atmospheric pressure, winds, and circulation patterns

Atmospheric pressure

The force per unit area exerted by the weight of air above a surface.

Mercury barometer

An instrument that measures air pressure using the height of a mercury column.

Standard sea-level pressure

1013.2 millibars (mb), 29.92 inches of mercury, or about 14.7 pounds per square inch (PSI).

Aneroid barometer

An instrument whose sealed, accordion-shaped cell expands or contracts as air pressure changes.

Elevation versus altitude

Elevation is the height of the ground; altitude is height in the air.

How does air pressure change with altitude?

Air pressure decreases as altitude increases because less air remains above you.

How does air density change with altitude?

Air density generally decreases rapidly with increasing altitude.

Air pressure on Mount Everest

About one-third of sea-level pressure, according to the slides.

Thermal pressure differences

Pressure differences caused by heating or cooling air, such as equatorial lows and polar highs.

Why does warm air tend to rise?

Warm air expands and becomes less dense than surrounding cooler air.

Why does cold air tend to sink?

Cold air is denser than surrounding warmer air.

Dynamic pressure differences

Pressure differences caused by atmospheric circulation, such as descending air at subtropical highs and rising air at subpolar lows.

Cyclone

A low-pressure system with surface convergence and rising air.

Anticyclone

A high-pressure system with sinking air and surface divergence.

Convergence

Air flowing together toward a region, such as toward a surface low.

Divergence

Air spreading outward from a region, such as away from a surface high.

Subsidence

The downward movement of air.

Isobar

A line on a map connecting points with equal atmospheric pressure.

Why are weather-map pressures adjusted to sea level?

To compare pressure across places with different elevations.

Pressure gradient

The change in air pressure over a distance.

Closely spaced isobars

A strong pressure gradient, usually associated with stronger winds.

Widely spaced isobars

A weak pressure gradient, usually associated with weaker winds.

Wind

Movement of air in response to differences in atmospheric pressure.

Pressure-gradient force

The force that pushes air from higher pressure toward lower pressure.

What three factors control wind direction?

Pressure gradient, Coriolis effect, and friction.

Advection

Horizontal transport of properties such as heat or moisture by moving air or water.

How do winds redistribute energy?

They transfer energy from areas with an insolation surplus toward areas with an energy deficit.

Coriolis effect

The apparent deflection of moving air and other freely moving objects caused by Earth's rotation.

Coriolis deflection in the Northern Hemisphere

To the right of the object's direction of motion.

Coriolis deflection in the Southern Hemisphere

To the left of the object's direction of motion.

Where is the Coriolis effect strongest?

At the poles; it weakens toward the equator.

Earth's rotation rate

15 degrees per hour. Linear surface speed is greatest at the equator.

Friction and wind

Surface drag slows wind, reducing the Coriolis effect and allowing wind to cross isobars toward lower pressure.

Friction layer

The lowest roughly 1,000 meters (3,300 feet) of the atmosphere, where surface friction most affects wind.

Geostrophic wind

Wind flowing parallel to isobars when the pressure-gradient force and Coriolis effect balance and friction is negligible.

Where is geostrophic flow most likely?

Above the surface friction layer, where surface drag has little influence.

How are winds named?

By the direction they come from. A northeast wind blows from the northeast toward the southwest.

Windward

The side of an object or landform facing the incoming wind.

Leeward

The sheltered, downwind side of an object or landform.

Prevailing winds

Winds that commonly or seasonally come from the same direction.

Northern Hemisphere surface winds around a cyclone

Spiral inward counterclockwise toward low pressure.

Northern Hemisphere surface winds around an anticyclone

Spiral outward clockwise away from high pressure.

Trough

An elongated area of low pressure.

Ridge

An elongated area of high pressure.

Global pressure belts from the North Pole to the South Pole

Polar high, subpolar low, subtropical high, equatorial low, subtropical high, subpolar low, polar high.

Equatorial low

A low-pressure belt near the equator associated with rising warm air and converging trade winds.

Subtropical highs

High-pressure belts near 30 degrees north and south, associated with descending air.

Subpolar lows

Low-pressure belts near 60 degrees north and south, associated with rising air.

Polar highs

High-pressure regions near the poles, associated with cold, dense, sinking air.

Siberian and Canadian Highs

Cold, stable high-pressure systems that develop over Northern Hemisphere land in winter.

Icelandic and Aleutian Lows

Subpolar low-pressure systems over the North Atlantic and North Pacific, especially prominent in Northern Hemisphere winter.

How do Northern Hemisphere pressure belts shift in summer?

Subpolar lows move poleward, and subtropical highs shift northward.

Bermuda High and Pacific High

Northern Hemisphere subtropical high-pressure systems over the Atlantic and Pacific oceans.

What drives global atmospheric circulation?

Differential heating, Earth's rotation, Coriolis deflection, and atmospheric dynamics.

Trade winds

Tropical easterly winds blowing toward the equatorial low, highlighted around 5–25 degrees latitude in the slides.

Northeast trade winds

Northern Hemisphere trade winds that blow from the northeast toward the equator.

Southeast trade winds

Southern Hemisphere trade winds that blow from the southeast toward the equator.

Intertropical convergence zone (ITCZ)

The equatorial zone where trade winds converge and warm air rises, producing heavy precipitation.

Doldrums (equatorial calms)

A zone of weak horizontal airflow near the ITCZ.

Why is the ITCZ rainy?

Converging warm, moist air rises, cools, and produces clouds and precipitation.

Horse latitudes

Areas near the subtropical highs characterized by weak winds and descending air.

Westerlies

Prevailing winds blowing from the west between about 30 and 60 degrees latitude, from subtropical highs toward subpolar lows.

Polar easterlies

Winds blowing from polar highs toward the polar front, generally from the east.

Polar front

The zone where relatively warm midlatitude air meets cold polar air, associated with subpolar lows and stormy weather.

Why do global wind and pressure belts migrate seasonally?

They shift with the latitude receiving maximum insolation.

Why can places around 5–15 degrees latitude have wet summers and dry winters?

The rainy ITCZ reaches them in summer, while subtropical high pressure dominates in winter.

Why does California have dry summers and wetter winters?

Subtropical high pressure dominates in summer; wetter westerlies influence it in winter.

Why are subtropical west coasts often dry?

Descending air from subtropical highs and cold ocean currents promote stable conditions.

Why are subtropical east coasts often moist?

Warm ocean water supplies moisture and favors rising, unstable air.

Jet stream

A narrow, high-velocity upper-air current, with speeds of about 75–250 mph in the slides.

Jet streams and geostrophic winds

Jet streams are high-altitude examples of approximately geostrophic flow within the upper-air westerlies.

Upper-air westerlies

The broad west-to-east airflow through much of the upper atmosphere.

Polar jet stream

A westerly jet near the tropopause, strongest in winter when temperature contrasts are steepest.

Subtropical jet stream

A westerly jet above subtropical highs that weakens and may disappear in summer.

Why are jet streams generally stronger in winter?

Greater temperature contrasts produce stronger upper-air pressure gradients.

How does pressure decrease in cold versus warm air columns?

Pressure decreases more rapidly with altitude in cold air because the column is denser and more compact.

Rossby waves

Large horizontal undulations in upper-air winds of the middle and high latitudes.

Rossby-wave cycle in the slides

The illustrated cycle takes roughly 4–8 weeks.

Why do upper-air winds matter for volcanic eruptions?

They can transport volcanic ash far from its source and create aviation hazards.

Monsoon

A seasonal reversal of wind direction caused by changing land-ocean pressure contrasts.

Summer monsoon

Warm, humid ocean air flows toward low pressure over heated land, bringing wet weather.

Winter monsoon

Cooler, drier air flows from land toward the sea.

Major monsoon regions

South Asia and East Asia.

Minor monsoon regions highlighted in the slides

Northern Australia and West Africa.

Diurnal wind reversal

A daily change in wind direction, such as land-sea or mountain-valley breezes.

What causes land and sea breezes?

Land heats and cools faster than water, creating daily pressure differences.

Sea breeze

A daytime surface wind from water toward warmer land.

Land breeze

A nighttime surface wind from cooler land toward relatively warmer water.

How can a sea or lake breeze cool a shoreline?

It brings cooler air from over the water and increases evaporation; the slides note 9–16 degrees Fahrenheit of cooling.

Valley breeze

A daytime upslope wind produced as mountain slopes heat and air rises.

Mountain breeze

A nighttime downslope wind produced as slopes cool and dense air drains downhill.

Katabatic (drainage) wind

A downslope flow of cold, dense air from higher to lower elevations.

Chinook (Foehn) wind

A warm, dry downslope wind on the leeward side of a mountain range.

Why does a Chinook become warm and dry?

Moist air rises and cools, condensation releases latent heat, and air then warms as it descends on the leeward side.

Santa Ana winds

A regional example of warming winds listed in the slides.

Latent heat release during condensation

Heat released when water vapor changes into liquid water.

Ocean water density compared with sea-level air

Ocean water is about 800 times denser than sea-level air.

What drives surface ocean currents?

Winds, especially prevailing winds.

Ocean gyre

A large system of surface ocean currents circulating around a subtropical high.

Warm ocean currents highlighted in the slides

The Gulf Stream and Kuroshio Current.