Weather variables are measured at weather stations, buoys, satellites, and radar. Temperature is measured with thermometers (Celsius, Fahrenheit, Kelvin). Atmospheric pressure is measured with barometers (millibars or hectopascals). Humidity is measured with hygrometers (relative humidity, dew point). Wind speed (anemometer) and direction (wind vane). Precipitation is measured with rain gauges (mm or inches) and radar (reflectivity). Weather maps show fronts (cold, warm, stationary, occluded), pressure systems (high pressure = fair weather, low pressure = storms), and isobars (lines of constant pressure). Weather forecasting uses numerical weather prediction (NWP) – supercomputers solve fluid dynamics equations (primitive equations) for the atmosphere. The first weather forecast (1861, Admiral FitzRoy, UK). Global forecasting models: GFS (US), ECMWF (Europe), ICON (Germany), UKMO (UK). Weather prediction accuracy has improved dramatically – a 5-day forecast today is as accurate as a 3-day forecast in 1995. Climate is defined by World Meteorological Organization (WMO) as 30-year averages (current normal period 1991-2020). Climate classifications include Köppen system (tropical, dry, temperate, continental, polar). Climate zones are influenced by latitude, altitude, proximity to oceans, prevailing winds, and ocean currents. Weather vs climate example: a cold day in winter is weather; the fact that winters are colder than summers is climate.

The water cycle is powered by solar energy and gravity. Evaporation from oceans (86% of atmospheric moisture) and other water bodies. Transpiration from plants (10%) – together called evapotranspiration. Sublimation (ice to vapor directly) from snow and ice. Condensation requires cloud condensation nuclei (dust, salt, pollen) for water vapor to form droplets. Clouds are classified by height: high (cirrus, cirrocumulus, cirrostratus – above 6 km), middle (altostratus, altocumulus – 2-6 km), low (stratus, stratocumulus, nimbostratus – below 2 km), and vertical development (cumulus, cumulonimbus – thunderstorm clouds). Precipitation forms via collision-coalescence (warm clouds) or ice crystal process (Bergeron-Findeisen – cold clouds). Types: rain (liquid), freezing rain (supercooled drops freezing on contact), sleet (ice pellets), hail (layered ice from strong updrafts in thunderstorms), snow (ice crystals). Runoff flows into rivers, lakes, oceans. Infiltration recharges groundwater aquifers. Groundwater moves slowly (meters to kilometers per year) and discharges into springs, rivers, or oceans. The water cycle residence times: atmosphere ~9 days, rivers ~2 weeks, soil moisture ~2-60 weeks, lakes ~10 years, groundwater ~200-10,000 years, glaciers ~1,000-100,000 years, oceans ~3,000 years. Humans affect the cycle through reservoirs, groundwater pumping, irrigation, and land use changes (urbanization increases runoff).

Thunderstorm lifecycle (single cell): (1) Cumulus stage – updraft dominates, no precipitation yet. (2) Mature stage – updrafts and downdrafts coexist, precipitation begins, lightning and thunder occur. (3) Dissipating stage – downdraft dominates, precipitation decreases, storm weakens. Single-cell storms last 30-60 minutes. Multi-cell clusters or squall lines can last hours. Supercells are long-lived (2-6 hours), rotating thunderstorms (mesocyclone) with a tilted updraft, capable of producing large hail (golf ball to softball size), strong winds (downbursts >100 km/h), and tornadoes. Lightning is a giant spark of static electricity between positive and negative charges within cloud (intra-cloud), cloud-to-cloud, or cloud-to-ground. Cloud-to-ground lightning strikes the tallest objects. Thunder is the sound of rapidly expanding superheated air (up to 30,000°C – 5x sun surface temperature). The time delay between lightning and thunder (5 seconds per mile, 3 seconds per km) gives distance. Hail forms when updrafts carry water droplets above freezing level, where they freeze and grow by colliding with supercooled water. Stronger updrafts produce larger hail. Hail can damage crops, roofs, and vehicles. Flash flooding occurs when heavy rain falls faster than ground can absorb or drain. The National Weather Service issues thunderstorm warnings for winds >93 km/h (58 mph) or hail >2.5 cm (1 inch). The United States experiences about 100,000 thunderstorms annually, with Florida having the most (80-100 days/year).

Tornado formation requires a supercell thunderstorm with a mesocyclone (rotating updraft). Strong wind shear (changing wind direction and speed with height) creates horizontal rotation. The updraft tilts this rotation into the vertical, forming a mesocyclone. If the mesocyclone tightens and descends, a wall cloud forms, then a tornado. Tornadoes most common 3-9 PM, April-June. Peak season for southern states is March-May; northern states June-July. Tornado alley has shifted eastward in recent decades to "Dixie Alley" (Arkansas, Louisiana, Mississippi, Alabama, Georgia, Tennessee). Warning lead time increased from <5 minutes (1980s) to 13-15 minutes (current) due to Doppler radar (NEXRAD – Next Generation Radar). Doppler radar measures rotation signatures (velocity couplet) and debris ball (tornado debris signature). Tornado outbreak – multiple tornadoes from same weather system (e.g., April 27, 2011, 199 tornadoes, 316 deaths). Waterspouts are tornadoes over water (usually weaker). Landspouts are non-supercell tornadoes from boundary layer convergence. Tornado safety: basement or interior room on lowest floor (bathroom, closet), away from windows. Mobile homes are dangerous – seek permanent shelter. The "tornado season" is a misnomer – tornadoes can occur any month. Oklahoma City has been hit more than any US city (over 100 times). The Tri-State Tornado (March 18, 1925) was the deadliest (695 deaths), crossing Missouri, Illinois, Indiana. Tornado records: widest (El Reno, OK 2013 – 4.2 km/2.6 miles), longest path (Tri-State – 352 km/219 miles), highest wind speed (Bridge Creek-Moore, OK 1999 – 484 km/h/301 mph).

Hurricane formation requires: warm ocean water (heat and moisture source), atmospheric instability, high humidity, Coriolis effect (provides spin – absent at equator, so no hurricanes within 5° latitude), low wind shear (no changing wind with height to disrupt organization), and a pre-existing disturbance (tropical wave). Hurricane categories (Saffir-Simpson scale): Category 1 (119-153 km/h) – very dangerous winds, some damage. Category 2 (154-177 km/h) – extensive damage. Category 3 (178-208 km/h) – devastating damage, major hurricane. Category 4 (209-251 km/h) – catastrophic damage. Category 5 (252+ km/h) – catastrophic damage, power outages weeks to months. Hurricane hazards: storm surge (rising water pushed by winds – greatest threat to life), high winds, heavy rainfall (flooding), tornadoes. Naming conventions: six-year rotating lists for Atlantic and Eastern North Pacific (male/female names). Lists retire names of deadly hurricanes (e.g., Katrina, Harvey, Irma, Maria, Florence, Dorian). Atlantic hurricane season: June 1 – November 30 (peak mid-August to late October). Eastern Pacific season: May 15 – November 30. Most active basin: Northwest Pacific (typhoons, year-round). Hurricane structure: Eye – 20-60 km diameter, clear, light winds, lowest pressure. Eyewall – ring of towering thunderstorms, highest winds. Rainbands – spiral bands of thunderstorms and showers. The right-front quadrant (relative to storm motion) has strongest winds in Northern Hemisphere. Hurricane intensity forecasting has improved, but rapid intensification (wind speed increase >55 km/h in 24 hours) remains challenging. Notable hurricanes: Labor Day 1935 (strongest US landfall, Category 5), Camille 1969 (second strongest US landfall), Katrina 2005 (costliest, $200 billion), Harvey 2017 (extreme rainfall >1.5 meters), Patricia 2015 (strongest ever recorded in Western Hemisphere, 345 km/h winds). Aircraft reconnaissance (NOAA Hurricane Hunters) fly into hurricanes to collect data.

High clouds (above 6 km / 20,000 ft): Cirrus (Ci) – thin, wispy, "mare's tails," indicate fair weather but possible approaching warm front. Cirrocumulus (Cc) – small ripples, "mackerel sky." Cirrostratus (Cs) – thin, veil-like, produces halos around sun/moon. Middle clouds (2-6 km / 6,500-20,000 ft): Altocumulus (Ac) – patchy, white/gray layers with shadow. Altostratus (As) – gray/blue sheets, can produce light precipitation. Low clouds (below 2 km / 6,500 ft): Stratus (St) – uniform, gray, fog-like, drizzle. Stratocumulus (Sc) – lumpy, gray, patches. Cumulus (Cu) – puffy, flat bottom, fair weather (Cumulus humilis). Vertical development clouds (surface to upper troposphere): Cumulonimbus (Cb) – thunderstorm clouds, anvil top, produce heavy rain, lightning, hail, tornadoes. Nimbostratus (Ns) – dark gray, continuous rain/snow, no lightning or thunder. Rare clouds: Nacreous (polar stratospheric clouds) – iridescent, polar regions only, responsible for ozone depletion. Noctilucent clouds – highest clouds (mesosphere, 80 km), visible at night during summer. Lenticular clouds – lens-shaped over mountains, often mistaken for UFOs. Clouds are identified by appearance and altitude. The World Meteorological Organization (WMO) maintains the International Cloud Atlas (first edition 1896). Luke Howard (1802) created the first cloud classification system (still used with modifications). Cloud cover measured in oktas (eighths of sky covered). Precipitation probability relates to cloud types – cumulonimbus = high chance of thunderstorm; cirrus = no immediate precipitation.

Pressure decreases exponentially with altitude – half at 5.5 km, half again at 16 km. Pressure at Mount Everest summit (8.8 km) is 330 mb (1/3 sea level). Evangelista Torricelli invented the mercury barometer (1643). Aneroid barometers use a flexible metal cell. Digital barometers use MEMS sensors. Pressure gradients (differences over distance) generate wind – wind blows from high to low pressure, with strength proportional to gradient. Coriolis effect (Earth's rotation) deflects wind to the right in Northern Hemisphere, left in Southern Hemisphere. Isobars (lines of constant pressure) on weather maps show pressure patterns. High pressure (H, blue) – sinking air (subsidence), suppresses cloud formation, clear skies, light winds, clockwise outflow in Northern Hemisphere. Low pressure (L, red) – rising air (convection), clouds, precipitation, counterclockwise inflow (cyclonic), strongest winds. Pressure tendency (rising/falling) indicates weather changes: rapidly falling pressure (1+ mb/hour) signals approaching storm. The Great Red Spot on Jupiter is a giant high-pressure system (anticyclone). Earth's lowest recorded sea-level pressure: 870 mb (Typhoon Tip, 1979). Highest recorded: 1085 mb (Mongolia, 2001). Air pressure affects human physiology – cabin pressure in commercial airliners is equivalent to 2,400 meters (8,000 ft) altitude (≈750 mb). Pressure is measured in millibars (mb), hectopascals (hPa – identical), inches of mercury (inHg), or atmospheres (atm). Barometric trend is the three-hour pressure change (rising/falling/steady).

Surface currents (upper 400 meters) are driven by prevailing winds (trade winds, westerlies) and deflected by Coriolis effect (Ekman transport), creating gyres – large circular current systems. Five major gyres: North Atlantic, South Atlantic, North Pacific, South Pacific, Indian Ocean. Gyres rotate clockwise in Northern Hemisphere, counterclockwise in Southern Hemisphere. The Gulf Stream transports warm water from Gulf of Mexico along US East Coast to Europe, warming northwestern Europe by up to 5-10°C. Gulf Stream flow: 30 million cubic meters per second (30 Sverdrups – 1 Sv = 1 million m³/s) – greater than all Earth's rivers combined. The Antarctic Circumpolar Current (ACC) flows west to east around Antarctica with volume transport of 125 Sv, connecting all three major oceans. Thermohaline circulation ("ocean conveyor belt") is driven by density differences: cold, salty water sinks in North Atlantic (deep water formation) and around Antarctica, traveling along the ocean floor, upwelling in Pacific and Indian Oceans after 1,000+ years. Upwelling (wind-driven upward movement of cold, nutrient-rich water) occurs along western coasts (California, Peru, Northwest Africa, Chile) – supports productive fisheries (anchovies, sardines). El Niño-Southern Oscillation (ENSO) is a climate pattern involving sea surface temperature changes in equatorial Pacific (El Niño – warm, La Niña – cool) affecting global weather. El Niño typically occurs every 2-7 years. Ocean currents affect marine ecosystems, shipping routes (time and fuel efficiency), and coastal temperatures. Thermohaline circulation is sensitive to freshwater input; collapse would alter global climate. ARGO floats (global array of 4,000 robotic profiling floats) measure temperature and salinity to 2,000 m depth. Satellite altimeters (TOPEX/Poseidon, Jason series, Sentinel-6) measure sea surface height for current mapping.

The Coriolis effect is a consequence of Earth's rotation (once per day) and the fact that points at different latitudes have different rotational speeds. At the equator, rotational speed is 1,670 km/h (1,040 mph); at 45° latitude, it's 1,180 km/h; at poles, zero. When an object moves north from the equator, it retains the higher eastward speed, appearing to deflect right relative to the ground. The Coriolis effect is crucial for meteorology: it causes large-scale wind patterns like the prevailing westerlies (30-60° latitude), trade winds (0-30°, blowing toward equator), and polar easterlies. It also creates cyclonic rotation: low-pressure systems rotate counterclockwise in Northern Hemisphere (cyclonic), clockwise in Southern Hemisphere (anticyclonic). Hurricanes and typhoons require Coriolis effect to spin – which is why they don't form within 5° of the equator. In oceanography, the Coriolis effect creates Ekman transport (surface water moves 45° relative to wind, forming gyres). In ballistics, long-range projectiles (artillery, missiles) and sniper shots must account for Coriolis deflection. Foucault pendulum (Léon Foucault, 1851) visually demonstrates Earth's rotation – the pendulum's plane of oscillation rotates due to Coriolis effect. For an object moving 1 km/s at 45° latitude, Coriolis acceleration is about 0.1 m/s² (much smaller than gravity's 9.8 m/s²). The Coriolis effect does NOT affect toilet flushing direction (tiny scale, other factors dominate), does NOT affect bathtub or sink drains, and is negligible for human-scale activities. The Rossby number (ratio of inertial to Coriolis forces) determines when Coriolis matters – for large-scale (weather) or long-time motions, Rossby number <1; for small-scale (tornadoes, dust devils), Rossby number >1 and Coriolis negligible.

Air masses are classified by source region: Arctic (A, extremely cold), Polar (P, cold), Tropical (T, warm), Equatorial (E, hot). Maritime (m, moist) or Continental (c, dry) modifiers – e.g., cP = continental polar (cold, dry from Canada/Siberia), mT = maritime tropical (warm, moist from Gulf of Mexico). On weather maps: cold fronts (blue line with triangles pointing movement direction), warm fronts (red line with semicircles), stationary fronts (alternating blue triangles and red semicircles), occluded fronts (purple line with alternating triangles/semicircles). Cold front passage: temperature drops sharply, pressure rises, wind shifts (usually NW in NH), skies clear after front passes. Cold fronts move 20-40 km/h. Warm front passage: temperature rises, pressure falls, wind shifts (SE to S to SW in NH), precipitation ends, clouds thin. Warm fronts move 10-20 km/h. Stationary fronts cause prolonged cloudy, rainy weather for days. Occluded fronts form when cold front catches warm front – cold occlusion (coldest air behind) or warm occlusion. Dryline is a moisture front (common in Texas/Oklahoma) separating dry air from moist air – triggers severe thunderstorms without temperature change. Frontogenesis is the process of front formation; frontolysis is weakening. Norwegian cyclone model (Bergen School, 1910s-1920s) established modern frontal analysis (Vilhelm Bjerknes, Jacob Bjerknes, Tor Bergeron). Surface fronts often have upper-level support from jet streams (Rossby waves). Conveyor belt models explain precipitation patterns: warm conveyor belt (WCB), cold conveyor belt (CCB), dry intrusion.