None of the terms on this page are marketing language or rough estimates. Each one is a precisely defined technical value with an official government formula behind it, developed specifically because the raw thermometer reading, or the single word "flood," does not tell a household what it actually needs to know to stay safe.
01 — Wind chill: what the number actually measures
Wind chill measures heat loss from skin, not a change in the actual air temperature
The National Weather Service defines wind chill as the rate of heat loss from exposed skin caused by the combined effect of wind and cold, not an actual drop in air temperature. A thermometer at 20°F reads 20°F regardless of wind; what changes is how quickly a body loses heat to the surrounding air, since wind continuously strips away the thin layer of warmed air the body maintains next to the skin and replaces it with cold air. The current NWS/Environment Canada formula, adopted in 2001, is Wind Chill = 35.74 + 0.6215T − 35.75(V^0.16) + 0.4275T(V^0.16), where T is air temperature in °F and V is wind speed in mph. Applied to a 20°F temperature with a 20 mph wind, this formula produces a wind chill of approximately 4°F, meaning exposed skin loses heat at the same rate it would in still air at 4°F.
This formula replaced a 1945 index that significantly overstated cooling, because it was originally derived from how quickly water froze in open cans, not from how human skin actually loses heat. The 2001 revision was developed using live human trial data from volunteers walking on treadmills inside a refrigerated wind tunnel, producing figures typically 10 to 15°F warmer than the old formula predicted at high wind speeds. The formula is only valid for temperatures at or below 50°F and wind speeds of 3 mph or greater; it also applies only to living organisms. An inanimate object like a car radiator or a water pipe cools faster in wind, but it will never drop below the actual air temperature the way exposed skin effectively does.
- Wind chill applies only to living things, not objects. A pipe or car radiator cools faster in wind but never goes below the actual air temperature; only skin experiences the accelerated heat-loss effect wind chill describes.
- The formula is valid only at or below 50°F and wind of 3+ mph. Outside that range, the NWS formula does not apply and produces meaningless results.
- The current formula (2001) is more accurate than the original 1945 version. The old index, based on freezing water in cans, overstated cooling by 10 to 15°F at high wind speeds compared to real human skin data.
- Wind chill directly predicts frostbite time on exposed skin. NWS wind chill charts pair specific values with frostbite exposure windows, ranging from about 30 minutes down to 5 minutes or less at the most extreme readings.
02 — Heat index: what the number actually measures
Heat index measures how humidity slows the body's ability to cool itself through sweat
Heat index measures how hot the air actually feels to the human body when relative humidity is factored in alongside air temperature. The body's primary cooling mechanism is the evaporation of sweat; high humidity slows that evaporation because the surrounding air is already close to saturated with moisture, which reduces the body's ability to shed heat even though the thermometer reading has not changed. The NWS heat index formula, known as the Rothfusz regression and built on 1979 research by physicist Robert Steadman, produces the specific, commonly cited example of a 90°F day at 70 percent relative humidity feeling like 106°F, a 16-degree gap driven entirely by humidity's effect on evaporative cooling.
Two limits matter for reading a heat index value correctly. First, the NWS formula is valid only for temperatures at or above 80°F and relative humidity at or above 40 percent; below that range, humidity has little practical effect on perceived temperature. Second, published heat index values assume shady conditions with light wind; full sun exposure can add up to 15°F to the effective heat a person actually experiences, meaning the number on a forecast can meaningfully understate the real risk for anyone working or exercising outdoors in direct sunlight. Heat is responsible for more weather-related deaths in the United States in a typical year than any other single hazard, which is why the heat index, not the raw air temperature, is the figure NWS heat alerts are actually based on.
- Heat index measures reduced sweat-evaporation efficiency, not just temperature. High humidity slows the body's primary cooling mechanism, which is why a humid 90°F feels far more dangerous than a dry 90°F.
- The formula applies at 80°F and above, with 40% or higher relative humidity. Below that range, humidity has little meaningful effect on perceived temperature.
- Full sun can add up to 15°F to the effective heat index. Published values assume shade and light wind; anyone working or exercising in direct sunlight faces meaningfully higher real heat stress than the forecast number suggests.
- Heat kills more people in a typical year than any other weather hazard. NWS heat alerts and advisories are based on heat index values specifically because they capture the actual physiological danger better than air temperature alone.
03 — Flash flood, river flood, and coastal flood are three different hazards
Each flood type develops on a different timeline and requires a different response
NOAA's National Severe Storms Laboratory and the U.S. Geological Survey both distinguish flooding by cause and by how quickly it develops, and that distinction matters directly for what response is correct. A flash flood develops in minutes to hours from intense, often very localized rainfall, and the USGS notes that flash floods generally cause the greatest loss of life specifically because their speed leaves so little time to react; a dangerous wall of fast-moving water can appear with almost no warning, carrying debris capable of sweeping away vehicles.
A river flood develops over a longer timeline, typically days, as sustained or repeated heavy rain, snowmelt, an ice jam, or a dam or levee failure causes a river to rise past its banks. Because this process unfolds more slowly, the USGS notes river floods generally cause greater property loss rather than greater loss of life, since the longer timeline usually allows more time to evacuate but affects a wider area for a longer duration. A coastal flood is driven by storm surge, ocean water pushed inland by a storm's winds and low pressure, worsened by higher-than-average tides and onshore wind; NOAA notes storm surge specifically can raise water levels in just a few minutes once a storm's wind field arrives, giving coastal residents a narrower window to act than the term "flood" alone suggests.
- Flash floods: minutes to hours, from intense localized rainfall. The speed of onset is what makes flash floods the leading cause of flood-related deaths; never drive through a flooded road, since floodwater depth and road integrity are both impossible to judge from a vehicle.
- River floods: days, from sustained rain, snowmelt, ice jams, or dam and levee issues. The slower timeline generally allows more evacuation time but affects property over a wider area and longer duration.
- Coastal floods: driven by storm surge, worsened by high tide and onshore wind. Storm surge specifically can raise water levels within just a few minutes once a storm's wind field reaches the coast.
- Each type calls for a different response. Flash flood safety means moving to high ground immediately; river flood safety means monitoring river forecasts and evacuating before roads become impassable; coastal flood safety means following evacuation zone orders before the storm arrives, not waiting to see the surge in person.
Quick reference
- Wind chill measures heat loss from exposed skin, not actual temperature. 20°F with 20 mph wind feels like about 4°F on skin. Applies only to living things and only at or below 50°F with wind of 3+ mph.
- Heat index measures how humidity slows sweat evaporation, making heat feel more dangerous than the thermometer alone suggests. 90°F at 70% humidity feels like 106°F. Full sun adds up to 15°F beyond the published value.
- Flash floods (minutes to hours) cause the most deaths due to speed. River floods (days) cause the most property damage due to duration and area. Coastal floods (storm surge) can rise within minutes of a storm's arrival.
Primary sources
- National Weather Service: Understanding Wind Chill: the official wind chill definition, the 2001 formula, its validity range, and confirmation that wind chill affects only living organisms.
- National Weather Service: Wind Chill Chart: the official formula and the frostbite-time correlation by wind chill value.
- National Weather Service: Heat Index: the heat index definition, the 90°F/70% humidity/106°F example, and the shade and light-wind assumption behind published values.
- Calculator.net, citing NWS methodology: Heat Index Calculator: the Rothfusz regression basis, Robert Steadman's 1979 research, and the formula's validity range (80°F+, 40%+ relative humidity).
- NOAA National Severe Storms Laboratory: Severe Weather 101, Flood Types: the definitions of river flooding, coastal flooding, storm surge, and inland flooding.
- U.S. Geological Survey: What Are the Two Types of Floods?: the finding that flash floods generally cause greater loss of life while river floods generally cause greater loss of property.