Glossary of Weather Terms - Beginning with "H"

Q: What is the difference between a Hurricane Watch and a Hurricane Warning?

A Hurricane Watch means hurricane conditions (sustained winds of 74 mph or greater) are possible within 48 hours - prepare now. A Hurricane Warning means hurricane conditions are expected within 36 hours - complete all preparations immediately and follow any evacuation orders. A Watch means be ready; a Warning means act now.

Q: What is the difference between a High Wind Advisory and a High Wind Warning?

A High Wind Advisory is issued for sustained winds of 26–39 mph or gusts of 45–57 mph - hazardous but manageable with caution. A High Wind Warning is issued for sustained winds of 40 mph or greater or gusts of 58 mph or greater, representing a more dangerous situation where significant property damage is possible and high-profile vehicles should stay off the road.

Q: When does the heat index become dangerous?

The National Weather Service considers a heat index of 90–103°F to be Extreme Caution where heat cramps and heat exhaustion are possible. Above 103°F is the Danger range where heat stroke is possible. Above 125°F is Extreme Danger where heat stroke is highly likely. A Heat Advisory is typically issued when the heat index reaches 100–105°F for 2 or more consecutive hours.

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Looking for the difference between a Hurricane Watch and Hurricane Warning, what the heat index really means, or the exact High Wind Warning criteria? This page defines all National Weather Service severe weather alerts and meteorological terms beginning with “H” – in plain English, with specific guidance on what each alert means and what to do when one is issued for your area.

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Weather Terms Beginning with “H”

Haboob

A haboob is an intense wall of dust and sand driven forward by strong outflow winds from a collapsing thunderstorm. Haboobs can rise to heights of 5,000 feet or more and reduce visibility to near zero within seconds. They are most common in the desert Southwest – particularly Arizona – but can also occur across the Great Plains and other arid regions. Motorists caught in a haboob should pull completely off the road, turn off their lights, and wait for it to pass.

Hail

Hail is precipitation in the form of balls or irregular chunks of ice produced by severe thunderstorms. Hailstones form when strong updrafts carry water droplets into the frigid upper atmosphere where they freeze and accumulate layers of ice. They range in size from small pea-sized pellets (1/4 inch) to grapefruit-sized stones (4.5 inches or larger). Large hail can total vehicles, destroy crops, and cause serious injury. A hailstone the size of a golf ball (1.75 inches) falls at approximately 44 mph.

Hail Warning / Severe Thunderstorm Warning (Hail)

The National Weather Service issues a Severe Thunderstorm Warning when radar indicates or a trained spotter reports hail of 1 inch (quarter size) or larger. Hail of 3/4 inch (penny size) or larger is sufficient to trigger a warning in many NWS offices. When hail reaches 2 inches or more in diameter, the NWS may add a Considerable or Destructive tag to the warning. Seek sturdy shelter immediately – a vehicle is not safe shelter from large hail or damaging winds.

Hard Freeze Warning

A Hard Freeze Warning is issued when temperatures at or below 28°F are expected to persist for at least 2 consecutive hours, typically during the growing season. These temperatures can kill crops and severely damage or destroy unprotected vegetation. This is a higher-impact alert than a standard Freeze Warning (which uses a threshold of 32°F). Gardeners, farmers, and anyone with exposed pipes should act before the warning period begins.

Hard Freeze Watch

A Hard Freeze Watch is issued when conditions are favorable for a hard freeze (temperatures at or below 28°F for 2 or more consecutive hours) within the next 36 to 48 hours. A Watch means conditions are possible but not yet certain – it is the time to prepare plants, cover sensitive crops, and protect exposed plumbing.

Haze

Haze is reduced atmospheric visibility caused by fine dust particles, smoke, pollutants, or salt aerosols suspended in the air. Unlike fog, haze does not require high relative humidity and can persist in dry conditions. It can significantly impair visibility for drivers, pilots, and boaters, and poses health risks for people with asthma or other respiratory conditions. During periods of dense smoke from wildfires, the NWS may issue an Air Quality Alert alongside haze advisories.

Heat Advisory

A Heat Advisory is issued by the National Weather Service when heat index values are expected to reach 100–105°F for 2 or more consecutive hours (thresholds vary by region and are calibrated to local climate normals). During a Heat Advisory, residents should limit outdoor physical activity, drink plenty of water, and check on elderly neighbors, children, and pets. Never leave a person or animal in a parked vehicle.

Heat Burst

A heat burst is a rare and dramatic meteorological event in which temperature suddenly spikes – sometimes 20°F or more within minutes – accompanied by strong, dry winds. Heat bursts typically occur late at night and are associated with dying thunderstorms: rain falling from a collapsing storm completely evaporates before reaching the ground, causing the surrounding air to rapidly heat and plunge downward. Though brief, heat bursts can produce localized temperatures exceeding 100°F in the middle of the night.

Heat Index

The heat index – also called the “feels like” temperature or apparent temperature – combines the actual air temperature with relative humidity to indicate how hot it actually feels to the human body. High humidity prevents sweat from evaporating effectively, which is the body’s primary cooling mechanism. A temperature of 90°F with 90% relative humidity produces a heat index near 122°F. The NWS uses the following general heat index thresholds:

80–90°F – Caution: fatigue possible with prolonged activity
90–103°F – Extreme Caution: heat cramps and heat exhaustion possible
103–124°F – Danger: heat cramps and heat exhaustion likely; heat stroke possible
Above 125°F – Extreme Danger: heat stroke highly likely

Heat Lightning

Heat lightning is a common informal term for lightning from a distant thunderstorm that illuminates the sky without audible thunder. Thunder cannot be heard beyond approximately 10–15 miles, so when a storm is 25 miles or more away, the lightning is visible but the thunder dissipates before it arrives. There is no special “heat lightning” – it is ordinary cloud-to-cloud or cloud-to-ground lightning. If you can see lightning, the storm is closer than it may appear, and you should move indoors immediately if it is within 10 miles.

Heat Wave

A heat wave is a prolonged period of excessively hot weather relative to the expected climate for an area, generally lasting two days or more. Heat waves are the deadliest weather phenomenon in the United States – they kill more people annually on average than tornadoes, hurricanes, and floods combined. Overnight heat is especially dangerous because the body needs cooler nighttime temperatures to recover. Urban areas are particularly vulnerable due to the heat island effect, where pavement and buildings trap and radiate heat.

Heavy Snow Warning

A Heavy Snow Warning is a specific type of Winter Storm Warning issued when heavy snowfall is forecast. Criteria vary by region but generally call for 6 or more inches in 12 hours, or 8 or more inches in 24 hours. Heavy Snow Warnings indicate travel will be hazardous to impossible. Motorists should avoid driving if possible; if travel is necessary, carry emergency supplies including blankets, water, and a flashlight.

High Pressure (Anticyclone)

A high pressure system is an area of the atmosphere where air pressure is higher than the surrounding region. High pressure is associated with descending, sinking air which suppresses cloud formation – resulting in clear skies and generally fair weather. In the Northern Hemisphere, winds around a high pressure system circulate clockwise and outward from the center. High pressure systems are sometimes called “anticyclones” and are the opposite of low pressure systems (cyclones), which are associated with storms and precipitation.

High Wind Advisory

A High Wind Advisory is issued when sustained non-convective winds of 26–39 mph and/or gusts of 45–57 mph are expected (criteria vary by NWS forecast office and local terrain). These winds can make driving hazardous – particularly for high-profile vehicles such as trucks, RVs, and buses – and may cause minor property damage, downed branches, and localized power outages. Secure outdoor furniture and objects that could blow around.

High Wind Warning

A High Wind Warning is issued when sustained non-convective winds of 40 mph or greater, and/or gusts of 58 mph or greater, are expected (criteria vary by NWS region and local terrain). High Wind Warnings indicate potentially dangerous conditions that could cause significant property damage, down large trees, and make travel extremely hazardous or impossible for high-profile vehicles. Outdoor activities should be suspended during a High Wind Warning.

High Wind Watch

A High Wind Watch is issued when conditions are favorable for a High Wind event – sustained winds of 40 mph or greater and/or gusts of 58 mph or greater – within the next 24 to 48 hours. A Watch means conditions are possible, not certain. Use the Watch period to secure outdoor items, restock emergency supplies, and prepare for the possibility of power outages.

High Wind Advisory vs. Watch vs. Warning – Quick Reference

	High Wind Advisory	High Wind Watch	High Wind Warning
Sustained winds	26–39 mph	≥40 mph possible	≥40 mph
Gusts	45–57 mph	≥58 mph possible	≥58 mph
When issued	Imminent or occurring	24–48 hours ahead	Imminent or occurring
Severity	Moderate	Elevated Risk	High – Dangerous
What to do	Use caution; secure loose outdoor items; avoid high-profile vehicles	Prepare now; secure outdoor objects; review plans	Stay indoors if possible; high-profile vehicles off the road; prepare for outages

Hodograph

A hodograph is a meteorological diagram that plots the wind speed and direction at multiple levels of the atmosphere onto a single graph. As altitude increases, each wind reading is plotted and connected to the next, creating a curve. The shape of this curve reveals the amount and type of wind shear present in the atmosphere. Strongly curved hodographs indicate large amounts of horizontal vorticity and are associated with supercell thunderstorms and significantly increased tornado potential.

Hook Echo

A hook echo is a distinctive hook-shaped appendage that appears on weather radar, typically on the southwest side of a supercell thunderstorm. It forms when precipitation is drawn around the storm’s rotating updraft (mesocyclone) by the inflow/outflow circulation. A hook echo is one of the strongest radar signatures indicating tornado development, though not every hook echo produces a tornado. Forecasters use hook echoes together with dual-polarization radar data and storm-relative velocity data to assess tornado probability.

Humidity / Relative Humidity

Humidity refers to the amount of water vapor present in the air. Relative humidity – the figure most commonly reported in weather forecasts – is the ratio of actual water vapor in the air to the maximum amount the air can hold at a given temperature, expressed as a percentage. At 100% relative humidity, air is saturated and condensation occurs in the form of clouds, fog, or dew. High humidity combined with high temperatures makes heat far more dangerous (see Heat Index above).

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Hurricane

A hurricane is a type of tropical cyclone – a large rotating storm system – with sustained surface winds of 74 mph (64 knots) or greater. Hurricanes form over warm tropical ocean waters and are rated on the Saffir-Simpson Hurricane Wind Scale:

Category 1: 74–95 mph – Minimal damage to well-constructed homes; some damage to mobile homes and trees
Category 2: 96–110 mph – Extensive damage; major roof and siding damage; power outages lasting days to weeks
Category 3: 111–129 mph – Devastating damage; well-built homes may sustain major damage; water and electricity unavailable for days to weeks
Category 4: 130–156 mph – Catastrophic damage; most of the area uninhabitable for weeks to months
Category 5: 157 mph or higher – Catastrophic damage; complete roof failure; total destruction of mobile homes; area uninhabitable for weeks to months

In addition to wind, hurricanes generate life-threatening storm surge (ocean water pushed inland), torrential rainfall and flooding, and can spawn tornadoes far inland.

Hurricane Force Wind Warning (Marine)

Issued for offshore and coastal marine areas when non-tropical system winds of 64 knots (74 mph) or greater are expected. This is the highest-level marine wind warning and indicates extremely dangerous sea conditions. All vessels should seek safe harbor immediately.

Hurricane Local Statement (HLS)

A Hurricane Local Statement is an informational product issued by local National Weather Service forecast offices to provide area-specific guidance when a tropical cyclone threatens. It translates the national tropical advisories into local timing, rainfall totals, storm surge ranges, and preparedness recommendations for specific counties and communities. The HLS is updated frequently as the storm approaches and is one of the most useful products for residents deciding whether to evacuate.

Hurricane Season

Hurricane season is the official period when tropical storms and hurricanes are most likely to develop. In the Atlantic Basin (including the Gulf of Mexico and Caribbean), hurricane season runs June 1 through November 30. In the Eastern Pacific, it runs May 15 through November 30. Statistically, the peak of Atlantic hurricane season is September 10. While storms can form outside of these dates, the vast majority occur within this window.

Hurricane Warning

A Hurricane Warning is issued by the National Hurricane Center when hurricane conditions – sustained winds of 74 mph or greater – are expected somewhere within a specified area within 36 hours. When a Hurricane Warning is issued, all preparations should be completed immediately. If local officials have ordered evacuation, leave at once – do not wait. Evacuating after a warning is issued becomes increasingly dangerous as roads become congested and weather deteriorates.

Hurricane Watch

A Hurricane Watch is issued when hurricane conditions are possible within a specified area, typically within 48 hours. A Watch is not a certainty – it means conditions could develop, giving residents time to prepare and finalize evacuation plans. During a Hurricane Watch, begin preparation: stock emergency supplies, fuel your vehicle, know your evacuation route, and identify your nearest shelter.

Hurricane Watch vs. Hurricane Warning – Key Differences

	Hurricane Watch	Hurricane Warning
What it means	Hurricane conditions possible	Hurricane conditions expected
Timeframe	Within 48 hours	Within 36 hours
Wind threshold	≥74 mph possible	≥74 mph expected
Urgency	Elevated – Prepare now	High – Act now; evacuate if ordered
Key action	Finalize supplies; know your evacuation route; be ready to leave	Rush all preparations to completion; leave immediately if ordered

Hydrologic Advisory (Flood Advisory)

A Hydrologic Advisory (commonly called a Flood Advisory) is issued to advise the public of minor flooding that may cause significant inconvenience or localized hazard but is not considered life-threatening. It is a lower-level notification than a Flood Warning and is often issued for nuisance flooding of streets, underpasses, low-lying areas, and intersections. Motorists should avoid driving through flooded roadways – just 6 inches of moving water can knock a person down, and 2 feet of moving water can carry away most vehicles.

Hydrologic Outlook (Flood Potential Outlook)

A Hydrologic Outlook provides advance notice of the potential for flooding beyond the standard short-range forecast period, generally beyond 3 days. It is used when a combination of heavy rainfall, snowmelt, saturated soils, or other conditions may produce significant flooding. Hydrologic Outlooks are intended to alert emergency managers, public officials, and the public early enough to allow for advanced preparation.

Hygrometer

A hygrometer is a meteorological instrument used to measure the relative humidity of the air. Modern digital hygrometers provide accurate real-time humidity readings and are used in professional weather stations, homes, and HVAC systems. The dew point temperature is a closely related measurement – the temperature at which the air would become saturated and moisture would begin to condense. Dew points above 65°F feel humid and uncomfortable; dew points above 70°F are oppressive.

Hyperthermia

Hyperthermia is a dangerous condition in which the body’s temperature rises to abnormally high levels due to heat exposure. It encompasses a spectrum of heat-related illnesses:

Heat Cramps: Painful muscle cramps caused by electrolyte imbalance from sweating; an early warning sign
Heat Exhaustion: Heavy sweating, cool/pale/clammy skin, weakness, and nausea; body temperature may reach 104°F – move the person to a cool area immediately
Heat Stroke: Body temperature above 104°F, hot/red/dry or damp skin, rapid pulse, possible unconsciousness – this is a life-threatening emergency; call 911 immediately and cool the person rapidly

Children, the elderly, and outdoor workers are at greatest risk. During Heat Advisories and Excessive Heat Warnings, check on at-risk individuals frequently.

Hypothermia

Hypothermia is a medical emergency in which the body loses heat faster than it produces it, causing core body temperature to drop below 95°F (35°C). Contrary to popular belief, hypothermia does not require freezing temperatures – it can occur in water or wet, windy conditions at temperatures well above 32°F. Symptoms progress from shivering and slurred speech to loss of coordination, confusion, drowsiness, and eventually unconsciousness. Anyone suspected of hypothermia should be moved out of the cold, have wet clothing replaced with dry coverings, and receive emergency medical attention immediately.

Frequently Asked Questions – “H” Weather Terms

What is the difference between a Hurricane Watch and a Hurricane Warning?

A Hurricane Watch means hurricane conditions (winds ≥74 mph) are possible within 48 hours – prepare now. A Hurricane Warning means hurricane conditions are expected within 36 hours – complete all preparations immediately and follow any evacuation orders. A Watch is “be ready.” A Warning is “act now.”

What is the difference between a High Wind Advisory and a High Wind Warning?

A High Wind Advisory covers sustained winds of 26–39 mph or gusts of 45–57 mph – hazardous but manageable with caution. A High Wind Warning is issued for sustained winds of 40 mph or greater or gusts of 58 mph or greater – a more dangerous situation where significant property damage is possible and high-profile vehicles should be kept off the road entirely.

When does the heat index become dangerous?

The NWS considers a heat index of 90–103°F to be in the “Extreme Caution” range where heat cramps and heat exhaustion are possible. Above 103°F is the “Danger” range where heat stroke is possible. Above 125°F is “Extreme Danger” where heat stroke is highly likely. A Heat Advisory is typically triggered when the heat index is expected to reach 100–105°F for 2 or more consecutive hours.

What should I do during a High Wind Warning?

During a High Wind Warning: avoid driving high-profile vehicles (trucks, RVs, trailers, buses); bring indoors or secure any outdoor furniture, decorations, or objects that could become airborne; stay away from trees and power lines; and prepare for possible power outages – keep flashlights and backup power accessible.

At what temperature does hypothermia occur?

Hypothermia begins when core body temperature drops below 95°F (35°C). It does not require freezing temperatures – hypothermia can develop in wet and windy conditions at temperatures as mild as 50°F. Cold water immersion is especially dangerous and can cause hypothermia within minutes even in moderately cool water. The elderly, young children, and those who are outdoors or wet are at highest risk.

What does a Hard Freeze Warning mean for my plants?

A Hard Freeze Warning means temperatures are expected to drop to 28°F or below for at least 2 consecutive hours. At these temperatures, most annual plants and vegetables are killed outright and many perennials sustain serious damage. Cover sensitive plants with frost cloth or bring containers indoors before the freeze begins – do not wait until after the temperatures have already dropped.

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Glossary of Weather Terms – Beginning with “H”