Meteorology

Meteorology in the 2027 season is about severe weather and storms: how forecasters see a storm coming, why the atmosphere sometimes builds a supercell instead of a passing shower, what a hook echo or a debris ball on radar means, how a hurricane drives the sea onto land, and what people should do when a warning goes out. The Division B event is a data event first. At least half of every test is built on maps, radar and satellite images, soundings, graphs and tables, so the team that wins is the one that can look at a display, say what is happening, and explain the physics behind it in a few correct sentences.

This guide follows the 2027 Division B rules manual topic by topic, explains the mechanism behind each storm type, works the formulas you need, and ends with real tournament questions.

Event format

In a station rotation you cannot take pages out of the binder, and you cannot go back to a station once you leave it. You can, however, revise or add to an earlier answer while you sit at a later station, so write provisional answers and keep a list of the ones you want to revisit. Because reasoning earns points, a short "because" clause after every answer is worth the extra ten seconds.

What the rules cover

The 2027 rules give the event the theme Severe Weather & Storms, replacing last season's Everyday Weather outline. Fair-weather topics that used to anchor the event (sky optics, fog types, temperature indices, space weather, degree days) are gone, and storms that were explicitly excluded last season (tornadoes, hurricanes, severe thunderstorms, derechos, downbursts, waterspouts) are now the center of it. The event is Division B only. The table groups the 2027 topic list (section 3c, items i to xv) into study areas; the wording is a paraphrase.

Strategy

Split by display type, then cross-train. One partner can own radar and satellite imagery, the other soundings, hodographs and calculations; both need to read a surface map. Station rotations punish a team that can only work one kind of station, so trade a practice test each week.

Build the binder for speed. Tab it in the order of the rules outline, and put these on the first pages:

• A radar key: reflectivity (dBZ) colors, velocity colors (inbound and outbound), correlation-coefficient values, and labeled examples of a hook echo, a bow echo, a TVS and a debris ball.
• The EF scale, the old F scale, the Saffir-Simpson scale, the hail-size chart and the NWS severe-thunderstorm criteria.
• A blank Stüve and Skew-T with dry adiabats, moist adiabats and mixing-ratio lines labeled, plus the LCL and CAPE formulas.
• A station-model key and a METAR decoder.
• The required U.S. map with state names, marked with Tornado Alley, Dixie Alley, the Great Lakes snowbelts and the Gulf and Atlantic coasts.
• One page per historic event in the rules, with the date, the storm type, and what made it notable, so you can recognize the data you are given.

Time management. Answer quick identifications first, then the data-interpretation questions (they are at least half the points), then long explanations. If a calculation stalls, write the setup and move on; partial reasoning still earns credit.

Observing severe weather

Every forecast starts with observations. The rules list the main observing platforms, and questions often ask what each one measures and what it cannot see.

Doppler radar products

The U.S. network of WSR-88D (NEXRAD) radars scans the sky at several elevation angles. The rules name the products you must read:

• Base reflectivity is the returned power from one tilt, usually the lowest (0.5°), in dBZ. More and larger targets mean higher dBZ: about 20 dBZ is light rain, 40 dBZ moderate to heavy rain, 50 to 60 dBZ very heavy rain, and values above about 60 dBZ usually mean hail.
• Composite reflectivity shows the highest value found anywhere in the vertical column above each point. It reveals hail cores aloft that the lowest tilt misses, but it hides the low-level structure.
• Base velocity shows only the radial component of motion, toward or away from the radar. The usual color convention is green (cool) for inbound and red (warm) for outbound. Wind blowing across the beam reads zero.
• Storm-relative velocity subtracts the motion of the storm itself, so rotation inside a fast-moving storm stands out.
• Correlation coefficient (CC) is a dual-polarization product (added to NEXRAD in the early 2010s) that measures how similar the targets in a volume are in size and shape. Pure rain or snow gives CC near 1 (above about 0.97). Melting snow, mixed hail and rain, and especially non-weather targets give lower values. Lofted tornado debris gives a small blob of very low CC.
• Wind profiles (the VAD wind profile) use the velocity data around the radar to estimate wind speed and direction at each height, a radar-made hodograph that updates every few minutes.

Satellite imagery

• Visible imagery shows reflected sunlight, so it works only in daylight. It shows texture: the bubbly tops of growing cumulus, overshooting tops, the shadows thrown by tall towers in low sun, and the clear eye of a hurricane.
• Infrared (IR) imagery shows emitted radiation as a brightness temperature. Colder means higher cloud tops, so the coldest pixels mark the deepest thunderstorms. A ring of very cold tops around a warm spot is a hurricane eye; a cold "U" or "V" shape on a storm top marks an intense updraft.
• Water-vapor imagery senses moisture in the middle and upper troposphere even where there are no clouds. It shows jet streams, troughs, dry slots and the plumes of moisture that feed atmospheric rivers.

Forecast data: maps, codes, hodographs and models

Surface maps, station models and METAR

A station model packs one observation into one symbol: temperature at the upper left, dew point at the lower left, sea-level pressure at the upper right (the last three digits in tenths of hPa, so 1008.4 hPa is written 084), and wind as a barb pointing into the direction the wind comes from (short barb 5 kt, long barb 10 kt, flag 50 kt). Sky cover fills the center circle. On a severe-weather map, look for the boundaries where storms start: fronts, drylines (a sharp dew-point drop over a short distance) and outflow boundaries.

METAR is the coded hourly aviation report, in a fixed order: station, day and time (UTC), wind, visibility, present weather, sky cover, temperature and dew point, altimeter, remarks. Present-weather codes you will see on severe days include TS (thunderstorm), +TSRA (thunderstorm with heavy rain), GR (hail), FC (funnel cloud; +FC is a tornado or waterspout), SQ (squall), FZRA (freezing rain), PL (ice pellets) and BLSN (blowing snow).

Hodographs

A hodograph plots the wind at several heights as vectors from a common origin and connects their tips. It is a picture of vertical wind shear: the length of the line is how much the wind changes, and its curvature shows how the direction turns with height.

• A long hodograph means strong shear, which organizes storms and keeps the updraft separate from the downdraft.
• A straight hodograph favors storms that split into left- and right-moving supercells.
• A hodograph that curves clockwise (winds veer with height in the lowest few kilometers) favors right-moving supercells, the ones that produce most strong tornadoes in the Northern Hemisphere.

Computer models

Forecast models solve the equations of motion forward in time from an analysis of the current observations. Global models (such as the GFS and the European model) handle large-scale patterns days ahead; high-resolution convection-allowing models (such as the HRRR) simulate individual storms a few hours ahead. Two things to say in any model question: the forecast is only as good as its starting observations, and errors grow with lead time, so forecasters compare several runs or an ensemble and trust the patterns that agree.

Soundings and stability

A sounding is the vertical profile of temperature, dew point and wind measured by a radiosonde. Storm forecasting depends on what a rising parcel of air does compared with the air around it.

A rising unsaturated parcel cools at the dry adiabatic lapse rate, about 9.8 °C per km. Once it saturates, condensation releases latent heat and it cools more slowly, at the moist adiabatic lapse rate, roughly 4 to 7 °C per km depending on temperature. The actual temperature change with height in the surrounding air is the environmental lapse rate.

An inversion, a layer where temperature rises with height, is the most stable case. A warm, dry inversion a kilometer or two up (a cap) can hold storms back all day while heat and moisture build underneath; if a front or dryline breaks the cap in late afternoon, the storms that form are often explosive.

Stüve diagrams (all levels)

A Stüve diagram plots temperature on the horizontal axis against pressure (scaled so dry adiabats are straight lines) on the vertical axis. You read it by following lines:

• Where the temperature and dew-point traces nearly touch, the layer is saturated: that is cloud.
• Where the temperature trace bends to the right with height, there is an inversion.
• Compare the slope of the temperature trace with the dry and moist adiabats to judge stability layer by layer.
• Wind barbs on the side show shear: speed increasing and direction turning with height.

State & National only: Skew-T, LCL and CAPE

A Skew-T/log-p diagram carries the same information with the isotherms tilted 45° to the right, so the traces spread out and the area between parcel and environment is easier to see. The skills are the same as on a Stüve.

The lifting condensation level (LCL) is where a lifted surface parcel cools to its dew point and cloud forms; it is the cloud-base height. On a diagram, follow the dry adiabat up from the surface temperature and the mixing-ratio line up from the surface dew point; they cross at the LCL. Low LCLs (under about 1,000 m) are associated with tornadic supercells because the downdrafts are less cold.

Convective available potential energy (CAPE) is the energy a rising parcel gains from being warmer than its surroundings, from the level of free convection (LFC), where it first becomes warmer, to the equilibrium level (EL), where it no longer is. On a Skew-T it is the area between the parcel path and the environmental trace. Negative area below the LFC is convective inhibition (CIN), the cap.

What sets up severe weather

Forecasters look for four ingredients: moisture, instability, lift and, for organized storms, wind shear. The rules list the large-scale features that supply them.

• Air masses. Warm, humid maritime tropical (mT) air from the Gulf of Mexico supplies the moisture; continental polar (cP) air behind cold fronts and hot, dry continental tropical (cT) air from the desert Southwest supply contrast. Dry desert air riding over the moist Gulf air at mid-levels creates the steep lapse rates and the cap typical of the Plains.
• Fronts. A cold front undercuts warm air and forces it up along a narrow line, the classic trigger for a squall line. Warm fronts and stationary fronts can focus heavy rain and, near the triple point where fronts meet a low, tornadoes.
• Drylines. A dryline separates moist Gulf air from dry desert air, most often over west Texas, Oklahoma and Kansas in spring. Across it the dew point can drop 10 °C or more in a few kilometers. It mixes eastward in the afternoon and retreats west at night, and isolated supercells often fire along it.
• Jet streams. The polar jet near 300 hPa steers storms and adds deep-layer shear. Air spreads out (diverges) in parts of a jet streak, and that upper-level divergence pulls air up from below, deepening surface lows. A low-level jet, a stream of fast southerly wind near 850 hPa at night over the Plains, carries moisture north and feeds nighttime storms and MCCs.
• Large-scale circulation. Long waves in the westerlies (Rossby waves) set where troughs and ridges sit; storms favor the region ahead (east) of an upper trough. A persistent ridge does the opposite, suppressing storms and building heat waves and drought.
• Wind shear. Shear tilts the updraft so rain falls outside it, letting a storm live for hours, and turns horizontal spin into vertical rotation in supercells. For hurricanes, the effect is the reverse: strong shear tilts and tears the storm apart.
• Atmospheric rivers. Long, narrow corridors of intense water-vapor transport from the tropics. When one stalls against mountains, days of rain or snow follow, producing floods, debris flows and heavy mountain snowpack.

Oceans and large lakes

Warm water is fuel. Ocean heat content, the heat stored in the upper ocean, matters more to a hurricane than the surface temperature alone: a storm mixes up water from below, and if the warm layer is shallow, it brings up cold water and weakens itself. That stirred, cooled track left behind a hurricane is its cold wake, and a slow or stalled storm can sit over its own wake and weaken. Ocean currents such as the Gulf Stream and the Loop Current carry heat poleward and can fuel rapid intensification when a storm crosses them. Fetch, the distance wind blows over open water, controls wave height and, for lake-effect snow, how much heat and moisture the air picks up before reaching shore.

Terrain

Mountains lift air (orographic lift), so windward slopes get heavier rain and snow, and leeward sides lie in a rain shadow. Terrain can focus flash floods in steep canyons, strengthen downslope windstorms, and weaken hurricanes that cross mountainous islands by cutting off their warm, moist inflow and disrupting their circulation. On the Plains, the gentle upslope toward the Rockies helps lift moist air on spring afternoons.

Severe-weather clouds

The rules limit clouds to those tied to severe weather. Learn what each one says about the storm.

Severe thunderstorms

The NWS calls a thunderstorm severe when it produces any of: hail 1 inch or larger in diameter, wind gusts of 58 mph (50 kt) or more, or a tornado. Heavy rain and frequent lightning are dangerous but do not by themselves make a storm severe.

Every thunderstorm passes through three stages. In the cumulus (developing) stage, updrafts dominate and the cloud grows. In the mature stage, precipitation drags air down, so updrafts and downdrafts exist side by side; this is when rain, hail, lightning and wind are strongest. In the dissipating stage, the rain-cooled downdraft spreads under the storm and cuts off its warm inflow.

An MCC has a formal satellite definition (Maddox, 1980): a cold cloud shield of at least 100,000 km² colder than −32 °C, with an inner core of at least 50,000 km² colder than −52 °C, lasting at least 6 hours, and nearly round in shape. MCCs often grow overnight on the Plains in summer, fed by the low-level jet.

A supercell's updraft rotates because shear creates horizontal spin (like a rolling pencil between two hands moving at different speeds), and the updraft tilts that spin into the vertical. A mature supercell has a forward-flank downdraft in the main rain area and a rear-flank downdraft wrapping around the back of the mesocyclone. On radar, the rain wrapping around the mesocyclone makes the hook echo. The updraft is so strong that precipitation has no time to form in its core, leaving a vault of weak echo capped by heavy echo aloft: the bounded weak echo region (BWER) seen in vertical slices, above the low-level inflow notch where warm air enters the storm.

Lightning

Thunderstorm clouds become electrified in the mixed-phase region, where supercooled water, ice crystals and graupel (soft hail) coexist. When graupel collides with small ice crystals, charge is transferred: at the temperatures of the main charging zone (roughly −10 to −20 °C), graupel usually becomes negatively charged and the light ice crystals positively charged. The updraft lifts the light crystals, so the cloud ends up with a positive region near the top, a negative region in the middle, and often a small positive region near the base. The ground under the storm develops a positive charge that follows the storm.

When the electric field is strong enough, a negatively charged stepped leader zigzags downward in short steps. Positive streamers rise from tall objects to meet it, and when they connect, a bright return stroke surges up the channel. The channel heats to roughly 30,000 °C in microseconds, and the shock wave of that sudden expansion is thunder.

Detection. Ground-based networks such as the National Lightning Detection Network locate cloud-to-ground strikes by combining the direction to each strike from magnetic direction finders with the arrival time of its radio pulse at several sensors. From space, the Geostationary Lightning Mapper on the GOES satellites records the optical flashes of all lightning, including intracloud, across the Americas. A sudden jump in the flash rate often comes before severe weather because it tracks a strengthening updraft.

Precipitation from severe storms

How precipitation forms

Cloud droplets are far too small to fall; a raindrop holds about a million of them. Two processes grow precipitation:

• Collision-coalescence dominates in warm clouds (above freezing throughout): larger droplets fall faster, collide with smaller ones and merge.
• The Bergeron (ice-crystal) process dominates in cold clouds: where ice crystals and supercooled droplets coexist, the saturation vapor pressure over ice is lower than over water, so vapor leaves the droplets and deposits on the crystals, which grow quickly.
• Riming is supercooled droplets freezing onto falling ice; heavy riming makes graupel. Aggregation is ice crystals sticking together into snowflakes, most effective near 0 °C where crystal surfaces are sticky.

What reaches the ground then depends on the temperature layers between cloud and ground:

Hail

Hail grows only in thunderstorms with strong updrafts. A hailstone embryo (graupel or a frozen drop) is carried up through the region rich in supercooled water, collecting layers of ice. Slow, wet growth makes clear ice; fast freezing traps air and makes cloudy ice, so a cut hailstone shows rings. The stone falls when it is too heavy for the updraft to hold, so hail size tracks updraft strength: golf-ball hail needs an updraft of roughly 25 m/s or more. Size is reported by comparison objects: penny 0.75 in, quarter 1.00 in (the severe threshold), golf ball 1.75 in, baseball 2.75 in, softball 4.00 in. Hail of 2 inches or more is called significant. The U.S. record stone, at Vivian, South Dakota, in 2010, was 8 inches across. Size matters because the energy of impact rises steeply with diameter: large hail breaks roofs, windshields and aircraft, and destroys crops.

Floods, debris flows and mudslides

Flooding kills more Americans in an average year than tornadoes or lightning. The rules separate several kinds:

• Flash floods rise within about six hours of heavy rain (or a dam or levee failure), often in minutes in steep terrain. They come from slow-moving or training storms (cells repeatedly crossing the same spot) and tropical-cyclone rain.
• Urban floods occur where pavement and roofs stop rain from soaking in, so runoff overwhelms storm drains and fills underpasses and basements.
• Stream and river floods develop over days as rain or snowmelt across a whole watershed fills the channel; the crest moves downstream and can be forecast.
• Debris flows and mudslides are fast-moving slurries of water, soil and rock on steep slopes, most likely after intense rain on saturated ground or on burn scars, where fire has left soil that sheds water.

Impacts of heavy precipitation

Heavy wet snow can collapse roofs (snow load is about the weight, not the depth). Ice accretion snaps trees and power lines. Floodwater undermines roads, washes out bridges and overtops or breaches dams and levees. Air travel is disrupted by low ceilings, icing and runway closures; ground travel by flooded roads, ice and whiteouts. Most flood deaths happen in vehicles.

State & National only: IDF curves and return periods

An intensity-duration-frequency (IDF) curve shows how much rain (as an intensity in in/h or mm/h, or as a depth) falls in a given duration for a given return period at one place. Each curve is one return period; intensity drops as duration grows, because short bursts are always heavier than a long-duration average, and the curves for rarer events sit higher. NOAA publishes these values (NOAA Atlas 14) and engineers use them to size culverts, storm drains and dams.

A return period (recurrence interval) is the average time between events of a given size. A "100-year rain" is not one that happens once a century: it has a 1% chance of happening in any given year, and it can happen in consecutive years.

Damaging winds

Most severe-weather reports are wind, not tornadoes. The rules list these wind types:

• Straight-line winds are any thunderstorm winds without rotation; the damage is blown in one direction, unlike the convergent pattern of a tornado.
• Downdrafts are sinking air in a storm, driven by rain and hail dragging air down and by evaporative cooling, which makes the air denser.
• A downburst is a strong downdraft that hits the ground and spreads out. Fujita split them by size: a microburst covers less than 4 km (2.5 mi) and lasts a few minutes, with winds that can exceed 100 mph; a macroburst covers 4 km or more and can last up to about 20 minutes. Wet microbursts come with heavy rain; dry microbursts fall from high-based storms over dry air, often with only virga, and are dangerous to aircraft on approach because the headwind suddenly becomes a tailwind.
• A gust front is the leading edge of a storm's cold outflow. Passing it brings a wind shift, a gust, a temperature drop and a pressure jump; a shelf cloud often rides on it, and new storms form where it lifts warm air.
• A squall line is a line of storms along a gust front or cold front, often with a narrow band of strong wind at its leading edge.
• A derecho is a long-lived, widespread windstorm from a fast-moving line or cluster of storms. NOAA's Storm Prediction Center currently uses a damage swath of more than about 400 miles (650 km) long and about 60 miles wide, with gusts of at least 58 mph along most of it and several gusts of 75 mph or more.
• Downslope winds form when air descends the lee of a mountain range and warms by compression. Examples are the Chinook east of the Rockies, Santa Ana winds in Southern California, and the bora-type cold downslope winds; strong ones can gust past 100 mph near Boulder, Colorado, and drive wildfires.

Wind signatures on radar

• Bow echo. A line segment that bows outward, pushed by a strong rear-inflow jet. The worst winds are at the apex of the bow; a rear-inflow notch (a weak-echo notch behind the line) marks where the jet descends. Bookend vortices at the ends can spin up brief tornadoes, especially on the northern end.
• Gust fronts appear on reflectivity as a thin line of weak echo (a fine line) ahead of the storms, because insects and dust are swept up along the boundary; on velocity they show as a sharp wind shift.
• Squall lines and derechos show as long lines or a series of bow echoes; velocity shows a broad band of strong inbound or outbound winds (often 50 kt or more) just above the ground.

Tornadoes and waterspouts

A tornado is a violently rotating column of air in contact with the ground and extending from a convective cloud. The visible funnel is condensation and debris; the circulation can reach the ground before the funnel does. A waterspout is a tornado over water. Tornadic waterspouts form under supercells or strong storms just like land tornadoes; fair-weather waterspouts form under growing cumulus in light winds, rotate from the surface upward, and are usually weak. A tornado can also contain several smaller suction vortices orbiting the main circulation, which explains narrow streaks of extreme damage next to lighter damage.

Life cycle. Most strong tornadoes come from supercells. Under the mesocyclone, the rear-flank downdraft wraps around and tightens the rotation near the ground. A typical life cycle runs organizing (a funnel or debris whirl beneath the wall cloud), mature (widest and strongest, often vertical), shrinking (the funnel narrows and tilts as cold outflow wraps around it) and decaying (a thin, twisted rope stage). The parent storm may then produce a new tornado, so long tracks of separate tornadoes from one storm are common.

Conditions. The ingredients are high instability (CAPE), strong deep-layer shear, strong low-level shear (a clockwise-curved hodograph in the lowest kilometer), a low cloud base (low LCL) and a trigger such as a dryline, front or outflow boundary.

Where and when. The United States has more tornadoes than any other country, about 1,200 a year, because cold dry air from Canada, hot dry air from the desert and moist Gulf air meet over flat land. Tornado Alley in the southern and central Plains peaks in May and June; Dixie Alley in the Southeast has more tornadoes in early spring and late fall, more at night, and more hidden in rain and trees, which makes them deadlier. Activity shifts north into the northern Plains and Midwest through summer. Most tornadoes occur between late afternoon and early evening (about 3 to 9 p.m. local time), when heating is greatest. Tracks run most often from southwest to northeast, following the steering winds.

F and EF scales

Tornadoes are rated after the event from damage, not from a direct wind measurement. The original Fujita (F) scale was used in the United States until February 1, 2007, when the Enhanced Fujita (EF) scale replaced it. The EF scale uses 28 damage indicators (types of buildings, towers and trees), each with degrees of damage, and its wind estimates are lower at the top end because engineering studies showed that the F-scale overstated the winds needed for a given level of damage.

Tornadoes on radar

• Reflectivity: a hook echo on the right-rear flank of a supercell, where rain and hail wrap around the mesocyclone. A debris ball, a small round spot of high reflectivity at the tip of the hook, is lofted debris.
• Velocity: a tight couplet of strong inbound and outbound velocities next to each other. A broad couplet is a mesocyclone; a small, intense one with gate-to-gate shear is a tornadic vortex signature (TVS), which shows strong rotation but does not prove a tornado is on the ground.
• Correlation coefficient: a small area of very low CC collocated with the couplet and hook is a tornado debris signature, near-confirmation that a tornado is on the ground and doing damage.
• A bow echo can also produce tornadoes, usually brief ones at the comma head or along the leading edge.

Hurricanes, typhoons and cyclones

A tropical cyclone is a warm-core, low-pressure system that forms over tropical oceans and draws its energy from the latent heat released when water vapor evaporated from the sea condenses in its thunderstorms. The name changes with the basin: hurricane in the North Atlantic and the eastern and central North Pacific, typhoon in the western North Pacific, and cyclone in the Indian Ocean and South Pacific. In the Northern Hemisphere they spin counterclockwise; in the Southern, clockwise.

Life cycle

The storm then dissipates over land or cold water, or becomes post-tropical or extratropical as it moves into the mid-latitudes and takes on fronts. An extratropical storm can still be enormous and dangerous, as Sandy showed.

Formation needs: sea-surface temperatures of at least about 26.5 °C (80 °F) through a layer roughly 50 m deep; a moist middle troposphere; low vertical wind shear; a pre-existing disturbance such as an African easterly wave; and a location at least about 5° from the equator, where the Coriolis effect is strong enough to start the spin. When and where: the Atlantic season runs June 1 to November 30 and peaks around September 10. Many of the strongest Atlantic hurricanes begin as waves off the coast of West Africa (Cape Verde storms); early- and late-season storms often form closer to home in the Gulf and Caribbean. The western North Pacific produces the most and strongest storms of any basin.

Tracks and what steers them

Tropical cyclones are steered by the large-scale flow around them. In the Atlantic, the trade winds on the south side of the Bermuda-Azores High carry storms west; as a storm reaches the western edge of the high it turns north and then recurves northeast into the westerlies. A strong high pushes storms into the Gulf or the Caribbean; a trough over the eastern United States pulls them north and out to sea. ENSO changes the odds: El Niño increases upper-level westerly shear over the Atlantic and suppresses hurricanes there while favoring the central and eastern Pacific; La Niña does the reverse. Mountains (Hispaniola, Cuba, the Sierra Madre) disrupt a storm's core and can weaken it rapidly, and landfall cuts off its warm-ocean fuel. Rapid intensification (an increase of at least 35 mph in 24 hours) is favored by warm, deep water, low shear and moist air.

Structure

At the center is the eye, 20 to 60 km across, where air sinks and warms, skies are often clear and winds are light. Around it is the eyewall, a ring of tall thunderstorms with the strongest winds and heaviest rain. Spiral rainbands curve inward toward the eyewall and can produce tornadoes, especially in the right-front quadrant at landfall. Air spirals in at the surface, rises in the eyewall and flows out at the top, where it turns clockwise (anticyclonic). In the Northern Hemisphere the right side of the storm (relative to its motion) has the strongest winds and highest surge, because the forward motion adds to the rotating winds there. Strong hurricanes sometimes go through eyewall replacement cycles, in which an outer eyewall forms and chokes off the inner one, temporarily weakening the storm while its wind field broadens.

On satellite: a clear eye surrounded by a ring of cold cloud tops in IR, a central dense overcast, and curved banding. On radar: a ring of high reflectivity (the eyewall) around a weak-echo hole (the eye), with spiral rainbands, and on velocity a pair of very strong inbound and outbound maxima on opposite sides of the eye.

The Saffir-Simpson scale

The Saffir-Simpson Hurricane Wind Scale rates a hurricane from 1 to 5 by its 1-minute sustained wind only. It does not include storm surge, rainfall or size, so a Category 1 storm can be deadlier than a Category 4 if it stalls and floods.

Storm surge

Storm surge is the rise of seawater above the normal tide, driven by a storm. Most of it comes from the wind stress pushing water toward the coast; a smaller part comes from the low pressure, which lets the sea bulge upward. The storm tide is the surge plus the astronomical tide. Surge is highest where the storm is strong and large, approaches perpendicular to the coast, makes landfall at high tide, and pushes into a shallow, gently sloping shelf or a funnel-shaped bay (the Gulf Coast is especially vulnerable). Height matters because water is heavy: a few feet of moving water can float cars and knock down walls, and historically surge has caused about half of U.S. hurricane deaths. Every foot of extra height extends flooding farther inland on a flat coast.

Severe winter storms

• Blizzards are defined by wind and visibility, not snowfall: sustained winds or frequent gusts of 35 mph or more with falling or blowing snow cutting visibility below 1/4 mile for at least 3 hours. A "ground blizzard" can occur with no new snow.
• Nor'easters are strong extratropical lows that move up the U.S. East Coast, mostly from October to April, named for the strong northeast winds blowing in from the ocean ahead of them. They draw cold air from Canada and moisture from the warm Gulf Stream, and they bring heavy snow inland, rain near the coast, high winds, beach erosion and coastal flooding. When pressure falls 24 hPa or more in 24 hours (at mid-latitudes), the storm is said to undergo bombogenesis (a "bomb cyclone").
• Lake-effect snowstorms form when very cold air crosses a relatively warm, unfrozen Great Lake. The lake warms and moistens the bottom of the air mass, making it unstable; snow forms in narrow bands that unload on the downwind shore. A long fetch across the lake and a large difference between the lake temperature and the air temperature aloft (around 13 °C or more at the 850 hPa level) make heavier bands, and hills downwind add orographic lift. The snowbelts east of Lakes Erie and Ontario, such as the Tug Hill Plateau and Buffalo, can get several feet in a day.
• Ice storms are freezing-rain events with damaging ice accretion, generally about 1/4 inch or more for warning criteria in much of the country. Ice adds great weight to trees and lines, so outages can last days or weeks, and roads become glazed.

Droughts and heat waves

A drought is a prolonged shortage of water compared with normal for a place. It is defined by its impacts:

• Meteorological drought: precipitation well below normal for an extended time.
• Agricultural drought: soil moisture too low for crops and pasture.
• Hydrological drought: low streamflow, reservoirs and groundwater, usually lagging the rainfall deficit.
• Socioeconomic drought: when the shortage affects the supply and demand of goods such as water, food or hydropower.

Droughts usually begin with a persistent ridge of high pressure that blocks storms and causes sinking, drying air. Heat increases evaporation, which dries soil, which then lets the sun heat the air instead of evaporating water: a feedback that deepens both drought and heat. ENSO patterns shift drought odds (La Niña often dries the southern United States). The U.S. Drought Monitor rates drought from D0 (abnormally dry) through D1 (moderate), D2 (severe), D3 (extreme) to D4 (exceptional). Effects include crop and livestock losses, wildfire, water restrictions, dust storms and, as in the Dust Bowl, wind erosion of exposed farmland.

A heat wave is a period of abnormally hot weather, usually lasting two days or more and often with high humidity. There is no single national threshold; the NWS issues heat advisories and extreme heat warnings using local criteria, usually based on the heat index (how hot it feels when humidity slows sweat evaporation). Heat waves usually come from a strong, slow-moving upper ridge (a heat dome) where sinking air warms by compression and suppresses clouds. Warm nights are the most dangerous part because the body never recovers. Cities are hotter than their surroundings (the urban heat island). Heat is the leading weather-related killer in the United States in most years.

Hazards of each storm type

Weather safety

Watches and warnings

The SPC also issues convective outlooks days ahead, with risk categories from marginal to high, and the NHC issues the forecast cone (which shows the probable track of the center, not the size of the storm or where impacts will happen) and storm-surge warnings.

Reliable sources

Get warnings from official sources: NOAA Weather Radio, the National Weather Service (weather.gov), the Storm Prediction Center, the National Hurricane Center, Wireless Emergency Alerts on phones, and local emergency management and broadcast meteorologists who relay them. Social-media posts showing a single model run days ahead are not forecasts. Have at least two ways to receive a warning, including one that wakes you at night.

Preparedness and actions

Preparedness means knowing your risks, having a plan and a designated shelter, building a kit (water, food, medications, flashlight, batteries, radio), and knowing your evacuation zone if you live on the coast.

Historic severe events and recent data

The rules limit historic case studies to a closed list, and they say you will be given the data rather than asked to recall it. What you need is enough background to recognize what a map, graph or table is showing and to explain it.

The rules also call for recent storm data, preferably from 2022 to 2027. Expect current radar loops, SPC storm reports, Drought Monitor maps, NHC track and cone graphics and surge maps from recent seasons. Practice reading these from SPC, NHC and NWS event-summary pages: name the feature, give the cause, and state the hazard.

Common pitfalls

How tournaments test this

The archive holds 1,963 Division B Meteorology questions from four seasons (2017, 2020, 2021 and 2024), 926 of them from 2024, and the practice bank holds about 16,600 more prompts from 2014 to 2026. They come from a mix of themes, so use them for phrasing and practice and the 2027 rules for deciding what is in scope.

• Format. Free response is the largest type, at 976 of 1,963 (about 50%). Multiple choice is 720 (about 37%), true-false 159 (8%), select-all 85 (4%) and numeric 23 (1%). Explanations carry the most points, which matches the rule that reasoning is scored.
• Tasks. "Explain why" is the most common task (about 9.9% of archived questions), then "which of the following" (6.7%), compare and contrast (2.6%), interpret a graph or table (2.0%), true-false (2.0%) and calculations (1.8%). Because at least half the 2027 test must rest on data displays, expect far more image and map interpretation than these tags suggest.
• Severe-weather topics in the archive and bank. The word "hurricane" appears in 658 prompts, "thunderstorm" in 518 and "tornado" in 413, so there is a deep pool of practice for the new theme.
• Recurring terms across the archive and bank include air mass (293 questions), wind speed (188), low pressure (154), cold front (116), jet stream (97), Coriolis effect (105), Doppler radar (85) and lake effect (68).
• Most common correct answers include cumulonimbus (68 questions), cold front (30), infrared (24), hail (22) and freezing rain (19).

The largest archive sources are the Sierra Vista Invitational 2024 (146 questions) and the Rustin Invitational 2020 (143). Typical severe-weather questions: identify a radar feature from an image, compute the distance to lightning, rank storms on a scale from damage descriptions, explain why a hurricane weakened, choose the protective action for a scenario, or read a sounding for stability.

Test yourself

These are real tournament questions from the public archive; each links to its full test. The explanations add the reasoning the official key does not give.

Test yourselfObserving and diagnosing storms

0 / 0 correct

1. 1

   How do Doppler radars work?

   Question 1 options

   ABy emitting radio waves and measuring their reflections off objectsBBy analyzing changes in temperature/humidity in the atmosphereCBy using laser beams to detect air movementsDBy tracking the movement of satellites in space

   From: UT Austin Invitational, 2023-24, Division B

Practice more Meteorology questions →

Test yourselfThunderstorms, lightning, precipitation and winter storms

0 / 0 correct

1. 1

   How many stages are there in a thunderstorm's life cycle

   Question 1 options

   A2B3C4D5

   From: Camas MiniSO Invitational, 2020-21, Division B

Practice more Meteorology questions →

Test yourselfTornadoes, hurricanes and safety

0 / 0 correct

1. 1

   Tornadoes can be made up of 2 or more smaller tornadoes called suction vortices. Circle fact or myth.

   Question 1 options

   AMythBFact

   From: COBRA Invitational, 2019-20, Division B

Practice more Meteorology questions →

References and further reading

• Science Olympiad, 2027 Division B Meteorology (Severe Weather & Storms) rules manual, available from soinc.org; the official rules govern the event.
• SciOly Wiki, Meteorology (a cross-check, not a source of rules): https://scioly.org/wiki/index.php/Meteorology
• NWS JetStream, online school for weather (thunderstorms, tornadoes, hurricanes, radar, soundings): https://www.weather.gov/jetstream/
• NOAA National Severe Storms Laboratory, Severe Weather 101: https://www.nssl.noaa.gov/education/svrwx101/
• Storm Prediction Center: outlooks, watches, storm reports, the EF scale and the derecho pages: https://www.spc.noaa.gov/
• National Hurricane Center: the Saffir-Simpson scale, storm surge overview and Tropical Cyclone Reports for each historic storm: https://www.nhc.noaa.gov/
• NWS Radar Operations Center and the NWS dual-polarization training pages (correlation coefficient and debris signatures): https://www.roc.noaa.gov/
• NWS Safety pages (tornado, lightning, flood, winter, heat): https://www.weather.gov/safety/
• NOAA Atlas 14 Precipitation Frequency Data Server (IDF curves and return periods): https://hdsc.nws.noaa.gov/pfds/
• U.S. Drought Monitor: https://droughtmonitor.unl.edu/
• Maddox, R. A. (1980), "Mesoscale Convective Complexes", Bulletin of the American Meteorological Society 61 (the MCC criteria).
• American Meteorological Society, Glossary of Meteorology: https://glossary.ametsoc.org/
• Practice more: the Scio.ly test archive has public Meteorology tests from several seasons; filter by division to find the Division B questions used here.