar spacing offers insight into wind dynamics, as closely packed isobars indicate stronger pressure gradients, resulting in enhanced winds that blow from high to low pressure.

Atmospheric Properties of Air Columns

• Behavior in Warm Air: As the temperature increases, isobaric surfaces tend to rise, whereas they decrease in height during cold air conditions. This relationship is critical for understanding weather patterns and vertical air motion.

• Analysis Method: Changes in isobaric surface heights are measured through advanced meteorological tools, depicted using contour lines that provide a visual representation of atmospheric conditions.

Troughs and Ridges Revisited

• Wave-like Isobaric Surface Behavior: Continued analysis shows how these pressure dynamics impact overall atmospheric conditions, shaping local climates and weather systems.

• Temperature Relationships:

  • Lower heights correspond with cooler temperatures, often signaling changing weather patterns.

  • Higher heights usually associate with warmer conditions, contributing to stable and clear weather.

Wind Dynamics

• Pressure Gradient Relation: The intensity of wind is directly influenced by the strength of pressure gradients, where strong gradients result in swift winds and weak gradients produce subtle air movements.

• Coriolis Force: The Coriolis force describes how Earth's rotation impacts airflow direction, causing a noticeable deflection in wind patterns.

  • In the Northern Hemisphere, winds are deflected to the right, while in the Southern Hemisphere, they are deflected to the left, impacting regional weather systems.

Geostrophic Wind Concept

• Description: Wind at upper atmospheric levels tends to stabilize, achieving a balance between the PGF and the Coriolis force, which allows it to flow parallel to isobars rather than directly from high to low pressure.

• Surface Winds: Ground-level wind patterns are influenced by friction, where frictional drag slows wind speeds, particularly at lower altitudes, while allowing for stronger winds at increased heights.

• Surface Influence: The interactions of surface features with wind can alter speed and direction, attributing to local weather phenomena.

Understanding Wind Flow

• Flow Patterns: Around low pressure systems, air tends to rotate counterclockwise, whereas high-pressure systems are characterized by clockwise rotations.

  • Cyclonic Flow: This wind rotation pattern associated with low pressure systems differs from the typical rotation associated with high pressure.

  • Anticyclonic Flow: Wind patterns exhibit clockwise rotation within high pressure systems, leading to subsiding air and stable conditions.

Southern Hemisphere Wind Flow

• High and Low Mechanics: In the Southern Hemisphere, the flow of wind patterns reverses compared to the Northern Hemisphere, where air rotates clockwise around low pressure systems and counterclockwise around high pressure systems.

Surface and Upper-level Chart Analysis

• Representation: Forecast and analysis charts utilize solid black lines to depict altitude in meters, while dashed red lines illustrate temperature isotherms measured in °C, aiding meteorologists in interpreting atmospheric conditions.

• Upper-level Winds: These winds exhibit a counterclockwise flow around upper-level lows and a clockwise flow around highs, maintaining a parallel orientation to height lines on maps.

Vertical Air Motion

• Low Pressure Systems: In low pressure systems, air converges at the surface leading to upward vertical movement, which contributes to cloud and storm formation as it diverges at higher altitudes.

• High Pressure Systems: In contrast, high pressure areas are characterized by air that diverges at the surface and sinks towards the ground, creating stable weather conditions as it spreads out.