Atmospheric Pressure and Wind: Principles and Mechanisms

Atmospheric Pressure and Wind

Introduction

Atmospheric pressure and wind are fundamental concepts in meteorology and environmental biology, describing how air moves and exerts force within Earth's atmosphere. Understanding these principles is essential for interpreting weather patterns and the physical environment.

Atmospheric Pressure

Definition and Measurement

• Atmospheric Pressure is the force exerted by the weight of the air column above a given point on Earth's surface.

• Measured as Force/Area, typically in units of millibars (mb) or mm Hg.

• Standard atmospheric pressure at sea level is 1013.25 mb or 760 mm Hg.

• Barometers are used to measure atmospheric pressure.

• Formula: $P = \frac{F}{A}$ where $P$ is pressure, $F$ is force, and $A$ is area.

• Example: The pressure exerted by the atmosphere on a surface is equivalent to the weight of the air above that surface.

Variation with Altitude

• Atmospheric pressure decreases rapidly with increasing altitude.

• At sea level (MSLP): 1013.25 mb

• Denver, CO (5,280 ft): 850 mb

• Mount Everest (29,035 ft): 300 mb

• Example: High-altitude locations experience lower atmospheric pressure due to less air above them.

Wind: Speed and Direction

Definition and Naming

• Wind is the movement of air relative to Earth's surface, caused by imbalances in atmospheric pressure.

• Winds move from areas of higher pressure to lower pressure.

• Winds are named for the direction from which they originate (e.g., Easterly Winds come FROM the East).

• Example: A northerly wind comes from the north and blows toward the south.

Forces Affecting Air Movement

Main Forces

• Pressure Gradient Force (PGF): Drives air from high to low pressure.

• Coriolis Force (Fco): Caused by Earth's rotation, deflects moving air to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

• Frictional Force (f): Acts opposite to the direction of air movement, significant near Earth's surface.

Pressure Gradient Force (PGF)

• PGF is the difference in pressure over a given distance between two points.

• Represented by isobars (lines of constant pressure) on weather maps.

• PGF direction is perpendicular to isobars and points toward lower pressure.

• Formula: $PGF = -\frac{1}{\rho} \frac{\Delta P}{\Delta d}$ where $\rho$ is air density, $\Delta P$ is pressure difference, and $\Delta d$ is distance.

• Example: Closely spaced isobars indicate a strong PGF and faster winds.

Causes of Pressure Differences

• Solar Radiation: Uneven heating of Earth's surface.

• Temperature: Warm air is less dense and creates lower pressure.

• Humidity: Moist air is lighter than dry air, affecting pressure.

• Air Movement: Dynamic changes in air masses.

Interpreting Isobars and Wind Speed

Isobars on Weather Maps

• Isobars that are tightly packed together indicate strong winds due to a steep pressure gradient.

• Isobars that are spread apart indicate weak winds due to a gentle pressure gradient.

• On upper-level maps, isoheights are used instead of isobars.

• Example: A weather map showing closely spaced isobars around a low-pressure system predicts strong winds in that area.

Isobar Spacing	Wind Speed	Pressure Gradient
Tightly packed	Strong wind	Steep
Spread apart	Weak wind	Gentle

Summary Table: Forces Affecting Wind

Force	Effect	Location of Influence
Pressure Gradient Force (PGF)	Moves air from high to low pressure	Everywhere in the atmosphere
Coriolis Force (Fco)	Deflects air movement due to Earth's rotation	Greater at higher latitudes
Frictional Force (f)	Slows and alters wind direction	Near Earth's surface

Additional info: These notes expand on the brief points in the original slides, providing definitions, formulas, and examples for clarity and completeness.