Chapter 6: Atmospheric Pressure, Wind & Global Circulation

Air Pressure

The measurement of the weight that the air exerts on Earth's surface

Air Pressure's controlling factors

Density, altitude and temperature

Low Pressure System

Converging warm air flows vertically upward in a clockwise (Southern Hemisphere) or counterclockwise (Northern Hemisphere) motion

High Pressure System

Diverging cold air flows vertically towards the ground in a clockwise (Northern Hemisphere) or counterclockwise (Southern Hemisphere) motion

Cyclone

Another word for a low pressure system

Anticyclone

Another word for a high pressure system

Advection

The horizontal mixing of air due to differences in pressure

Airflow influences

Unequal heating of land surfaces, the pressure gradient force, the Coriolis force and various frictional forces

Convection

The vertical mixing of air due to differences in temperature

Unequal heating of land surfaces

The ultimate cause for all wind patterns on Earth that results from variations in the amount of solar radiation between latitudes

Pressure gradient force

The variable that drives the movement of air between two areas of different pressures; the greater the difference in pressure, the faster the air flows from high pressure to low pressure

Coriolis force

This force causes objects in the atmosphere (included the air) to appear to be deflected sideways as the Earth rotates under them

Geostrophic wind

Airflow that moves around pressure systems in the upper atmosphere

Frictional forces

These forces occur at ground level and are the result of the drag and impediments created by features on the Earth such as trees, mountains, buildings, etc that act to oppose the wind's direction

Hadley cell

Large-scale convection loop in the tropical latitudes that connects the Intertropical Convergence Zone (ITCZ) and the Subtropical High Pressure System (STH)

Intertropical Convergence Zone (ITCZ)

A narrow band of low pressure, calm winds, frequent rains, volatile weather and clouds along the equator due to direct year-round radiation

Trade winds

The primary wind system in the tropics (also known as the tropical easterlies) that flows toward the southwest in the Northern Hemisphere and towards the northwest in the Southern Hemisphere

Subtropical High Pressure System

A system of high air pressure that consists of the cooled, dry air that spirals downward towards the Earth's surface, diverging there to create the trade winds

Prevailing Westerlies

This airflow is formed when warm, dry air associated with the STH pressure system spirals down to Earth's surface and diverges, the pole-ward winds flow from west to east as part of a Ferrel Cell

Polar front

The focal point of midlatitude circulation that is the point of contact between cold air from higher latitudes and the relatively warm air from the tropics

Rossby Waves (meridional flow)

These occur when significant temperature differences exist between tropical and polar air masses and cause ripples or undulations to develop in the polar front jet stream that cause air to flow parallel to the meridians in many places, bringing cold air further south than normal.

Polar front jet stream (zonal flow)

River of high-speed air in the upper atmosphere that flows along the polar front, which is tightly confined to the high latitudes, forming a circular to semi-circular polar view

Polar High

Zone of high atmospheric pressure at high latitudes that consists of rotating systems of descending, cool & dry air that forms the polar easterlies

Polar easterlies

Band of easterly winds at high latitudes

Seasonal migrations of pressure systems

The ITCZ starts at the equator during the spring equinox, moves over the tropic of cancer in the summer solstice, returns back to the equator during the fall equinox, and ventures to the tropic of Capricorn during the winter solstice - causing all pressure systems to be moved with it

Monsoon

The seasonal change in wind direction that occurs in subtropical locations due to the migration of the ITCZ and the STH pressure systems.

Land-sea wind systems

These exist along the shores of major bodies of water such as oceans or large lakes due to the different heating and cooling characteristics of continents and water

Sea breeze

This occurs during the day when air over land heats up faster than the air over water, resulting in a low pressure zone while a high pressure zone forms over the water; this causes the cool air to flow to the land

Land breeze

This occurs at night when air over land cools more rapidly than the air over water, resulting in a high pressure zone while a low pressure zone forms over the water, causing the cool air to flow to the water

Valley breeze

During the day, the sun's rays and re-radiation heat the mountain slopes creating a low pressure zone while high pressure builds in the lowlands below, this causes the air to flow from the lowlands to the high lands

Mountain breeze

At night, the mountain slopes cool more rapidly than the lowlands, resulting in airflow to the low pressure valley

Gyres

Large oceanic circulatory systems that form because currents are deflected by landmasses

Thermohaline circulation

The global circulatory system that is driven differences in salinity (also called the oceanic conveyor belt)

El Nino

An atmospheric anomaly that weakens easterly winds every 3-5 years and reverses surface ocean water flow from west to east