Glossary of Weather Terms – Beginning with “C”

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Looking for a complete cloud types chart, the difference between a Coastal Flood Advisory and Warning, what a cold front means for your weather, or how a cumulonimbus cloud produces a tornado? This page defines National Weather Service alerts and meteorological terms beginning with “C” – including a full cloud types reference and coastal flood alert breakdown.

Weather Terms Beginning with “C”

Ceiling

In aviation meteorology, ceiling is the height above the ground of the lowest layer of clouds covering more than half the sky (called broken or overcast coverage). Ceiling is reported in hundreds of feet – a ceiling of 3,000 feet means the cloud base is 3,000 feet above ground level. Low ceilings are a major aviation hazard; instrument flight rules (IFR) apply when the ceiling is below 1,000 feet or visibility is below 3 miles. A ceiling of zero (fog to the ground) effectively closes airports without precision instrument approaches.

Cirrus

Cirrus clouds are thin, wispy, hair-like clouds composed entirely of ice crystals that form at high altitudes – typically 20,000 to 40,000 feet. They appear white and are often described as resembling streaks, curls, or “mare’s tails.” Cirrus clouds generally indicate fair weather but can be a precursor to an approaching warm front and deteriorating conditions within 24-48 hours. When cirrus thickens into a veil covering the sky (cirrostratus), it is often a reliable indicator of rain or snow approaching within 12-24 hours.

Cirrocumulus

Cirrocumulus clouds appear as a thin, white, rippled or granular layer of very small cloudlets at high altitudes (above 20,000 feet), often arranged in rows resembling fish scales – hence the phrase “mackerel sky.” Like all high clouds, they are composed of ice crystals. Cirrocumulus clouds indicate fair but unstable air at high levels. A sky covered in cirrocumulus is said to have a “mackerel sky,” which was historically used as a predictor of changing weather – “Mackerel sky, mackerel sky, never long wet, never long dry.”

Cirrostratus

Cirrostratus is a thin, translucent, sheet-like high cloud composed of ice crystals that covers large portions of the sky, often the entire sky, at altitudes above 20,000 feet. It is thin enough that the sun or moon is usually visible through it, but the ice crystals refract light to create halos around the sun or moon – one of the most reliable indicators of an approaching warm front. When you see a halo around the sun or moon, rain or snow within 12-24 hours is a strong possibility.

Clear Air Turbulence (CAT)

Clear Air Turbulence (CAT) is turbulence that occurs at high altitude (typically above 15,000 feet, usually near the jet stream) in cloudless air with no visible warning. It is caused by wind shear – abrupt changes in wind speed or direction over a short distance near the boundaries of the jet stream. Because CAT is invisible to both pilots and weather radar, it is one of the most hazardous weather phenomena for aviation and is responsible for most non-fatal airline injuries each year. CAT forecasts are issued by the Aviation Weather Center and are a standard part of pilot weather briefings.

Climate vs. Weather

Weather is the current state of the atmosphere at a specific time and place – today’s temperature, humidity, wind, and precipitation. Climate is the long-term average of weather conditions over a standard 30-year period (called a “climate normal”) for a given location. A simple way to remember the distinction: “Climate is what you expect; weather is what you get.” When a meteorologist says Chicago’s average high in July is 84°F, that is a climate statement. When they say it will be 97°F tomorrow with a heat advisory, that is a weather forecast.

Cloud Types – Reference Chart

Clouds are classified by the altitude of their base and their shape. The World Meteorological Organization recognizes ten basic cloud genera arranged into four altitude groups:

See also: Full cloud types guide

Coastal Flood Advisory

A Coastal Flood Advisory is issued when minor coastal flooding is occurring or expected in low-lying areas near the coast. The flooding typically causes some road closures, minor property damage, and nuisance conditions, but is not considered life-threatening. It is the lowest level of coastal flood alert. Residents in vulnerable low-lying areas should move vehicles to higher ground and avoid driving through flooded roadways – even 6 inches of moving water can knock a person down and 2 feet can carry away a vehicle.

Coastal Flood Warning

A Coastal Flood Warning is issued when significant coastal flooding is occurring or expected, posing a threat to life and property. It is the highest level coastal flood alert and indicates widespread, moderate to major flooding of low-lying coastal areas. Storm surge – water pushed onshore by wind – is the primary mechanism during tropical cyclones and is responsible for the majority of hurricane-related fatalities. During a Coastal Flood Warning, evacuate low-lying areas immediately if ordered and do not attempt to drive through flooded streets.

Coastal Flood Watch

A Coastal Flood Watch is issued when conditions are favorable for significant coastal flooding within 24 to 48 hours. A Watch means coastal flooding is possible but not yet certain – it is the time to prepare: move valuables to higher floors, check your evacuation route, stock emergency supplies, and monitor weather updates. Coastal Flood Watches are commonly issued ahead of significant storm systems, nor’easters, or tropical cyclones approaching the coast.

Coastal Flood Advisory vs. Watch vs. Warning

Cold Front

A cold front is the leading edge of an advancing mass of cold, dense air that displaces warmer air. As the cold air wedges under the warm air and forces it rapidly upward, the result is often a narrow but intense band of showers and thunderstorms along or just ahead of the front. Cold fronts typically move faster than warm fronts and are associated with a sharp temperature drop, a wind shift (often from southwest to northwest), rising pressure, and clearing skies after the front passes. In summer, cold fronts are often the trigger for severe thunderstorm and tornado outbreaks as they clash with warm, humid air masses.

Condensation

Condensation is the process by which water vapor in the air converts to liquid water. It occurs when air cools to its dew point temperature – the temperature at which the air becomes saturated and can no longer hold all of its water vapor. Condensation is responsible for cloud formation, fog, dew on grass, and the droplets on the outside of a cold glass on a humid day. In clouds, condensation occurs on tiny particles called condensation nuclei – dust, sea salt, pollen, or pollution. The release of latent heat during condensation is the energy source that powers thunderstorms and hurricanes.

Convection

Convection in meteorology refers to the vertical transport of heat and moisture through the atmosphere by the rising of warm, less-dense air and the sinking of cool, denser air. When the ground heats up on a summer afternoon, thermals of warm air rise, cool, and condense to form cumulus clouds. If the atmosphere is sufficiently unstable, convection can become deep and explosive, producing cumulonimbus clouds, thunderstorms, hail, and tornadoes. The term “convective weather” refers to weather produced by this process. Forecasters measure convective potential using indices like CAPE (Convective Available Potential Energy) – the higher the CAPE, the more explosive the potential thunderstorm development.

Cumulonimbus

A cumulonimbus (Cb) is a towering, vertical cloud that extends from a base as low as 1,000 feet up to the tropopause at 40,000-60,000 feet or higher. It is the only cloud type capable of producing all forms of precipitation – rain, snow, sleet, hail, and freezing rain – as well as lightning, strong gusty winds, and tornadoes. The classic cumulonimbus has a flat, anvil-shaped top (the anvil) caused when the rising air hits the stable tropopause and spreads outward horizontally. A rotating cumulonimbus with a persistent, organized updraft is called a supercell – the most dangerous type of thunderstorm.

Cumulus

Cumulus clouds are the classic, puffy, cotton-ball shaped clouds with flat bases and rounded tops that most people picture when they think of “clouds.” They form when thermals of warm, moist air rise, cool, and condense at a uniform altitude (the lifting condensation level), creating the characteristic flat base. Fair-weather cumulus – small, scattered, and not growing vertically – indicate an unstable but harmless atmosphere. When cumulus develop significant vertical extent (towering cumulus or cumulus congestus), they can quickly evolve into cumulonimbus thunderstorms given sufficient moisture and instability.

Cyclone

In meteorology, a cyclone is any area of low atmospheric pressure around which winds blow counterclockwise in the Northern Hemisphere (clockwise in the Southern Hemisphere). The term encompasses several different types of storms: extratropical cyclones (the large mid-latitude low-pressure systems responsible for most of the United States’ weather), tropical cyclones (hurricanes and tropical storms), and mesocyclones (the rotating updraft within a supercell thunderstorm). In common usage in the Indian Ocean and South Pacific, “cyclone” specifically refers to what the Atlantic calls a hurricane. Cyclones are associated with clouds, precipitation, and stormy conditions – the opposite of anticyclones (high pressure systems).

Frequently Asked Questions – “C” Weather Terms

What is the difference between a Coastal Flood Advisory, Watch, and Warning?

A Coastal Flood Advisory means minor flooding is occurring – nuisance conditions, some road closures, not life-threatening. A Coastal Flood Watch means significant coastal flooding is possible within 24-48 hours – prepare now. A Coastal Flood Warning means significant flooding is occurring or imminent and poses a direct threat to life and property – evacuate immediately if ordered and never drive through floodwater. The key rule: if you can’t see the road surface, turn around, don’t drown.

What is the difference between weather and climate?

Weather is the current state of the atmosphere – today’s temperature, humidity, wind, and precipitation. Climate is the long-term average of weather over a 30-year period for a specific location. The difference: weather is what you experience on any given day; climate is the expected range of conditions for a region and season. Climate normals are updated every 10 years by NOAA and serve as the baseline for determining whether any given day, month, or season is warmer, colder, wetter, or drier than average.

What is a cold front and what weather does it bring?

A cold front is the boundary where an advancing mass of cold, dense air meets and undercuts warmer air. The warm air is forced rapidly upward, often producing a narrow band of heavy showers and thunderstorms along or just ahead of the front. After the front passes, expect a sharp temperature drop, a wind shift from southwest to northwest, falling humidity, and clearing skies. In the spring and fall, cold fronts clashing with warm, humid air masses can trigger severe thunderstorm and tornado outbreaks across the central United States.

What are the different types of clouds?

Clouds are organized into ten main types by altitude and shape. High clouds (above 20,000 ft) include cirrus, cirrocumulus, and cirrostratus – all ice-crystal clouds. Middle clouds (6,500-20,000 ft) include altostratus and altocumulus. Low clouds (surface to 6,500 ft) include stratus, stratocumulus, and nimbostratus (the steady rain cloud). Vertically developed clouds that span multiple levels include fair-weather cumulus and the towering cumulonimbus – the thunderstorm cloud capable of producing lightning, hail, and tornadoes.

What is a cumulonimbus cloud?

A cumulonimbus is the towering thunderstorm cloud that can extend from near the surface to the tropopause at 40,000-60,000 feet. It is the only cloud capable of producing lightning, hail, heavy rain, and tornadoes. The distinctive anvil-shaped top forms when the rising air hits the stable stratosphere and spreads outward. A rotating cumulonimbus is called a supercell – the most dangerous thunderstorm type. The presence of a cumulonimbus almost always means a Severe Thunderstorm Warning or Tornado Warning could be issued imminently.

What causes a cyclone?

A cyclone forms when air converges toward a low-pressure center and rises. As the rising air diverges at high levels, more air rushes in at the surface to replace it, which due to the Coriolis effect (Earth’s rotation), begins to rotate counterclockwise in the Northern Hemisphere. Tropical cyclones (hurricanes) are powered by warm ocean water and latent heat release; extratropical cyclones are driven by temperature contrasts between air masses. The lower the central pressure, the stronger the storm and the more intense the winds circling the low.

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