Weather, Climate, and Air Pressure: Key Concepts for Meteorology, Hurricanes: Structure, Damage, Classification, and Case Study, Global Atmospheric Circulation & Prevailing Winds: Key Concepts and Patterns, Advanced Thunderstorm and Tornado Dynamics:…

Weather

The short-term state of the atmosphere in a specific place and time. Can change within minutes, hours, or days. Includes things like temperature, precipitation, cloud cover, and wind speed.

Climate

The long-term average of weather conditions over decades or centuries. Gives the "expected" pattern of weather in a region.

Key Difference between Weather and Climate

Weather = what you get day-to-day. Climate = what you expect over long periods.

Influences on Climate

Natural cycles (like Earth's orbit, ocean currents, volcanic eruptions) and human activity (especially burning fossil fuels → greenhouse gases → global warming).

Elements of Weather and Climate

Meteorologists usually track 5 main elements: Temperature, Humidity, Clouds, Precipitation, Wind.

Air Pressure

The driving force behind most weather patterns. Controls whether conditions are stable (clear) or unstable (stormy).

Definition of Air Pressure

The force per unit area exerted by the weight of the atmosphere.

Cause of Air Pressure

Gravity pulls atmospheric gases (mostly nitrogen [N₂] and oxygen [O₂]) toward Earth, creating weight that pushes down on surfaces.

Standard Air Pressure at Sea Level

1,013.25 millibars (mb) or 29.92 inches of mercury (Hg).

Tools for Measuring Air Pressure

Barometer → mercury column rises/falls with pressure changes. Altimeter in planes = modified barometer for altitude.

Variation in Air Pressure

Changes by elevation, temperature, and humidity.

Effect of Elevation on Air Pressure

Higher altitudes = less atmosphere above = lower pressure.

Effect of Temperature on Air Pressure

Warm air → expands, less dense → lower pressure. Cold air → contracts, denser → higher pressure.

Effect of Humidity on Air Pressure

Moist air is lighter than dry air (water vapor has lower molecular weight than N₂ or O₂). More humidity = lower pressure.

Why Air Pressure Matters

Pressure differences drive winds. Rising air (low pressure) leads to clouds and storms. Sinking air (high pressure) leads to clear, calm skies.

High Pressure Systems (Anticyclones)

Characteristics: Cold, dry, heavy air sinks toward the ground. Stable, calm weather. Few clouds → clear skies, little precipitation.

Example of High Pressure System

Crisp, cool winter day with sunshine.

Low Pressure Systems (Cyclones)

Characteristics: Warm, moist, lighter air rises. As it rises, it cools → water vapor condenses → clouds form. Storms and precipitation are common.

Intensity of Low Pressure Systems

The lower the pressure, the more intense the storm.

Example of Low Pressure System

Hurricanes and typhoons are extreme low-pressure systems.

Hurricane Katrina

Dropped to ~920 mb → very strong winds and storm surge.

Temperature

Measure of the average kinetic energy (motion) of particles.

Cold air

Molecules move slower, closer together → denser, heavier → higher pressure.

Warm air

Molecules move faster, spread farther apart → lighter → lower pressure.

Absolute Zero

The point where all molecular motion stops (0 Kelvin).

Warm, moist air

The 'fuel' for storms.

Cold, dry air

Stabilizes weather.

Conduction

Heat transfer by direct contact between particles.

Convection

Heat transfer by the movement of fluids (liquids or gases).

Radiation

Heat transfer by electromagnetic waves.

Latent Heat

Hidden heat stored in water molecules during phase changes.

Melting

Solid → liquid → absorbs heat.

Freezing

Liquid → solid → releases heat.

Evaporation

Liquid → gas → absorbs heat (cooling process).

Condensation

Gas → liquid → releases heat (warming process).

Absolute Humidity

Actual amount (grams of water per cubic meter of air).

Relative Humidity

Ratio of how much vapor is present vs. how much the air could hold at that temperature.

Humidity

Amount of water vapor in the air.

Water vapor molecules

H₂O are lighter (molecular weight 18) than N₂ (28) or O₂ (32).

High pressure

Cold, dry, stable, clear weather.

Low pressure

Warm, moist, unstable, stormy weather.

Key Takeaways

Air pressure is the main control of weather and climate.

Hurricane

Organized tropical cyclones with thunderstorms rotating counterclockwise (NH).

Eye

Calm center of storm.

Eyewall

Surrounding intense thunderstorms and strongest winds.

Rainbands

Spiral bands of storms around the eye.

Small hurricanes

Stronger winds due to a steeper pressure gradient.

Large hurricanes

Cover more area and push more water ashore (storm surge).

High Winds

Strongest on eastern side of storm; forward motion adds to sustained wind on east side.

Storm Surge

Abnormal rise of sea level during hurricane; worst in low-lying coastal areas.

Hurricane size

Influences storm surge.

Wind speed & pressure

Factors that influence storm surge.

Tide level at landfall

Influences storm surge.

Hurricane Katrina (2005)

Category 3 winds, but 21 ft storm surge → destruction like Category 5.

Saffir-Simpson Scale

Classification based only on sustained wind speed.

Category 1

74-95 mph winds.

Category 2

96-110 mph winds.

Category 3

111-129 mph winds.

Category 4

130-156 mph winds.

Category 5

157+ mph winds.

Tropical Disturbance

Unorganized thunderstorms.

Tropical Depression

20-38 mph winds, weak rotation.

Tropical Storm

39-73 mph winds, eye starts to form.

Naming of Hurricanes

Early US hurricanes named by location; now use male & female names on a 7-year rotating list.

Destructive hurricanes' names

Retired after significant damage (e.g., Katrina, Camille, Matthew, Isaac).

Hadley Cell

Convection cell located between 0°-30° latitude.

Ferrel Cell

Convection cell located between 30°-60° latitude.

Polar Cell

Convection cell located between 60°-90° latitude.

Coriolis Effect

Apparent deflection of moving air (and objects) due to Earth's rotation.

Polar Easterlies

Winds that originate in the east and move from polar high (90°) toward 60° low, deflected right in the Northern Hemisphere.

Westerlies

Winds that move from 30° high toward 60° low, deflected right in the Northern Hemisphere, originating in the west.

Trade Winds

Winds that move from 30° high toward equatorial low (ITCZ), deflected right in the Northern Hemisphere, blowing from northeast to southwest.

Doldrums

Calm, weak winds near the equator (0°) where winds slow and stall.

Pressure Belts

High and low pressure zones that alternate by latitude.

High Pressure (90°)

Cold, dry, desert-like conditions found at the poles.

Low Pressure (60°)

Clouds and rain belts found at 60° latitude.

High Pressure (30°)

Dry, desert regions found at 30° latitude.

Low Pressure (0°)

Warm, rainy tropics found at the equator (ITCZ).

Deserts

Regions found at 30° N/S and 90° (poles), such as the Sahara Desert.

Rain Belts

Regions found at 0° (Equator) and 60° N/S, such as tropical rainforests.

Friction

Slows and deflects winds, caused by mountains, forests, and rough terrain.

Non-uniform surface of Earth

Land heats/cools faster than water, affecting temperature moderation.

Seasonal shifts

Changes in sunlight distribution due to Earth's tilt, moving pressure belts north/south.

January (NH Winter)

Pressure centers include strong Polar Highs and low pressures over oceans.

July (NH Summer)

Pressure centers shift northward with the sun over Tropic of Cancer.

Local Example: Louisiana (30°N)

Humid and rainy due to proximity to Gulf of Mexico and influence of low-pressure systems.

Big Picture: Why This Matters

Prevailing winds and pressure belts control movement of weather systems and local climates.

Cumulonimbus Stage

Slightly more advanced stage of thunderstorms.

Hail formation

Updrafts suspend raindrops in cold air → freezing → additional layers accumulate → hail grows.

Cold fronts

Don't produce a single line of storms. Storms form along the boundary, die out, and new storms develop as the front moves.

Supercell Thunderstorms

Supercells are thunderstorms with an internal rotation called a mesocyclone (3-5 km wide).

Mesocyclone

Internal rotation in the storm; 'meso' = mid-scale, 'cyclone' = rotation.

Wind shear

Upper-level cold air moves opposite to lower-level warm, humid air.

Updrafts

Lift surface-parallel rotation into the storm → rotation becomes perpendicular → can spawn tornadoes.

Subtropical jet stream

30°-60° latitude.

Polar jet stream

60°-90° latitude.

Seasonal tornado risk

Louisiana & North Texas: Feb-Apr; Oklahoma & Kansas: Later spring; Dakotas: Late May-July.

Flat land

Reduces friction, allowing stronger rotation.

Tornado Watch

A storm with internal rotation has potential to spawn a tornado.

Tornado Warning

Rotation confirmed reaching the ground.