Comprehensive Meteorology: Air Pressure, Wind Patterns, and Circulation

What happens when a gas is heated?

Molecules move faster, air expands, density decreases, and pressure can drop.

What happens when a gas is cooled?

Molecules move slower, air contracts, density increases, and pressure can rise.

Relationship between temperature, density, and pressure

Warmer air = less dense, lower pressure; cooler air = more dense, higher pressure.

Air pressure

The force exerted by the weight of air molecules above a given point.

Average sea level pressure

1013.25 mb (millibars) or 29.92 inHg.

How air pressure changes vertically

Pressure decreases with altitude because less air is above you.

How air pressure changes horizontally

Pressure differences across space drive winds.

How air pressure causes vertical motion

Low pressure allows air to rise, high pressure forces air to sink.

How air pressure causes horizontal motion

Air moves from high pressure to low pressure, creating wind.

Pressure Gradient Force

The force that moves air from areas of high pressure to areas of low pressure.

Convergence

When air flows together and rises.

Divergence

When air spreads apart and sinks.

Friction and wind at Earth's surface

Friction slows wind, reduces Coriolis effect, and causes air to cross isobars toward low pressure.

Friction and wind aloft

Little friction, so winds flow more parallel to isobars.

Coriolis effect aloft

Deflects winds to the right in the Northern Hemisphere, to the left in the Southern Hemisphere, creating geostrophic flow.

Coriolis effect at surface

Still deflects winds, but friction weakens it, so winds angle toward low pressure.

Atmospheric circulation differences between hemispheres

Northern Hemisphere winds deflect right, Southern Hemisphere winds deflect left.

Wind patterns around low pressure at surface

Air converges and rises, counterclockwise in Northern Hemisphere, clockwise in Southern.

Wind patterns around high pressure at surface

Air diverges and sinks, clockwise in Northern Hemisphere, counterclockwise in Southern.

Wind patterns aloft around low pressure

Winds circulate parallel to isobars around troughs.

Wind patterns aloft around high pressure

Winds circulate parallel to isobars around ridges.

Coriolis effect

Apparent deflection of moving objects due to Earth's rotation.

Geostrophic wind

Wind aloft that flows parallel to isobars when Coriolis force balances pressure gradient force.

Earth's energy balance

Surplus energy near equator, deficit near poles.

Thermal high pressure

Cold, dense air creates high pressure (e.g., polar high).

Thermal low pressure

Warm, rising air creates low pressure (e.g., equatorial low).

Dynamic high pressure

Created by descending air in circulation cells (e.g., subtropical high).

Dynamic low pressure

Created by rising air in circulation cells (e.g., subpolar low).

Major wind systems

Trade winds, westerlies, polar easterlies, named for direction they come from.

Land vs water pressure differences

Land heats/cools faster, creating stronger thermal highs/lows than water.

Bermuda High

Subtropical high in the Atlantic that steers hurricanes and controls summer weather.

Equatorial Low

Rising air at the ITCZ, thermal low.

Subtropical High

Descending air around 30° latitude, dynamic high.

Subpolar Low

Rising air around 60° latitude, dynamic low.

Polar High

Cold, dense sinking air at poles, thermal high.

Westerlies

Winds from west in mid-latitudes.

Polar Easterlies

Cold winds blowing from east near poles.

Trade Winds

Steady winds from east in tropics.

Main circulation patterns in tropics

Hadley cells with rising air at ITCZ and trade winds at surface.

Main circulation patterns in mid-latitudes

Westerlies and Ferrell cell circulation.

Main circulation patterns in high latitudes

Polar cells with easterly winds.

ITCZ migration

Moves north in summer, south in winter, following solar heating.

Air circulation aloft in mid-latitudes

Westerly jet streams with Rossby waves.

Air circulation aloft in tropics

Rising motion at ITCZ and outflow aloft.

Air circulation aloft in poles

Sinking cold air with weak circulation.

Polar Front Jet Stream

Strong winds near tropopause around mid-latitudes, separating cold and warm air.

Rossby Waves

Large meanders in jet stream that influence U.S. weather patterns.

Tropopause height by latitude

Higher in tropics (warm air expands), lower near poles (cold air contracts).

Trade Winds

Steady easterly surface winds in tropics.

ITCZ

Intertropical Convergence Zone where trade winds meet and air rises.

Hadley Cell

Circulation cell with rising air at equator and sinking at subtropics.

Polar Cell

Circulation cell with rising air at subpolar lows and sinking at poles.

Ferrell Cell

Mid-latitude circulation cell driven by Hadley and Polar cells.

Jet Stream

Fast winds aloft near tropopause caused by strong pressure gradients.

Polar Front Jet Stream

Strongest jet stream, at mid-latitudes.

Subtropical Jet Stream

Jet stream near 30° latitude.

Ridges

Northward bulges in jet stream, bring warm weather.

Troughs

Southward dips in jet stream, bring cold weather.

Sea Breeze

Daytime breeze from ocean to land due to land heating faster.

Land Breeze

Nighttime breeze from land to ocean due to land cooling faster.

Valley Breeze

Daytime upslope wind as slopes heat faster than valleys.

Mountain Breeze

Nighttime downslope wind as slopes cool faster.

Urban Winds

Modified circulation due to heat island effect.

Chinook Winds

Warm, dry downslope winds east of mountains.

Santa Ana Winds

Hot, dry downslope winds in California, increasing fire risk.

Monsoon Winds

Seasonal reversal of winds due to land-ocean heating contrasts.

Effects of local winds on people

Influence temperature, humidity, storms, agriculture, and hazards like fires.

Why land and water heat differently

Land heats/cools quickly, water slowly, due to specific heat and mixing.

Where is water in Earth's systems?

Atmosphere, oceans, ice, groundwater, rivers, lakes, and living organisms.

How water changes from vapor to liquid in atmosphere

Cooling to dew point causes condensation.

Polarity of water

Water molecules have positive and negative ends, leading to hydrogen bonding.

Hydrogen bonds

Bonds between water molecules that give water cohesion, surface tension, and capillary action.

Four ways to measure humidity

Absolute humidity, specific humidity, vapor pressure, relative humidity.

Relative humidity temperature dependence

Warmer air can hold more water vapor; RH decreases as temperature rises.

Two ways air can reach dew point

Cooling air to saturation or adding more water vapor.

General patterns of dew point in U.S.

Higher in the Southeast, lower in the West and at higher elevations.

Is dew point changing?

Yes, it has been increasing with warming climate.

Surface tension

Property of water due to hydrogen bonding at surface.

Capillary action

Water movement through small pores due to cohesion/adhesion.

Polarity

Unequal distribution of charges in a molecule.

Hydrogen bond

Weak bond between positive hydrogen and negative oxygen in water molecules.

Evaporation

Liquid water to vapor.

Condensation

Vapor to liquid.

Saturation

Maximum water vapor air can hold at a given temperature.

Dew point

Temperature at which air becomes saturated.

Dew point depression

Difference between air temperature and dew point.

Vapor pressure

Pressure exerted by water vapor in the air.

Relative humidity

Ratio of water vapor in air to maximum possible at that temperature.

Absolute humidity

Mass of vapor per unit volume of air.

Adiabatic process

Temperature changes in air without heat exchange, caused by expansion or compression.

Ideal Gas Law and air parcels

Rising air expands and cools, sinking air compresses and warms.

Dew point at LCL

Rising air cools to dew point, clouds begin forming.

How adiabatic processes move energy

Rising air carries heat upward, fueling weather systems.

Unsaturated lapse rate

10°C/km cooling for rising unsaturated air.

Saturated lapse rate

6°C/km cooling for rising saturated air.

Environmental lapse rate

Actual temperature change with height in atmosphere.

Stable atmosphere characteristics

Suppresses vertical motion, clear skies, inversions.

Unstable atmosphere characteristics

Promotes convection, clouds, storms.

Things that increase instability

Surface heating, urban heat, warm ocean currents.

Things that increase stability

Temperature inversions, cold ocean currents.