Glossary of Weather Terms - Beginning with "F"

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Searching for the difference between a flash flood watch and a flash flood warning? Wondering about flash flood warning meaning, what is frostbite, how the Fujita Scale rates tornado damage, what is freezing rain versus sleet, or when a fire weather watch is issued? This page defines National Weather Service alert types and meteorological terms beginning with “F”, in plain English, with guidance on what each alert means and what to do.

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Weather Terms Beginning with “F”

Fire Weather Watch

A Fire Weather Watch is issued by the NWS when critical fire weather conditions, including low relative humidity (under 15-25%), strong winds (15-25+ mph), and low fuel moisture, are expected within 24-72 hours. The watch tells fire managers to prepare resources and review plans before conditions deteriorate. A Fire Weather Watch is upgraded to a Red Flag Warning when those conditions are occurring or imminent. Fire Weather Watches are critical for wildland firefighters and homeowners in fire-prone areas of the West and South, where a combination of drought, dry vegetation, and gusty winds can allow a small spark to become a rapidly spreading wildland fire in minutes.

Flash Flood

A flash flood is the rapid flooding of low-lying areas caused by heavy rainfall over a short period (usually 6 hours or less), dam or levee failure, or the sudden release of water held behind a natural ice jam or debris dam. Flash floods are the leading weather-related killer in the United States, responsible for roughly 100 deaths per year. The NWS “Turn Around Don’t Drown” safety message exists because just 6 inches of fast-moving water can knock an adult off their feet, and as little as 2 feet of water can sweep a vehicle off a road. Urban flash floods are especially dangerous because paved, impervious surfaces concentrate runoff rapidly, overwhelming storm drains and sending water surging through streets and underpasses with little warning. Read more: What is the difference between a Flash Flood and a Flood? and Turn Around, Don’t Drown.

Flash Flood Watch

A Flash Flood Watch is issued when conditions are favorable for flash flooding to develop within the watch area, though flooding is not yet occurring or certain. Recipients should monitor the weather closely, know the location of high ground near their home and workplace, avoid flood-prone areas such as creek banks and low-water crossings, and be ready to move to safety quickly if conditions deteriorate. Flash Flood Watches are typically issued 12-36 hours in advance of the potential event. Watches cover broad areas, often multiple counties, and are based on forecasted rainfall totals combined with soil moisture and drainage conditions.

Flash Flood Warning

A Flash Flood Warning is issued when flash flooding is occurring or imminent, based on heavy rain reports, stream gauge readings, or Doppler radar rainfall estimates. This is the highest level of flash flood alert and requires immediate action, move to higher ground immediately. Do not attempt to walk, swim, or drive through flooded roads or streams. Flash Flood Warnings may also be issued for dam breaks or levee failures regardless of rainfall amounts. Warnings are typically issued for specific areas (often smaller than the county) and remain in effect for 1-6 hours, though they can be extended if flooding continues.

	Flash Flood Watch	Flash Flood Warning	Flood Warning
What it means	Conditions favorable for flash flooding	Flash flooding occurring or imminent	Flooding occurring or expected along streams/rivers
Timeframe	12-36 hours out	Now or within hours	Hours to days
Water source	Heavy rain, local runoff	Same as watch but happening	Rising rivers, streams, drainage areas
Area affected	Large area (county or region)	Specific area	Specific river/stream reaches
Action	Be ready; know your escape route	Move to high ground NOW	Avoid flooded areas; be ready to evacuate
Duration	Hours to 1-2 days	Usually 1-6 hours	Can last days

Flood Warning

A Flood Warning is issued when flooding is occurring or imminent along streams and rivers, in low-lying areas, or in urban areas. Unlike a Flash Flood Warning, which covers sudden, intense local flooding, a Flood Warning is based on stream gauge data, rainfall forecasts, and observed river conditions that suggest a longer-duration flooding event. Flood Warnings can remain in effect for hours to several days depending on how slowly rivers recede. Recipients should stay out of flood-prone areas, avoid walking or driving through floodwaters, and monitor river stage forecasts from the NWS Advanced Hydrologic Prediction Service (AHPS) at water.weather.gov.

Fog

Fog is a cloud at ground level that reduces horizontal visibility to less than 5/8 of a mile (1 km). It forms through several mechanisms: radiation fog develops overnight when clear skies allow the ground to cool rapidly, chilling the overlying air to its dew point (common in valleys and low-lying areas); advection fog forms when warm, moist air moves over a cooler surface (classic along the California coast and over the Great Lakes); and evaporation fog occurs when cold air moves over warm water. Freezing Fog (FZFG) is particularly hazardous, it deposits rime ice on roads, bridges, aircraft, and power lines. Patchy fog can create sudden, severe visibility drops on highways, leading to multi-vehicle accidents. A Dense Fog Advisory is issued when visibility drops to 1/4 mile or less.

Freezing Rain

Freezing rain is liquid rain that falls through a shallow layer of subfreezing air near the surface and freezes on contact with cold surfaces such as roads, bridges, trees, and power lines, producing a coating of smooth, transparent glaze ice. A quarter-inch of ice accumulation can bring down tree limbs and power lines, causing widespread outages that can last days. Freezing rain producing ice accumulations of 0.25 inches or more is one of the triggers for a Winter Storm Warning. The critical difference between freezing rain and sleet: sleet freezes entirely in the air before hitting the ground and falls as small, bouncing ice pellets; freezing rain remains liquid until it makes contact with a below-freezing surface, making roads appear wet rather than icy, and far more treacherous.

Frost

Frost consists of ice crystals that form directly on surfaces when their temperature falls below the freezing point (32°F) and below the dew point, causing water vapor to deposit as ice (a process called deposition). The NWS issues three cold-related alerts during the growing season: a Frost Advisory when temperatures are expected to fall to 33-36°F, threatening sensitive vegetation; a Freeze Warning when temperatures are expected to drop to 32°F or below; and a Hard Freeze Warning when temperatures are expected to reach 28°F or below, which can kill most cultivated vegetation. Frost forming on car windshields overnight is a reliable indicator of clear skies and strong radiational cooling near the surface.

Frostbite

Frostbite is a cold-weather injury caused by the freezing of skin and underlying tissues when exposed to temperatures below 32°F, especially in wind. It progresses through three stages: frostnip (superficial, reversible, skin appears red and feels numb but is still soft and pliable); superficial frostbite (skin turns white or grayish-yellow with a hard outer layer but soft underneath, blisters may form within 24 hours); and deep frostbite (full-thickness freezing of skin and underlying tissue, resulting in hard, blackened skin, requires immediate medical treatment and can result in permanent tissue loss). Frostbite most commonly affects fingers, toes, the nose, ears, and cheeks. Wind chill dramatically accelerates risk, at a wind chill of -20°F, exposed skin can develop frostbite in as little as 30 minutes. See also: What is Frostbite?, full article and OSHA Cold Temperature Stress guidance.

Enhanced Fujita Scale (EF Scale)

The Enhanced Fujita Scale is the official tornado intensity rating system in the United States, replacing the original Fujita Scale in 2007. It rates tornado damage based on 28 specific damage indicators across six categories: EF0 (65-85 mph winds, minor damage, broken branches, shallow-rooted trees toppled); EF1 (86-110 mph, moderate damage, roofs peeled, mobile homes overturned); EF2 (111-135 mph, considerable damage, roofs torn off frame homes, large trees snapped); EF3 (136-165 mph, severe damage, entire stories of well-constructed homes destroyed); EF4 (166-200 mph, devastating damage, well-built homes leveled); EF5 (over 200 mph, incredible damage, complete destruction of well-built homes, reinforced concrete structures damaged). About 80% of all tornadoes are EF0-EF1; EF4-EF5 tornadoes account for less than 1% of all tornadoes but cause roughly 70% of tornado fatalities. See also: Fujita Scale FAQ and Criteria for a Tornado Warning.

Funnel Cloud

A funnel cloud is a rotating, cone-shaped column of condensation that extends downward from the base of a cumulonimbus cloud but does NOT reach the ground. The moment it makes contact with the ground, or when a debris swirl is visible at the surface beneath it even if the visible funnel has not fully extended down, it is classified as a tornado. A funnel cloud indicates active rotation in the thunderstorm and may be accompanied by damaging wind even if it never touches down. The NWS will often issue or maintain a Tornado Warning for a confirmed funnel cloud that has not yet touched ground because of the imminent danger. If you see a funnel cloud, treat it as a tornado and take immediate shelter.

Flash Freeze

A flash freeze is a rapid, dramatic drop in temperature over a matter of hours, often associated with a fast-moving Arctic cold front, that causes wet roads and standing water to freeze almost instantaneously. Flash freezes are especially dangerous because drivers may be caught completely off guard: roads that were merely wet minutes earlier become ice-covered with no transition period. A Special Weather Statement or Winter Weather Advisory is often issued in anticipation of a flash freeze. They are particularly common across the central and southern Plains when a potent Arctic cold front sweeps through after a warm rain event, dropping temperatures 30-40 degrees in a few hours.

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Frequently Asked Questions, “F” Weather Terms

What is the difference between a Flash Flood Watch and a Flash Flood Warning?

A Flash Flood Watch means conditions are favorable for flash flooding to develop; recipients should be prepared and know escape routes but flooding is not yet occurring. A Flash Flood Warning means flash flooding is occurring or imminent and you should take immediate action, move to higher ground now. A Flood Warning (different from a Flash Flood Warning) applies to slower-developing river and stream flooding rather than the sudden, local flooding of a Flash Flood Warning. When a Flash Flood Warning is issued for your area, do not wait, act immediately.

What causes flash floods and why are they so deadly?

Flash floods are caused by intense, heavy rainfall in a short period, usually over 1-3 hours, that overwhelms drainage systems, streams, and dry washes. Urban areas with paved surfaces are especially vulnerable because water cannot absorb into the ground. Flash floods are the leading weather-related killer in the US because they develop with little warning and people underestimate the force of moving water. Just 6 inches of fast-moving water can knock an adult off their feet, and 2 feet of water can sweep away a vehicle. The NWS “Turn Around Don’t Drown” campaign exists specifically because most flash flood deaths occur in vehicles on flooded roads.

What is the difference between freezing rain and sleet?

Both occur in winter storms with a shallow layer of freezing air near the surface, but they freeze at different stages. Freezing rain starts as rain or snow, falls through a warm air layer that melts it to liquid, then falls through a shallow sub-freezing layer near the surface that is too thin to re-freeze the drops, so the rain hits the ground as liquid and then freezes on contact with surfaces below 32°F, creating glaze ice. Sleet (ice pellets) freezes entirely while still in the air, falling as small ice pellets that bounce on hard surfaces. Freezing rain is generally more hazardous because roads look wet rather than icy, reducing driver awareness.

What is a Fire Weather Watch and when is it issued?

A Fire Weather Watch is issued by the NWS when critical fire weather conditions are expected within 24-72 hours. These conditions include low relative humidity (typically below 15-25%), strong or gusty winds (15-25+ mph), low fuel moisture, and sometimes low rainfall combined with high temperatures. A Fire Weather Watch is upgraded to a Red Flag Warning when these conditions are occurring or are imminent. Fire Weather Watches are most common in the western US during summer and fall, and in the Southeast in winter and spring, any time dry, windy conditions combine with drought-stressed or cured vegetation.

What is frostbite and how quickly can it develop?

Frostbite occurs when skin and underlying tissue freeze, most commonly affecting fingers, toes, ears, the nose, and cheeks. The speed of onset depends heavily on wind chill, at a wind chill of -20°F, frostbite can develop on exposed skin in as little as 30 minutes; at -40°F, in under 10 minutes. Frostnip (early stage) causes red, cold, numb skin but is reversible with rewarming. Superficial frostbite causes white, waxy skin. Deep frostbite freezes all layers of tissue and requires immediate medical treatment. The best prevention is covering all exposed skin and limiting time outdoors when wind chills are dangerously low.

What is a funnel cloud and when does it become a tornado?

A funnel cloud is a rotating, cone-shaped cloud that extends downward from a severe thunderstorm but does not reach the ground. It becomes a tornado the moment it makes contact with the ground, or when debris or dust is visible at the surface beneath it even if the visible funnel has not fully extended down. A Tornado Warning may be issued for a confirmed funnel cloud that has not yet touched down because of the imminent danger. If you see a funnel cloud, treat it as a tornado warning and take immediate shelter, do not wait to see if it touches down before acting.

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Glossary of Weather Terms – Beginning with “F”